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diff --git a/Documentation/scsi/st.txt b/Documentation/scsi/st.txt new file mode 100644 index 00000000000..20e30cf3187 --- /dev/null +++ b/Documentation/scsi/st.txt @@ -0,0 +1,499 @@ +This file contains brief information about the SCSI tape driver. +The driver is currently maintained by Kai Mäkisara (email +Kai.Makisara@kolumbus.fi) + +Last modified: Mon Mar 7 21:14:44 2005 by kai.makisara + + +BASICS + +The driver is generic, i.e., it does not contain any code tailored +to any specific tape drive. The tape parameters can be specified with +one of the following three methods: + +1. Each user can specify the tape parameters he/she wants to use +directly with ioctls. This is administratively a very simple and +flexible method and applicable to single-user workstations. However, +in a multiuser environment the next user finds the tape parameters in +state the previous user left them. + +2. The system manager (root) can define default values for some tape +parameters, like block size and density using the MTSETDRVBUFFER ioctl. +These parameters can be programmed to come into effect either when a +new tape is loaded into the drive or if writing begins at the +beginning of the tape. The second method is applicable if the tape +drive performs auto-detection of the tape format well (like some +QIC-drives). The result is that any tape can be read, writing can be +continued using existing format, and the default format is used if +the tape is rewritten from the beginning (or a new tape is written +for the first time). The first method is applicable if the drive +does not perform auto-detection well enough and there is a single +"sensible" mode for the device. An example is a DAT drive that is +used only in variable block mode (I don't know if this is sensible +or not :-). + +The user can override the parameters defined by the system +manager. The changes persist until the defaults again come into +effect. + +3. By default, up to four modes can be defined and selected using the minor +number (bits 5 and 6). The number of modes can be changed by changing +ST_NBR_MODE_BITS in st.h. Mode 0 corresponds to the defaults discussed +above. Additional modes are dormant until they are defined by the +system manager (root). When specification of a new mode is started, +the configuration of mode 0 is used to provide a starting point for +definition of the new mode. + +Using the modes allows the system manager to give the users choices +over some of the buffering parameters not directly accessible to the +users (buffered and asynchronous writes). The modes also allow choices +between formats in multi-tape operations (the explicitly overridden +parameters are reset when a new tape is loaded). + +If more than one mode is used, all modes should contain definitions +for the same set of parameters. + +Many Unices contain internal tables that associate different modes to +supported devices. The Linux SCSI tape driver does not contain such +tables (and will not do that in future). Instead of that, a utility +program can be made that fetches the inquiry data sent by the device, +scans its database, and sets up the modes using the ioctls. Another +alternative is to make a small script that uses mt to set the defaults +tailored to the system. + +The driver supports fixed and variable block size (within buffer +limits). Both the auto-rewind (minor equals device number) and +non-rewind devices (minor is 128 + device number) are implemented. + +In variable block mode, the byte count in write() determines the size +of the physical block on tape. When reading, the drive reads the next +tape block and returns to the user the data if the read() byte count +is at least the block size. Otherwise, error ENOMEM is returned. + +In fixed block mode, the data transfer between the drive and the +driver is in multiples of the block size. The write() byte count must +be a multiple of the block size. This is not required when reading but +may be advisable for portability. + +Support is provided for changing the tape partition and partitioning +of the tape with one or two partitions. By default support for +partitioned tape is disabled for each driver and it can be enabled +with the ioctl MTSETDRVBUFFER. + +By default the driver writes one filemark when the device is closed after +writing and the last operation has been a write. Two filemarks can be +optionally written. In both cases end of data is signified by +returning zero bytes for two consecutive reads. + +If rewind, offline, bsf, or seek is done and previous tape operation was +write, a filemark is written before moving tape. + +The compile options are defined in the file linux/drivers/scsi/st_options.h. + +4. If the open option O_NONBLOCK is