510 lines
24 KiB
Plaintext
510 lines
24 KiB
Plaintext
This file contains brief information about the SCSI tape driver.
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The driver is currently maintained by Kai Mäkisara (email
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Kai.Makisara@kolumbus.fi)
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Last modified: Sun Feb 24 21:59:07 2008 by kai.makisara
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BASICS
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The driver is generic, i.e., it does not contain any code tailored
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to any specific tape drive. The tape parameters can be specified with
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one of the following three methods:
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1. Each user can specify the tape parameters he/she wants to use
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directly with ioctls. This is administratively a very simple and
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flexible method and applicable to single-user workstations. However,
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in a multiuser environment the next user finds the tape parameters in
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state the previous user left them.
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2. The system manager (root) can define default values for some tape
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parameters, like block size and density using the MTSETDRVBUFFER ioctl.
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These parameters can be programmed to come into effect either when a
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new tape is loaded into the drive or if writing begins at the
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beginning of the tape. The second method is applicable if the tape
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drive performs auto-detection of the tape format well (like some
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QIC-drives). The result is that any tape can be read, writing can be
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continued using existing format, and the default format is used if
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the tape is rewritten from the beginning (or a new tape is written
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for the first time). The first method is applicable if the drive
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does not perform auto-detection well enough and there is a single
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"sensible" mode for the device. An example is a DAT drive that is
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used only in variable block mode (I don't know if this is sensible
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or not :-).
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The user can override the parameters defined by the system
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manager. The changes persist until the defaults again come into
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effect.
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3. By default, up to four modes can be defined and selected using the minor
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number (bits 5 and 6). The number of modes can be changed by changing
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ST_NBR_MODE_BITS in st.h. Mode 0 corresponds to the defaults discussed
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above. Additional modes are dormant until they are defined by the
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system manager (root). When specification of a new mode is started,
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the configuration of mode 0 is used to provide a starting point for
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definition of the new mode.
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Using the modes allows the system manager to give the users choices
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over some of the buffering parameters not directly accessible to the
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users (buffered and asynchronous writes). The modes also allow choices
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between formats in multi-tape operations (the explicitly overridden
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parameters are reset when a new tape is loaded).
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If more than one mode is used, all modes should contain definitions
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for the same set of parameters.
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Many Unices contain internal tables that associate different modes to
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supported devices. The Linux SCSI tape driver does not contain such
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tables (and will not do that in future). Instead of that, a utility
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program can be made that fetches the inquiry data sent by the device,
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scans its database, and sets up the modes using the ioctls. Another
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alternative is to make a small script that uses mt to set the defaults
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tailored to the system.
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The driver supports fixed and variable block size (within buffer
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limits). Both the auto-rewind (minor equals device number) and
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non-rewind devices (minor is 128 + device number) are implemented.
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In variable block mode, the byte count in write() determines the size
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of the physical block on tape. When reading, the drive reads the next
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tape block and returns to the user the data if the read() byte count
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is at least the block size. Otherwise, error ENOMEM is returned.
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In fixed block mode, the data transfer between the drive and the
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driver is in multiples of the block size. The write() byte count must
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be a multiple of the block size. This is not required when reading but
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may be advisable for portability.
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Support is provided for changing the tape partition and partitioning
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of the tape with one or two partitions. By default support for
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partitioned tape is disabled for each driver and it can be enabled
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with the ioctl MTSETDRVBUFFER.
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By default the driver writes one filemark when the device is closed after
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writing and the last operation has been a write. Two filemarks can be
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optionally written. In both cases end of data is signified by
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returning zero bytes for two consecutive reads.
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If rewind, offline, bsf, or seek is done and previous tape operation was
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write, a filemark is written before moving tape.
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The compile options are defined in the file linux/drivers/scsi/st_options.h.
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4. If the open option O_NONBLOCK is used, open succeeds even if the
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drive is not ready. If O_NONBLOCK is not used, the driver waits for
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the drive to become ready. If this does not happen in ST_BLOCK_SECONDS
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seconds, open fails with the errno value EIO. With O_NONBLOCK the
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device can be opened for writing even if there is a write protected
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tape in the drive (commands trying to write something return error if
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attempted).
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MINOR NUMBERS
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The tape driver currently supports 128 drives by default. This number
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can be increased by editing st.h and recompiling the driver if
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necessary. The upper limit is 2^17 drives if 4 modes for each drive
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are used.
