st(4) — Linux manual page


ST(4)                     Linux Programmer's Manual                    ST(4)

NAME         top

       st - SCSI tape device

SYNOPSIS         top

       #include <sys/mtio.h>

       int ioctl(int fd, int request [, (void *)arg3]);
       int ioctl(int fd, MTIOCTOP, (struct mtop *)mt_cmd);
       int ioctl(int fd, MTIOCGET, (struct mtget *)mt_status);
       int ioctl(int fd, MTIOCPOS, (struct mtpos *)mt_pos);

DESCRIPTION         top

       The st driver provides the interface to a variety of SCSI tape
       devices.  Currently, the driver takes control of all detected devices
       of type “sequential-access”.  The st driver uses major device number

       Each device uses eight minor device numbers.  The lowermost five bits
       in the minor numbers are assigned sequentially in the order of
       detection.  In the 2.6 kernel, the bits above the eight lowermost
       bits are concatenated to the five lowermost bits to form the tape
       number.  The minor numbers can be grouped into two sets of four
       numbers: the principal (auto-rewind) minor device numbers, n, and the
       “no-rewind” device numbers, (n + 128).  Devices opened using the
       principal device number will be sent a REWIND command when they are
       closed.  Devices opened using the “no-rewind” device number will not.
       (Note that using an auto-rewind device for positioning the tape with,
       for instance, mt does not lead to the desired result: the tape is
       rewound after the mt command and the next command starts from the
       beginning of the tape).

       Within each group, four minor numbers are available to define devices
       with different characteristics (block size, compression, density,
       etc.)  When the system starts up, only the first device is available.
       The other three are activated when the default characteristics are
       defined (see below).  (By changing compile-time constants, it is
       possible to change the balance between the maximum number of tape
       drives and the number of minor numbers for each drive.  The default
       allocation allows control of 32 tape drives.  For instance, it is
       possible to control up to 64 tape drives with two minor numbers for
       different options.)

       Devices are typically created by:

           mknod -m 666 /dev/st0 c 9 0
           mknod -m 666 /dev/st0l c 9 32
           mknod -m 666 /dev/st0m c 9 64
           mknod -m 666 /dev/st0a c 9 96
           mknod -m 666 /dev/nst0 c 9 128
           mknod -m 666 /dev/nst0l c 9 160
           mknod -m 666 /dev/nst0m c 9 192
           mknod -m 666 /dev/nst0a c 9 224

       There is no corresponding block device.

       The driver uses an internal buffer that has to be large enough to
       hold at least one tape block.  In kernels before 2.1.121, the buffer
       is allocated as one contiguous block.  This limits the block size to
       the largest contiguous block of memory the kernel allocator can pro‐
       vide.  The limit is currently 128 kB for 32-bit architectures and
       256 kB for 64-bit architectures.  In newer kernels the driver allo‐
       cates the buffer in several parts if necessary.  By default, the max‐
       imum number of parts is 16.  This means that the maximum block size
       is very large (2 MB if allocation of 16 blocks of 128 kB succeeds).

       The driver's internal buffer size is determined by a compile-time
       constant which can be overridden with a kernel startup option.  In
       addition to this, the driver tries to allocate a larger temporary
       buffer at run time if necessary.  However, run-time allocation of
       large contiguous blocks of memory may fail and it is advisable not to
       rely too much on dynamic buffer allocation with kernels older than
       2.1.121 (this applies also to demand-loading the driver with kerneld
       or kmod).

       The driver does not specifically support any tape drive brand or
       model.  After system start-up the tape device options are defined by
       the drive firmware.  For example, if the drive firmware selects
       fixed-block mode, the tape device uses fixed-block mode.  The options
       can be changed with explicit ioctl(2) calls and remain in effect when
       the device is closed and reopened.  Setting the options affects both
       the auto-rewind and the nonrewind device.

       Different options can be specified for the different devices within
       the subgroup of four.  The options take effect when the device is
       opened.  For example, the system administrator can define one device
       that writes in fixed-block mode with a certain block size, and one
       which writes in variable-block mode (if the drive supports both

       The driver supports tape partitions if they are supported by the
       drive.  (Note that the tape partitions have nothing to do with disk
       partitions.  A partitioned tape can be seen as several logical tapes
       within one medium.)  Partition support has to be enabled with an
       ioctl(2).  The tape location is preserved within each partition
       across partition changes.  The partition used for subsequent tape
       operations is selected with an ioctl(2).  The partition switch is
       executed together with the next tape operation in order to avoid
       unnecessary tape movement.  The maximum number of partitions on a
       tape is defined by a compile-time constant (originally four).  The
       driver contains an ioctl(2) that can format a tape with either one or
       two partitions.