used, open succeeds even if the +drive is not ready. If O_NONBLOCK is not used, the driver waits for +the drive to become ready. If this does not happen in ST_BLOCK_SECONDS +seconds, open fails with the errno value EIO. With O_NONBLOCK the +device can be opened for writing even if there is a write protected +tape in the drive (commands trying to write something return error if +attempted). + + +MINOR NUMBERS + +The tape driver currently supports 128 drives by default. This number +can be increased by editing st.h and recompiling the driver if +necessary. The upper limit is 2^17 drives if 4 modes for each drive +are used. + +The minor numbers consist of the following bit fields: + +dev_upper non-rew mode dev-lower + 20 - 8 7 6 5 4 0 +The non-rewind bit is always bit 7 (the uppermost bit in the lowermost +byte). The bits defining the mode are below the non-rewind bit. The +remaining bits define the tape device number. This numbering is +backward compatible with the numbering used when the minor number was +only 8 bits wide. + + +SYSFS SUPPORT + +The driver creates the directory /sys/class/scsi_tape and populates it with +directories corresponding to the existing tape devices. There are autorewind +and non-rewind entries for each mode. The names are stxy and nstxy, where x +is the tape number and y a character corresponding to the mode (none, l, m, +a). For example, the directories for the first tape device are (assuming four +modes): st0 nst0 st0l nst0l st0m nst0m st0a nst0a. + +Each directory contains the entries: default_blksize default_compression +default_density defined dev device driver. The file 'defined' contains 1 +if the mode is defined and zero if not defined. The files 'default_*' contain +the defaults set by the user. The value -1 means the default is not set. The +file 'dev' contains the device numbers corresponding to this device. The links +'device' and 'driver' point to the SCSI device and driver entries. + +A link named 'tape' is made from the SCSI device directory to the class +directory corresponding to the mode 0 auto-rewind device (e.g., st0). + + +BSD AND SYS V SEMANTICS + +The user can choose between these two behaviours of the tape driver by +defining the value of the symbol ST_SYSV. The semantics differ when a +file being read is closed. The BSD semantics leaves the tape where it +currently is whereas the SYS V semantics moves the tape past the next +filemark unless the filemark has just been crossed. + +The default is BSD semantics. + + +BUFFERING + +The driver tries to do transfers directly to/from user space. If this +is not possible, a driver buffer allocated at run-time is used. If +direct i/o is not possible for the whole transfer, the driver buffer +is used (i.e., bounce buffers for individual pages are not +used). Direct i/o can be impossible because of several reasons, e.g.: +- one or more pages are at addresses not reachable by the HBA +- the number of pages in the transfer exceeds the number of + scatter/gather segments permitted by the HBA +- one or more pages can't be locked into memory (should not happen in + any reasonable situation) + +The size of the driver buffers is always at least one tape block. In fixed +block mode, the minimum buffer size is defined (in 1024 byte units) by +ST_FIXED_BUFFER_BLOCKS. With small block size this allows buffering of +several blocks and using one SCSI read or write to transfer all of the +blocks. Buffering of data across write calls in fixed block mode is +allowed if ST_BUFFER_WRITES is non-zero and direct i/o is not used. +Buffer allocation uses chunks of memory having sizes 2^n * (page +size). Because of this the actual buffer size may be larger than the +minimum allowable buffer size. + +NOTE that if direct i/o is used, the small writes are not buffered. This may +cause a surprise when moving from 2.4. There small writes (e.g., tar without +-b option) may have had good throughput but this is not true any more with +2.6. Direct i/o can be turned off to solve this problem but a better solution +is to use bigger write() byte counts (e.g., tar -b 64). + +Asynchronous writing. Writing the buffer contents to the tape is +started and the write call returns immediately. The status is checked +at the next tape operation. Asynchronous writes are not done with +direct i/o and not in fixed block mode. + +Buffered writes and asynchronous writes may in some rare cases cause +problems in multivolume operations if there is not enough space on the +tape after the early-warning mark to flush the driver buffer. + +Read ahead for fixed block mode (ST_READ_AHEAD). Filling the buffer is +attempted even if the user does not want to get all of the data at +this read command. Should be disabled for those drives that don't like +a filemark to truncate a