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The minor numbers consist of the following bit fields:
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dev_upper non-rew mode dev-lower
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20 - 8 7 6 5 4 0
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The non-rewind bit is always bit 7 (the uppermost bit in the lowermost
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byte). The bits defining the mode are below the non-rewind bit. The
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remaining bits define the tape device number. This numbering is
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backward compatible with the numbering used when the minor number was
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only 8 bits wide.
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SYSFS SUPPORT
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The driver creates the directory /sys/class/scsi_tape and populates it with
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directories corresponding to the existing tape devices. There are autorewind
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and non-rewind entries for each mode. The names are stxy and nstxy, where x
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is the tape number and y a character corresponding to the mode (none, l, m,
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a). For example, the directories for the first tape device are (assuming four
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modes): st0 nst0 st0l nst0l st0m nst0m st0a nst0a.
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Each directory contains the entries: default_blksize default_compression
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default_density defined dev device driver. The file 'defined' contains 1
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if the mode is defined and zero if not defined. The files 'default_*' contain
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the defaults set by the user. The value -1 means the default is not set. The
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file 'dev' contains the device numbers corresponding to this device. The links
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'device' and 'driver' point to the SCSI device and driver entries.
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Each directory also contains the entry 'options' which shows the currently
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enabled driver and mode options. The value in the file is a bit mask where the
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bit definitions are the same as those used with MTSETDRVBUFFER in setting the
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options.
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A link named 'tape' is made from the SCSI device directory to the class
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directory corresponding to the mode 0 auto-rewind device (e.g., st0).
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BSD AND SYS V SEMANTICS
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The user can choose between these two behaviours of the tape driver by
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defining the value of the symbol ST_SYSV. The semantics differ when a
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file being read is closed. The BSD semantics leaves the tape where it
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currently is whereas the SYS V semantics moves the tape past the next
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filemark unless the filemark has just been crossed.
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The default is BSD semantics.
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BUFFERING
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The driver tries to do transfers directly to/from user space. If this
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is not possible, a driver buffer allocated at run-time is used. If
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direct i/o is not possible for the whole transfer, the driver buffer
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is used (i.e., bounce buffers for individual pages are not
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used). Direct i/o can be impossible because of several reasons, e.g.:
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- one or more pages are at addresses not reachable by the HBA
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- the number of pages in the transfer exceeds the number of
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scatter/gather segments permitted by the HBA
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- one or more pages can't be locked into memory (should not happen in
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any reasonable situation)
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The size of the driver buffers is always at least one tape block. In fixed
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block mode, the minimum buffer size is defined (in 1024 byte units) by
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ST_FIXED_BUFFER_BLOCKS. With small block size this allows buffering of
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several blocks and using one SCSI read or write to transfer all of the
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blocks. Buffering of data across write calls in fixed block mode is
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allowed if ST_BUFFER_WRITES is non-zero and direct i/o is not used.
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Buffer allocation uses chunks of memory having sizes 2^n * (page
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size). Because of this the actual buffer size may be larger than the
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minimum allowable buffer size.
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NOTE that if direct i/o is used, the small writes are not buffered. This may
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cause a surprise when moving from 2.4. There small writes (e.g., tar without
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-b option) may have had good throughput but this is not true any more with
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2.6. Direct i/o can be turned off to solve this problem but a better solution
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is to use bigger write() byte counts (e.g., tar -b 64).
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Asynchronous writing. Writing the buffer contents to the tape is
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started and the write call returns immediately. The status is checked
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at the next tape operation. Asynchronous writes are not done with
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direct i/o and not in fixed block mode.
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Buffered writes and asynchronous writes may in some rare cases cause
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problems in multivolume operations if there is not enough space on the
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tape after the early-warning mark to flush the driver buffer.
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Read ahead for fixed block mode (ST_READ_AHEAD). Filling the buffer is
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attempted even if the user does not want to get all of the data at
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this read command. Should be disabled for those drives that don't like
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a filemark to truncate a read request or that don't like backspacing.
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Scatter/gather buffers (buffers that consist of chunks non-contiguous
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in the physical memory) are used if contiguous buffers can't be
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allocated. To support all SCSI adapters (including those not
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supporting scatter/gather), buffer allocation is using the following
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three kinds of chunks:
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1. The initial segment that is used for all SCSI adapters including
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those not supporting scatter/gather. The size of this buffer will be
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(PAGE_SIZE << ST_FIRST_ORDER) bytes if the system can give a chunk of
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this size (and it is not larger than the buffer size specified by
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ST_BUFFER_BLOCKS). If this size is not available, the driver halves
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the size and tries again until the size of one page. The default
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settings in st_options.h make the driver to try to allocate all of the
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buffer as one chunk.
