| This file contains brief information about the SCSI tape driver. |
| The driver is currently maintained by Kai M{kisara (email |
| Kai.Makisara@metla.fi) |
| |
| Last modified: Tue Jan 22 21:08:57 2002 by 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. Up to four modes can be defined and selected using the minor number |
| (bits 5 and 6). 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. |
| |
| 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. |
| |
| |
| 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 uses tape buffers allocated either at system initialization |
| or at run-time when needed. One buffer is used for each open tape |
| device. The size of the buffers is selectable at compile and/or boot |
| time. The buffers are used to store the data being transferred to/from |
| the SCSI adapter. The following buffering options are selectable at |
| compile time and/or at run time (via ioctl): |
| |
| Buffering of data across write calls in fixed block mode (define |
| ST_BUFFER_WRITES). |
| |
| 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. |
| |
| 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. |
| |
| The buffer size is defined (in 1024 byte units) by ST_BUFFER_BLOCKS or |
| at boot time. If this size is not large enough, the driver tries to |
| temporarily enlarge the buffer. Buffer allocation uses chunks of |
| memory having sizes 2^n * (page size). Because of this the actual |
| buffer size may be larger than the buffer size specified with |
| ST_BUFFER_BLOCKS. |
| |
| A small number of buffers are allocated at driver initialisation. The |
| maximum number of these buffers is defined by ST_MAX_BUFFERS. The |
| maximum can be changed with kernel or module startup options. One |
| buffer is allocated for each drive detected when the driver is |
| initialized up to the maximum. |
| |
| The driver tries to allocate new buffers at run-time if |
| necessary. These buffers are freed after use. If the maximum number of |
| initial buffers is set to zero, all buffer allocation is done at |
| run-time. The advantage of run-time allocation is that memory is not |
| wasted for buffers not being used. The disadvantage is that there may |
| not be memory available at the time when a buffer is needed for the |
| first time (once a buffer is allocated, it is not released). This risk |
| should not be big if the tape drive is connected to a PCI adapter that |
| supports scatter/gather (the allocation is not limited to "DMA memory" |
| and the buffer can be composed of several fragments). |
| |
| The threshold for triggering asynchronous write in fixed block mode |
| is defined by ST_WRITE_THRESHOLD. This may be optimized for each |
| use pattern. The default triggers asynchronous write after three |
| default sized writes (10 kB) from tar. |
| |
| 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. |
| |
| |
| 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 in kilobytes is set to xxx |
| write_threshold_kbs=xxx the write threshold in kilobytes set to xxx |
| max_buffers=xxx the maximum number of tape buffer set to xxx |
| max_sg_segs=xxx the maximum number of scatter/gather |
| segments |
| |
| 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 keywords 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,max_buffers:2 |
| |
| The following syntax used by the old kernel versions is also supported: |
| |
| st=aa[,bb[,cc[,dd]]] |
| |
| where |
| aa is the buffer size in 1024 byte units |
| bb is the write threshold in 1024 byte units |
| cc is the maximum number of tape buffers to allocate (the number of |
| buffers is bounded also by the number of drives detected) |
| 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. |
| 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 voluminuous. |
| |
| 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. |