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: Sun Apr 18 13:24:43 1999 by makisara@home 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 the data is truncated. 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. 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 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 enough, the driver tries to allocate a large enough temporary buffer that is released when the device is closed. 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. Allocation of the buffers is done at run-time when they are needed. Allocation of the specified number of buffers can be done at initialization if ST_RUNTIME_BUFFERS is defined non-zero. 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). The maximum number of buffers allocated at initialization is defined by ST_MAX_BUFFERS. One buffer is allocated for each drive detected when the driver is initialized up to the maximum. The minimum number of allocated buffers is ST_EXTRA_DEVS (in hosts.h). This ensures some functionality also for the drives found after tape driver initialization (a SCSI adapter driver is loaded as a module). The default for ST_EXTRA_DEVS is two. The driver tries to allocate new buffers at run-time if necessary. 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. BOOT TIME CONFIGURATION The buffer size, write threshold, and the maximum number of allocated buffers are configurable at boot time using, e.g., the LILO command line. The option syntax is the following: st=aa[,bb[,cc]] 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) MODULE PARAMETERS The same parameters can be also set 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 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_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 MT_ST_DEF_COMPRESSION Used to set or clear the density (8 bits), drive buffer state (3 bits), and compression (single bit). If the value is MT_ST_CLEAR_DEFAULT (0xfffff), the default will not be used any more. Otherwise the lower-most bits of the value contain the new value of the parameter. 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. 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.