read request or that don't like backspacing. + +Scatter/gather buffers (buffers that consist of chunks non-contiguous +in the physical memory) are used if contiguous buffers can't be +allocated. To support all SCSI adapters (including those not +supporting scatter/gather), buffer allocation is using the following +three kinds of chunks: +1. The initial segment that is used for all SCSI adapters including +those not supporting scatter/gather. The size of this buffer will be +(PAGE_SIZE << ST_FIRST_ORDER) bytes if the system can give a chunk of +this size (and it is not larger than the buffer size specified by +ST_BUFFER_BLOCKS). If this size is not available, the driver halves +the size and tries again until the size of one page. The default +settings in st_options.h make the driver to try to allocate all of the +buffer as one chunk. +2. The scatter/gather segments to fill the specified buffer size are +allocated so that as many segments as possible are used but the number +of segments does not exceed ST_FIRST_SG. +3. The remaining segments between ST_MAX_SG (or the module parameter +max_sg_segs) and the number of segments used in phases 1 and 2 +are used to extend the buffer at run-time if this is necessary. The +number of scatter/gather segments allowed for the SCSI adapter is not +exceeded if it is smaller than the maximum number of scatter/gather +segments specified. If the maximum number allowed for the SCSI adapter +is smaller than the number of segments used in phases 1 and 2, +extending the buffer will always fail. + + +EOM BEHAVIOUR WHEN WRITING + +When the end of medium early warning is encountered, the current write +is finished and the number of bytes is returned. The next write +returns -1 and errno is set to ENOSPC. To enable writing a trailer, +the next write is allowed to proceed and, if successful, the number of +bytes is returned. After this, -1 and the number of bytes are +alternately returned until the physical end of medium (or some other +error) is encountered. + + +MODULE PARAMETERS + +The buffer size, write threshold, and the maximum number of allocated buffers +are configurable when the driver is loaded as a module. The keywords are: + +buffer_kbs=xxx the buffer size for fixed block mode is set + to xxx kilobytes +write_threshold_kbs=xxx the write threshold in kilobytes set to xxx +max_sg_segs=xxx the maximum number of scatter/gather + segments +try_direct_io=x try direct transfer between user buffer and + tape drive if this is non-zero + +Note that if the buffer size is changed but the write threshold is not +set, the write threshold is set to the new buffer size - 2 kB. + + +BOOT TIME CONFIGURATION + +If the driver is compiled into the kernel, the same parameters can be +also set using, e.g., the LILO command line. The preferred syntax is +is to use the same keyword used when loading as module but prepended +with 'st.'. For instance, to set the maximum number of scatter/gather +segments, the parameter 'st.max_sg_segs=xx' should be used (xx is the +number of scatter/gather segments). + +For compatibility, the old syntax from early 2.5 and 2.4 kernel +versions is supported. The same keywords can be used as when loading +the driver as module. If several parameters are set, the keyword-value +pairs are separated with a comma (no spaces allowed). A colon can be +used instead of the equal mark. The definition is prepended by the +string st=. Here is an example: + + st=buffer_kbs:64,write_threhold_kbs:60 + +The following syntax used by the old kernel versions is also supported: + + st=aa[,bb[,dd]] + +where + aa is the buffer size for fixed block mode in 1024 byte units + bb is the write threshold in 1024 byte units + dd is the maximum number of scatter/gather segments + + +IOCTLS + +The tape is positioned and the drive parameters are set with ioctls +defined in mtio.h The tape control program 'mt' uses these ioctls. Try +to find an mt that supports all of the Linux SCSI tape ioctls and +opens the device for writing if the tape contents will be modified +(look for a package mt-st* from the Linux ftp sites; the GNU mt does +not open for writing for, e.g., erase). + +The supported ioctls are: + +The following use the structure mtop: + +MTFSF Space forward over count filemarks. Tape positioned after filemark. +MTFSFM As above but tape positioned before filemark. +MTBSF Space backward over count filemarks. Tape positioned before + filemark. +MTBSFM As above but ape positioned after filemark. +MTFSR Space forward over count records. +MTBSR Space backward over count records. +MTFSS Space forward over count setmarks. +MTBSS Space backward over count setmarks. +MTWEOF Write count filemarks. +MTWSM Write count setmarks. +MTREW Rewind tape. +MTOFFL Set device off line (often rewind plus