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2. The scatter/gather segments to fill the specified buffer size are
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allocated so that as many segments as possible are used but the number
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of segments does not exceed ST_FIRST_SG.
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3. The remaining segments between ST_MAX_SG (or the module parameter
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max_sg_segs) and the number of segments used in phases 1 and 2
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are used to extend the buffer at run-time if this is necessary. The
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number of scatter/gather segments allowed for the SCSI adapter is not
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exceeded if it is smaller than the maximum number of scatter/gather
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segments specified. If the maximum number allowed for the SCSI adapter
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is smaller than the number of segments used in phases 1 and 2,
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extending the buffer will always fail.
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EOM BEHAVIOUR WHEN WRITING
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When the end of medium early warning is encountered, the current write
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is finished and the number of bytes is returned. The next write
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returns -1 and errno is set to ENOSPC. To enable writing a trailer,
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the next write is allowed to proceed and, if successful, the number of
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bytes is returned. After this, -1 and the number of bytes are
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alternately returned until the physical end of medium (or some other
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error) is encountered.
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MODULE PARAMETERS
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The buffer size, write threshold, and the maximum number of allocated buffers
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are configurable when the driver is loaded as a module. The keywords are:
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buffer_kbs=xxx the buffer size for fixed block mode is set
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to xxx kilobytes
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write_threshold_kbs=xxx the write threshold in kilobytes set to xxx
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max_sg_segs=xxx the maximum number of scatter/gather
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segments
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try_direct_io=x try direct transfer between user buffer and
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tape drive if this is non-zero
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Note that if the buffer size is changed but the write threshold is not
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set, the write threshold is set to the new buffer size - 2 kB.
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BOOT TIME CONFIGURATION
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If the driver is compiled into the kernel, the same parameters can be
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also set using, e.g., the LILO command line. The preferred syntax is
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to use the same keyword used when loading as module but prepended
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with 'st.'. For instance, to set the maximum number of scatter/gather
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segments, the parameter 'st.max_sg_segs=xx' should be used (xx is the
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number of scatter/gather segments).
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For compatibility, the old syntax from early 2.5 and 2.4 kernel
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versions is supported. The same keywords can be used as when loading
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the driver as module. If several parameters are set, the keyword-value
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pairs are separated with a comma (no spaces allowed). A colon can be
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used instead of the equal mark. The definition is prepended by the
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string st=. Here is an example:
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st=buffer_kbs:64,write_threshold_kbs:60
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The following syntax used by the old kernel versions is also supported:
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st=aa[,bb[,dd]]
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where
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aa is the buffer size for fixed block mode in 1024 byte units
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bb is the write threshold in 1024 byte units
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dd is the maximum number of scatter/gather segments
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IOCTLS
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The tape is positioned and the drive parameters are set with ioctls
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defined in mtio.h The tape control program 'mt' uses these ioctls. Try
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to find an mt that supports all of the Linux SCSI tape ioctls and
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opens the device for writing if the tape contents will be modified
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(look for a package mt-st* from the Linux ftp sites; the GNU mt does
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not open for writing for, e.g., erase).
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The supported ioctls are:
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The following use the structure mtop:
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MTFSF Space forward over count filemarks. Tape positioned after filemark.
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MTFSFM As above but tape positioned before filemark.
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MTBSF Space backward over count filemarks. Tape positioned before
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filemark.
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MTBSFM As above but ape positioned after filemark.
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MTFSR Space forward over count records.
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MTBSR Space backward over count records.
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MTFSS Space forward over count setmarks.
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MTBSS Space backward over count setmarks.
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MTWEOF Write count filemarks.
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MTWSM Write count setmarks.
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MTREW Rewind tape.
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MTOFFL Set device off line (often rewind plus eject).
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MTNOP Do nothing except flush the buffers.
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MTRETEN Re-tension tape.
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MTEOM Space to end of recorded data.
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MTERASE Erase tape. If the argument is zero, the short erase command
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is used. The long erase command is used with all other values
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of the argument.
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MTSEEK Seek to tape block count. Uses Tandberg-compatible seek (QFA)
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for SCSI-1 drives and SCSI-2 seek for SCSI-2 drives. The file and
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block numbers in the status are not valid after a seek.
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MTSETBLK Set the drive block size. Setting to zero sets the drive into
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variable block mode (if applicable).
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MTSETDENSITY Sets the drive density code to arg. See drive
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documentation for available codes.
