| 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: Thu Feb 21 21:54:16 2008 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 |
| 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_threshold_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 semantics (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_SILI enables setting the SILI bit in SCSI commands when |
| reading in variable block mode to enhance performance when |
| reading blocks shorter than the byte count; set this only |
| if you are sure that the drive supports SILI and the HBA |
| correctly returns transfer residuals |
| 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. |