       Device /dev/tape is usually created as a hard or soft link to the
       default tape device on the system.

       Starting from kernel 2.6.2, the driver exports in the sysfs directory
       /sys/class/scsi_tape the attached devices and some parameters
       assigned to the devices.

   Data transfer
       The driver supports operation in both fixed-block mode and variable-
       block mode (if supported by the drive).  In fixed-block mode the
       drive writes blocks of the specified size and the block size is not
       dependent on the byte counts of the write system calls.  In variable-
       block mode one tape block is written for each write call and the byte
       count determines the size of the corresponding tape block.  Note that
       the blocks on the tape don't contain any information about the writ‐
       ing mode: when reading, the only important thing is to use commands
       that accept the block sizes on the tape.

       In variable-block mode the read byte count does not have to match the
       tape block size exactly.  If the byte count is larger than the next
       block on tape, the driver returns the data and the function returns
       the actual block size.  If the block size is larger than the byte
       count, an error is returned.

       In fixed-block mode the read byte counts can be arbitrary if buffer‐
       ing is enabled, or a multiple of the tape block size if buffering is
       disabled.  Kernels before 2.1.121 allow writes with arbitrary byte
       count if buffering is enabled.  In all other cases (kernel before
       2.1.121 with buffering disabled or newer kernel) the write byte count
       must be a multiple of the tape block size.

       In the 2.6 kernel, the driver tries to use direct transfers between
       the user buffer and the device.  If this is not possible, the
       driver's internal buffer is used.  The reasons for not using direct
       transfers include improper alignment of the user buffer (default is
       512 bytes but this can be changed by the HBA driver), one or more
       pages of the user buffer not reachable by the SCSI adapter, and so

       A filemark is automatically written to tape if the last tape opera‐
       tion before close was a write.

       When a filemark is encountered while reading, the following happens.
       If there are data remaining in the buffer when the filemark is found,
       the buffered data is returned.  The next read returns zero bytes.
       The following read returns data from the next file.  The end of
       recorded data is signaled by returning zero bytes for two consecutive
       read calls.  The third read returns an error.

       The driver supports three ioctl(2) requests.  Requests not recognized
       by the st driver are passed to the SCSI driver.  The definitions
       below are from /usr/include/linux/mtio.h:

   MTIOCTOP — perform a tape operation
       This request takes an argument of type (struct mtop *).  Not all
       drives support all operations.  The driver returns an EIO error if
       the drive rejects an operation.

           /* Structure for MTIOCTOP - mag tape op command: */
           struct mtop {
               short   mt_op;       /* operations defined below */
               int     mt_count;    /* how many of them */

       Magnetic tape operations for normal tape use:

       MTBSF  Backward space over mt_count filemarks.

       MTBSFM Backward space over mt_count filemarks.  Reposition the tape
              to the EOT side of the last filemark.

       MTBSR  Backward space over mt_count records (tape blocks).

       MTBSS  Backward space over mt_count setmarks.

              Enable compression of tape data within the drive if mt_count
              is nonzero and disable compression if mt_count is zero.  This
              command uses the MODE page 15 supported by most DATs.

       MTEOM  Go to the end of the recorded media (for appending files).

              Erase tape.  With 2.6 kernel, short erase (mark tape empty) is
              performed if the argument is zero.  Otherwise, long erase
              (erase all) is done.

       MTFSF  Forward space over mt_count filemarks.

       MTFSFM Forward space over mt_count filemarks.  Reposition the tape to
              the BOT side of the last filemark.

       MTFSR  Forward space over mt_count records (tape blocks).

       MTFSS  Forward space over mt_count setmarks.

       MTLOAD Execute the SCSI load command.  A special case is available
              for some HP autoloaders.  If mt_count is the constant
              MT_ST_HPLOADER_OFFSET plus a number, the number is sent to the
              drive to control the autoloader.

       MTLOCK Lock the tape drive door.

              Format the tape into one or two partitions.  If mt_count is
              positive, it gives the size of partition 1 and partition 0
              contains the rest of the tape.  If mt_count is zero, the tape
              is formatted into one partition.  From kernel version 4.6, a
              negative mt_count specifies the size of partition 0 and the
              rest of the tape contains partition 1.  The physical ordering
              of partitions depends on the drive.  This command is not
              allowed for a drive unless the partition support is enabled
              for the drive (see MT_ST_CAN_PARTITIONS below).

       MTNOP  No op—flushes the driver's buffer as a side effect.  Should be
              used before reading status with MTIOCGET.