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MTLOCK and MTUNLOCK Explicitly lock/unlock the tape drive door.
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MTLOAD and MTUNLOAD Explicitly load and unload the tape. If the
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command argument x is between MT_ST_HPLOADER_OFFSET + 1 and
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MT_ST_HPLOADER_OFFSET + 6, the number x is used sent to the
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drive with the command and it selects the tape slot to use of
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HP C1553A changer.
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MTCOMPRESSION Sets compressing or uncompressing drive mode using the
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SCSI mode page 15. Note that some drives other methods for
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control of compression. Some drives (like the Exabytes) use
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density codes for compression control. Some drives use another
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mode page but this page has not been implemented in the
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driver. Some drives without compression capability will accept
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any compression mode without error.
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MTSETPART Moves the tape to the partition given by the argument at the
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next tape operation. The block at which the tape is positioned
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is the block where the tape was previously positioned in the
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new active partition unless the next tape operation is
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MTSEEK. In this case the tape is moved directly to the block
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specified by MTSEEK. MTSETPART is inactive unless
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MT_ST_CAN_PARTITIONS set.
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MTMKPART Formats the tape with one partition (argument zero) or two
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partitions (the argument gives in megabytes the size of
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partition 1 that is physically the first partition of the
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tape). The drive has to support partitions with size specified
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by the initiator. Inactive unless MT_ST_CAN_PARTITIONS set.
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MTSETDRVBUFFER
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Is used for several purposes. The command is obtained from count
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with mask MT_SET_OPTIONS, the low order bits are used as argument.
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This command is only allowed for the superuser (root). The
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subcommands are:
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0
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The drive buffer option is set to the argument. Zero means
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no buffering.
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MT_ST_BOOLEANS
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Sets the buffering options. The bits are the new states
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(enabled/disabled) the following options (in the
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parenthesis is specified whether the option is global or
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can be specified differently for each mode):
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MT_ST_BUFFER_WRITES write buffering (mode)
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MT_ST_ASYNC_WRITES asynchronous writes (mode)
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MT_ST_READ_AHEAD read ahead (mode)
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MT_ST_TWO_FM writing of two filemarks (global)
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MT_ST_FAST_EOM using the SCSI spacing to EOD (global)
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MT_ST_AUTO_LOCK automatic locking of the drive door (global)
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MT_ST_DEF_WRITES the defaults are meant only for writes (mode)
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MT_ST_CAN_BSR backspacing over more than one records can
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be used for repositioning the tape (global)
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MT_ST_NO_BLKLIMS the driver does not ask the block limits
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from the drive (block size can be changed only to
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variable) (global)
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MT_ST_CAN_PARTITIONS enables support for partitioned
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tapes (global)
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MT_ST_SCSI2LOGICAL the logical block number is used in
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the MTSEEK and MTIOCPOS for SCSI-2 drives instead of
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the device dependent address. It is recommended to set
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this flag unless there are tapes using the device
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dependent (from the old times) (global)
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MT_ST_SYSV sets the SYSV semantics (mode)
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MT_ST_NOWAIT enables immediate mode (i.e., don't wait for
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the command to finish) for some commands (e.g., rewind)
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MT_ST_SILI enables setting the SILI bit in SCSI commands when
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reading in variable block mode to enhance performance when
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reading blocks shorter than the byte count; set this only
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if you are sure that the drive supports SILI and the HBA
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correctly returns transfer residuals
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MT_ST_DEBUGGING debugging (global; debugging must be
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compiled into the driver)
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MT_ST_SETBOOLEANS
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MT_ST_CLEARBOOLEANS
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Sets or clears the option bits.
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MT_ST_WRITE_THRESHOLD
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Sets the write threshold for this device to kilobytes
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specified by the lowest bits.
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MT_ST_DEF_BLKSIZE
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Defines the default block size set automatically. Value
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0xffffff means that the default is not used any more.
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MT_ST_DEF_DENSITY
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MT_ST_DEF_DRVBUFFER
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Used to set or clear the density (8 bits), and drive buffer
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state (3 bits). If the value is MT_ST_CLEAR_DEFAULT
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(0xfffff) the default will not be used any more. Otherwise
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the lowermost bits of the value contain the new value of
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the parameter.
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MT_ST_DEF_COMPRESSION
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The compression default will not be used if the value of
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the lowermost byte is 0xff. Otherwise the lowermost bit
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contains the new default. If the bits 8-15 are set to a
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non-zero number, and this number is not 0xff, the number is
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used as the compression algorithm. The value
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MT_ST_CLEAR_DEFAULT can be used to clear the compression
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default.