eject). +MTNOP Do nothing except flush the buffers. +MTRETEN Re-tension tape. +MTEOM Space to end of recorded data. +MTERASE Erase tape. If the argument is zero, the short erase command + is used. The long erase command is used with all other values + of the argument. +MTSEEK Seek to tape block count. Uses Tandberg-compatible seek (QFA) + for SCSI-1 drives and SCSI-2 seek for SCSI-2 drives. The file and + block numbers in the status are not valid after a seek. +MTSETBLK Set the drive block size. Setting to zero sets the drive into + variable block mode (if applicable). +MTSETDENSITY Sets the drive density code to arg. See drive + documentation for available codes. +MTLOCK and MTUNLOCK Explicitly lock/unlock the tape drive door. +MTLOAD and MTUNLOAD Explicitly load and unload the tape. If the + command argument x is between MT_ST_HPLOADER_OFFSET + 1 and + MT_ST_HPLOADER_OFFSET + 6, the number x is used sent to the + drive with the command and it selects the tape slot to use of + HP C1553A changer. +MTCOMPRESSION Sets compressing or uncompressing drive mode using the + SCSI mode page 15. Note that some drives other methods for + control of compression. Some drives (like the Exabytes) use + density codes for compression control. Some drives use another + mode page but this page has not been implemented in the + driver. Some drives without compression capability will accept + any compression mode without error. +MTSETPART Moves the tape to the partition given by the argument at the + next tape operation. The block at which the tape is positioned + is the block where the tape was previously positioned in the + new active partition unless the next tape operation is + MTSEEK. In this case the tape is moved directly to the block + specified by MTSEEK. MTSETPART is inactive unless + MT_ST_CAN_PARTITIONS set. +MTMKPART Formats the tape with one partition (argument zero) or two + partitions (the argument gives in megabytes the size of + partition 1 that is physically the first partition of the + tape). The drive has to support partitions with size specified + by the initiator. Inactive unless MT_ST_CAN_PARTITIONS set. +MTSETDRVBUFFER + Is used for several purposes. The command is obtained from count + with mask MT_SET_OPTIONS, the low order bits are used as argument. + This command is only allowed for the superuser (root). The + subcommands are: + 0 + The drive buffer option is set to the argument. Zero means + no buffering. + MT_ST_BOOLEANS + Sets the buffering options. The bits are the new states + (enabled/disabled) the following options (in the + parenthesis is specified whether the option is global or + can be specified differently for each mode): + MT_ST_BUFFER_WRITES write buffering (mode) + MT_ST_ASYNC_WRITES asynchronous writes (mode) + MT_ST_READ_AHEAD read ahead (mode) + MT_ST_TWO_FM writing of two filemarks (global) + MT_ST_FAST_EOM using the SCSI spacing to EOD (global) + MT_ST_AUTO_LOCK automatic locking of the drive door (global) + MT_ST_DEF_WRITES the defaults are meant only for writes (mode) + MT_ST_CAN_BSR backspacing over more than one records can + be used for repositioning the tape (global) + MT_ST_NO_BLKLIMS the driver does not ask the block limits + from the drive (block size can be changed only to + variable) (global) + MT_ST_CAN_PARTITIONS enables support for partitioned + tapes (global) + MT_ST_SCSI2LOGICAL the logical block number is used in + the MTSEEK and MTIOCPOS for SCSI-2 drives instead of + the device dependent address. It is recommended to set + this flag unless there are tapes using the device + dependent (from the old times) (global) + MT_ST_SYSV sets the SYSV sematics (mode) + MT_ST_NOWAIT enables immediate mode (i.e., don't wait for + the command to finish) for some commands (e.g., rewind) + MT_ST_DEBUGGING debugging (global; debugging must be + compiled into the driver) + MT_ST_SETBOOLEANS + MT_ST_CLEARBOOLEANS + Sets or clears the option bits. + MT_ST_WRITE_THRESHOLD + Sets the write threshold for this device to kilobytes + specified by the lowest bits. + MT_ST_DEF_BLKSIZE + Defines the default block size set automatically. Value + 0xffffff means that the default is not used any more. + MT_ST_DEF_DENSITY + MT_ST_DEF_DRVBUFFER + Used to set or clear the density (8 bits), and drive buffer + state (3 bits). If the value is MT_ST_CLEAR_DEFAULT + (0xfffff) the default will not be used any more. Otherwise + the lowermost bits of the value contain the new value of + the parameter. + MT_ST_DEF_COMPRESSION + The compression default will not be used if the value of + the lowermost byte is 0xff. Otherwise the lowermost bit + contains the new default. If the bits 8-15 are set to a + non-zero number, and this number is not 0xff, the number is + used as the compression algorithm. The value + MT_ST_CLEAR_DEFAULT can be used to clear the compression + default. + MT_ST_SET_TIMEOUT + Set the normal timeout in seconds for this device. The + default is 900 seconds (15 minutes). The timeout should be + long enough for the retries done by the device while + reading/writing. + MT_ST_SET_LONG_TIMEOUT + Set the long timeout that is used for operations that are + known to take a long time. The default is 14000 seconds + (3.9 hours). For erase this value is further multiplied by + eight. + MT_ST_SET_CLN + Set the cleaning request interpretation parameters using + the lowest 24 bits of the argument. The driver can set the + generic status bit GMT_CLN if a cleaning request bit pattern + is found from the extended sense data. Many drives set one or + more bits in the extended sense data when the drive needs + cleaning. The bits are device-dependent. The driver is + given the number of the sense data byte (the lowest eight + bits of the argument; must be >= 18 (values 1 - 17 + reserved) and <= the maximum requested sense data sixe), + a mask to select the relevant bits (the bits 9-16), and the + bit pattern (bits 17-23). If the bit pattern is zero, one + or more bits under the mask indicate cleaning request. If + the pattern is non-zero, the pattern must match the masked + sense data byte. + + (The cleaning bit is set if the additional sense code and + qualifier 00h 17h are seen regardless of the setting of + MT_ST_SET_CLN.) + +The following ioctl uses the structure mtpos: +MTIOCPOS Reads the current position from the drive. Uses + Tandberg-compatible QFA for SCSI-1 drives and the SCSI-2 + command for the SCSI-2 drives. + +The following ioctl uses the structure mtget to return the status: +MTIOCGET Returns some status information. + The file number and block number within file are returned. The + block is -1 when it can't be determined (e.g., after MTBSF). + The drive type is either MTISSCSI1 or MTISSCSI2. + The number of recovered errors since the previous status call + is stored in the lower word of the field mt_erreg. + The current block size and the density code are stored in the field + mt_dsreg (shifts for the subfields are MT_ST_BLKSIZE_SHIFT and + MT_ST_DENSITY_SHIFT). + The GMT_xxx status bits reflect the drive status. GMT_DR_OPEN + is set if there is no tape in the drive. GMT_EOD means either + end of recorded data or end of tape. GMT_EOT means end of tape. + + +MISCELLANEOUS COMPILE OPTIONS + +The recovered write errors are considered fatal if ST_RECOVERED_WRITE_FATAL +is defined. + +The maximum number of tape devices is determined by the define +ST_MAX_TAPES. If more tapes are detected at driver initialization, the +maximum is adjusted accordingly. + +Immediate return from tape positioning SCSI commands can be enabled by +defining ST_NOWAIT. If this is defined, the user should take care that +the next tape operation is not started before the previous one has +finished. The drives and SCSI adapters should handle this condition +gracefully, but some drive/adapter combinations are known to hang the +SCSI bus in this case. + +The MTEOM command is by default implemented as spacing over 32767 +filemarks. With this method the file number in the status is +correct. The user can request using direct spacing to EOD by setting +ST_FAST_EOM 1 (or using the MT_ST_OPTIONS ioctl). In this case the file +number will be invalid. + +When using read ahead or buffered writes the position within the file +may not be correct after the file is closed (correct position may +require backspacing over more than one record). The correct position +within file can be obtained if ST_IN_FILE_POS is defined at compile +time or the MT_ST_CAN_BSR bit is set for the drive with an ioctl. +(The driver always backs over a filemark crossed by read ahead if the +user does not request data that far.) + + +DEBUGGING HINTS + +To enable debugging messages, edit st.c and #define DEBUG 1. As seen +above, debugging can be switched off with an ioctl if debugging is +compiled into the driver. The debugging output is not voluminous. + +If the tape seems to hang, I would be very interested to hear where +the driver is waiting. With the command 'ps -l' you can see the state +of the process using the tape. If the state is D, the process is +waiting for something. The field WCHAN tells where the driver is +waiting. If you have the current System.map in the correct place (in +/boot for the procps I use) or have updated /etc/psdatabase (for kmem +ps), ps writes the function name in the WCHAN field. If not, you have +to look up the function from System.map. + +Note also that the timeouts are very long compared to most other +drivers. This means that the Linux driver may appear hung although the +real reason is that the tape firmware has got confused. |