       MTOFFL Rewind and put the drive off line.

              Reset drive.

              Re-tension tape.

       MTREW  Rewind.

       MTSEEK Seek to the tape block number specified in mt_count.  This
              operation requires either a SCSI-2 drive that supports the
              LOCATE command (device-specific address) or a Tandberg-compat‐
              ible SCSI-1 drive (Tandberg, Archive Viper, Wangtek, ...).
              The block number should be one that was previously returned by
              MTIOCPOS if device-specific addresses are used.

              Set the drive's block length to the value specified in
              mt_count.  A block length of zero sets the drive to variable
              block size mode.

              Set the tape density to the code in mt_count.  The density
              codes supported by a drive can be found from the drive docu‐

              The active partition is switched to mt_count.  The partitions
              are numbered from zero.  This command is not allowed for a
              drive unless the partition support is enabled for the drive
              (see MT_ST_CAN_PARTITIONS below).

              Execute the SCSI unload command (does not eject the tape).

              Unlock the tape drive door.

       MTWEOF Write mt_count filemarks.

       MTWSM  Write mt_count setmarks.

       Magnetic tape operations for setting of device options (by the supe‐

              Set various drive and driver options according to bits encoded
              in mt_count.  These consist of the drive's buffering mode, a
              set of Boolean driver options, the buffer write threshold,
              defaults for the block size and density, and timeouts (only in
              kernels 2.1 and later).  A single operation can affect only
              one item in the list below (the Booleans counted as one item.)

              A value having zeros in the high-order 4 bits will be used to
              set the drive's buffering mode.  The buffering modes are:

                   0   The drive will not report GOOD status on write com‐
                       mands until the data blocks are actually written to
                       the medium.

                   1   The drive may report GOOD status on write commands as
                       soon as all the data has been transferred to the
                       drive's internal buffer.

                   2   The drive may report GOOD status on write commands as
                       soon as (a) all the data has been transferred to the
                       drive's internal buffer, and (b) all buffered data
                       from different initiators has been successfully writ‐
                       ten to the medium.

              To control the write threshold the value in mt_count must
              include the constant MT_ST_WRITE_THRESHOLD bitwise ORed with a
              block count in the low 28 bits.  The block count refers to
              1024-byte blocks, not the physical block size on the tape.
              The threshold cannot exceed the driver's internal buffer size
              (see DESCRIPTION, above).

              To set and clear the Boolean options the value in mt_count
              must include one of the constants MT_ST_BOOLEANS, MT_ST_SET‐
              ORed with whatever combination of the following options is
              desired.  Using MT_ST_BOOLEANS the options can be set to the
              values defined in the corresponding bits.  With MT_ST_SET‐
              BOOLEANS the options can be selectively set and with
              MT_ST_DEFBOOLEANS selectively cleared.

              The default options for a tape device are set with MT_ST_DEF‐
              BOOLEANS.  A nonactive tape device (e.g., device with minor 32
              or 160) is activated when the default options for it are
              defined the first time.  An activated device inherits from the
              device activated at start-up the options not set explicitly.

              The Boolean options are:

              MT_ST_BUFFER_WRITES (Default: true)
                     Buffer all write operations in fixed-block mode.  If
                     this option is false and the drive uses a fixed block
                     size, then all write operations must be for a multiple
                     of the block size.  This option must be set false to
                     write reliable multivolume archives.

              MT_ST_ASYNC_WRITES (Default: true)
                     When this option is true, write operations return imme‐
                     diately without waiting for the data to be transferred
                     to the drive if the data fits into the driver's buffer.
                     The write threshold determines how full the buffer must
                     be before a new SCSI write command is issued.  Any
                     errors reported by the drive will be held until the
                     next operation.  This option must be set false to write
                     reliable multivolume archives.

              MT_ST_READ_AHEAD (Default: true)
                     This option causes the driver to provide read buffering
                     and read-ahead in fixed-block mode.  If this option is
                     false and the drive uses a fixed block size, then all
                     read operations must be for a multiple of the block

              MT_ST_TWO_FM (Default: false)
                     This option modifies the driver behavior when a file is
                     closed.  The normal action is to write a single file‐
                     mark.  If the option is true, the driver will write two
                     filemarks and backspace over the second one.

                     Note: This option should not be set true for QIC tape
                     drives since they are unable to overwrite a filemark.
                     These drives detect the end of recorded data by testing
                     for blank tape rather than two consecutive filemarks.
                     Most other current drives also detect the end of
                     recorded data and using two filemarks is usually neces‐
                     sary only when interchanging tapes with some other sys‐

              MT_ST_DEBUGGING (Default: false)
                     This option turns on various debugging messages from
                     the driver (effective only if the driver was compiled
                     with DEBUG defined nonzero).