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MT_ST_SET_TIMEOUT
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Set the normal timeout in seconds for this device. The
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default is 900 seconds (15 minutes). The timeout should be
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long enough for the retries done by the device while
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reading/writing.
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MT_ST_SET_LONG_TIMEOUT
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Set the long timeout that is used for operations that are
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known to take a long time. The default is 14000 seconds
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(3.9 hours). For erase this value is further multiplied by
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eight.
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MT_ST_SET_CLN
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Set the cleaning request interpretation parameters using
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the lowest 24 bits of the argument. The driver can set the
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generic status bit GMT_CLN if a cleaning request bit pattern
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is found from the extended sense data. Many drives set one or
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more bits in the extended sense data when the drive needs
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cleaning. The bits are device-dependent. The driver is
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given the number of the sense data byte (the lowest eight
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bits of the argument; must be >= 18 (values 1 - 17
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reserved) and <= the maximum requested sense data sixe),
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a mask to select the relevant bits (the bits 9-16), and the
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bit pattern (bits 17-23). If the bit pattern is zero, one
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or more bits under the mask indicate cleaning request. If
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the pattern is non-zero, the pattern must match the masked
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sense data byte.
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(The cleaning bit is set if the additional sense code and
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qualifier 00h 17h are seen regardless of the setting of
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MT_ST_SET_CLN.)
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The following ioctl uses the structure mtpos:
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MTIOCPOS Reads the current position from the drive. Uses
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Tandberg-compatible QFA for SCSI-1 drives and the SCSI-2
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command for the SCSI-2 drives.
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The following ioctl uses the structure mtget to return the status:
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MTIOCGET Returns some status information.
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The file number and block number within file are returned. The
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block is -1 when it can't be determined (e.g., after MTBSF).
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The drive type is either MTISSCSI1 or MTISSCSI2.
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The number of recovered errors since the previous status call
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is stored in the lower word of the field mt_erreg.
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The current block size and the density code are stored in the field
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mt_dsreg (shifts for the subfields are MT_ST_BLKSIZE_SHIFT and
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MT_ST_DENSITY_SHIFT).
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The GMT_xxx status bits reflect the drive status. GMT_DR_OPEN
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is set if there is no tape in the drive. GMT_EOD means either
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end of recorded data or end of tape. GMT_EOT means end of tape.
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MISCELLANEOUS COMPILE OPTIONS
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The recovered write errors are considered fatal if ST_RECOVERED_WRITE_FATAL
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is defined.
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The maximum number of tape devices is determined by the define
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ST_MAX_TAPES. If more tapes are detected at driver initialization, the
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maximum is adjusted accordingly.
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Immediate return from tape positioning SCSI commands can be enabled by
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defining ST_NOWAIT. If this is defined, the user should take care that
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the next tape operation is not started before the previous one has
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finished. The drives and SCSI adapters should handle this condition
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gracefully, but some drive/adapter combinations are known to hang the
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SCSI bus in this case.
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The MTEOM command is by default implemented as spacing over 32767
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filemarks. With this method the file number in the status is
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correct. The user can request using direct spacing to EOD by setting
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ST_FAST_EOM 1 (or using the MT_ST_OPTIONS ioctl). In this case the file
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number will be invalid.
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When using read ahead or buffered writes the position within the file
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may not be correct after the file is closed (correct position may
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require backspacing over more than one record). The correct position
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within file can be obtained if ST_IN_FILE_POS is defined at compile
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time or the MT_ST_CAN_BSR bit is set for the drive with an ioctl.
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(The driver always backs over a filemark crossed by read ahead if the
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user does not request data that far.)
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DEBUGGING HINTS
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To enable debugging messages, edit st.c and #define DEBUG 1. As seen
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above, debugging can be switched off with an ioctl if debugging is
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compiled into the driver. The debugging output is not voluminous.
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If the tape seems to hang, I would be very interested to hear where
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the driver is waiting. With the command 'ps -l' you can see the state
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of the process using the tape. If the state is D, the process is
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waiting for something. The field WCHAN tells where the driver is
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waiting. If you have the current System.map in the correct place (in
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/boot for the procps I use) or have updated /etc/psdatabase (for kmem
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ps), ps writes the function name in the WCHAN field. If not, you have
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to look up the function from System.map.
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Note also that the timeouts are very long compared to most other
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drivers. This means that the Linux driver may appear hung although the
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real reason is that the tape firmware has got confused.
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