              MT_ST_FAST_EOM (Default: false)
                     This option causes the MTEOM operation to be sent
                     directly to the drive, potentially speeding up the
                     operation but causing the driver to lose track of the
                     current file number normally returned by the MTIOCGET
                     request.  If MT_ST_FAST_EOM is false, the driver will
                     respond to an MTEOM request by forward spacing over

              MT_ST_AUTO_LOCK (Default: false)
                     When this option is true, the drive door is locked when
                     the device file is opened and unlocked when it is

              MT_ST_DEF_WRITES (Default: false)
                     The tape options (block size, mode, compression, etc.)
                     may change when changing from one device linked to a
                     drive to another device linked to the same drive
                     depending on how the devices are defined.  This option
                     defines when the changes are enforced by the driver
                     using SCSI-commands and when the drives auto-detection
                     capabilities are relied upon.  If this option is false,
                     the driver sends the SCSI-commands immediately when the
                     device is changed.  If the option is true, the SCSI-
                     commands are not sent until a write is requested.  In
                     this case, the drive firmware is allowed to detect the
                     tape structure when reading and the SCSI-commands are
                     used only to make sure that a tape is written according
                     to the correct specification.

              MT_ST_CAN_BSR (Default: false)
                     When read-ahead is used, the tape must sometimes be
                     spaced backward to the correct position when the device
                     is closed and the SCSI command to space backward over
                     records is used for this purpose.  Some older drives
                     can't process this command reliably and this option can
                     be used to instruct the driver not to use the command.
                     The end result is that, with read-ahead and fixed-block
                     mode, the tape may not be correctly positioned within a
                     file when the device is closed.  With 2.6 kernel, the
                     default is true for drives supporting SCSI-3.

              MT_ST_NO_BLKLIMS (Default: false)
                     Some drives don't accept the READ BLOCK LIMITS SCSI
                     command.  If this is used, the driver does not use the
                     command.  The drawback is that the driver can't check
                     before sending commands if the selected block size is
                     acceptable to the drive.

              MT_ST_CAN_PARTITIONS (Default: false)
                     This option enables support for several partitions
                     within a tape.  The option applies to all devices
                     linked to a drive.

              MT_ST_SCSI2LOGICAL (Default: false)
                     This option instructs the driver to use the logical
                     block addresses defined in the SCSI-2 standard when
                     performing the seek and tell operations (both with
                     MTSEEK and MTIOCPOS commands and when changing tape
                     partition).  Otherwise, the device-specific addresses
                     are used.  It is highly advisable to set this option if
                     the drive supports the logical addresses because they
                     count also filemarks.  There are some drives that sup‐
                     port only the logical block addresses.

              MT_ST_SYSV (Default: false)
                     When this option is enabled, the tape devices use the
                     System V semantics.  Otherwise, the BSD semantics are
                     used.  The most important difference between the seman‐
                     tics is what happens when a device used for reading is
                     closed: in System V semantics the tape is spaced for‐
                     ward past the next filemark if this has not happened
                     while using the device.  In BSD semantics the tape
                     position is not changed.

              MT_NO_WAIT (Default: false)
                     Enables immediate mode (i.e., don't wait for the com‐
                     mand to finish) for some commands (e.g., rewind).

              An example:

                  struct mtop mt_cmd;
                  mt_cmd.mt_op = MTSETDRVBUFFER;
                  mt_cmd.mt_count = MT_ST_BOOLEANS |
                          MT_ST_BUFFER_WRITES | MT_ST_ASYNC_WRITES;
                  ioctl(fd, MTIOCTOP, mt_cmd);

              The default block size for a device can be set with
              MT_ST_DEF_BLKSIZE and the default density code can be set with
              MT_ST_DEFDENSITY.  The values for the parameters are or'ed
              with the operation code.

              With kernels 2.1.x and later, the timeout values can be set
              with the subcommand MT_ST_SET_TIMEOUT ORed with the timeout in
              seconds.  The long timeout (used for rewinds and other com‐
              mands that may take a long time) can be set with
              MT_ST_SET_LONG_TIMEOUT.  The kernel defaults are very long to
              make sure that a successful command is not timed out with any
              drive.  Because of this, the driver may seem stuck even if it
              is only waiting for the timeout.  These commands can be used
              to set more practical values for a specific drive.  The time‐
              outs set for one device apply for all devices linked to the
              same drive.

              Starting from kernels 2.4.19 and 2.5.43, the driver supports a
              status bit which indicates whether the drive requests clean‐
              ing.  The method used by the drive to return cleaning informa‐
              tion is set using the MT_ST_SEL_CLN subcommand.  If the value
              is zero, the cleaning bit is always zero.  If the value is
              one, the TapeAlert data defined in the SCSI-3 standard is used
              (not yet implemented).  Values 2–17 are reserved.  If the low‐
              est eight bits are >= 18, bits from the extended sense data
              are used.  The bits 9–16 specify a mask to select the bits to
              look at and the bits 17–23 specify the bit pattern to look
              for.  If the bit pattern is zero, one or more bits under the
              mask indicate the cleaning request.  If the pattern is
              nonzero, the pattern must match the masked sense data byte.

   MTIOCGET — get status
       This request takes an argument of type (struct mtget *).

           /* structure for MTIOCGET - mag tape get status command */
           struct mtget {
               long     mt_type;
               long     mt_resid;
               /* the following registers are device dependent */
               long     mt_dsreg;
               long     mt_gstat;
               long     mt_erreg;
               /* The next two fields are not always used */
               daddr_t  mt_fileno;
               daddr_t  mt_blkno;

              The header file defines many values for mt_type, but the cur‐
              rent driver reports only the generic types MT_ISSCSI1 (Generic
              SCSI-1 tape) and MT_ISSCSI2 (Generic SCSI-2 tape).

              contains the current tape partition number.

              reports the drive's current settings for block size (in the
              low 24 bits) and density (in the high 8 bits).  These fields
              are defined by MT_ST_BLKSIZE_SHIFT, MT_ST_BLKSIZE_MASK,

              reports generic (device independent) status information.  The
              header file defines macros for testing these status bits:

              GMT_EOF(x): The tape is positioned just after a filemark
                  (always false after an MTSEEK operation).

              GMT_BOT(x): The tape is positioned at the beginning of the
                  first file (always false after an MTSEEK operation).

              GMT_EOT(x): A tape operation has reached the physical End Of

              GMT_SM(x): The tape is currently positioned at a setmark
                  (always false after an MTSEEK operation).

              GMT_EOD(x): The tape is positioned at the end of recorded

              GMT_WR_PROT(x): The drive is write-protected.  For some drives
                  this can also mean that the drive does not support writing
                  on the current medium type.

              GMT_ONLINE(x): The last open(2) found the drive with a tape in
                  place and ready for operation.

              GMT_D_6250(x), GMT_D_1600(x), GMT_D_800(x): This “generic”
                  status information reports the current density setting for
                  9-track ½" tape drives only.

              GMT_DR_OPEN(x): The drive does not have a tape in place.

              GMT_IM_REP_EN(x): Immediate report mode.  This bit is set if
                  there are no guarantees that the data has been physically
                  written to the tape when the write call returns.  It is
                  set zero only when the driver does not buffer data and the
                  drive is set not to buffer data.

              GMT_CLN(x): The drive has requested cleaning.  Implemented in
                  kernels since 2.4.19 and 2.5.43.

              The only field defined in mt_erreg is the recovered error
              count in the low 16 bits (as defined by MT_ST_SOFTERR_SHIFT
              and MT_ST_SOFTERR_MASK).  Due to inconsistencies in the way
              drives report recovered errors, this count is often not main‐
              tained (most drives do not by default report soft errors but
              this can be changed with a SCSI MODE SELECT command).

              reports the current file number (zero-based).  This value is
              set to -1 when the file number is unknown (e.g., after MTBSS
              or MTSEEK).

              reports the block number (zero-based) within the current file.
              This value is set to -1 when the block number is unknown
              (e.g., after MTBSF, MTBSS, or MTSEEK).

   MTIOCPOS — get tape position
       This request takes an argument of type (struct mtpos *) and reports
       the drive's notion of the current tape block number, which is not the
       same as mt_blkno returned by MTIOCGET.  This drive must be a SCSI-2
       drive that supports the READ POSITION command (device-specific
       address) or a Tandberg-compatible SCSI-1 drive (Tandberg, Archive
       Viper, Wangtek, ... ).

           /* structure for MTIOCPOS - mag tape get position command */
           struct mtpos {
               long mt_blkno;    /* current block number */

RETURN VALUE         top

       EACCES An attempt was made to write or erase a write-protected tape.
              (This error is not detected during open(2).)

       EBUSY  The device is already in use or the driver was unable to
              allocate a buffer.

       EFAULT The command parameters point to memory not belonging to the
              calling process.

       EINVAL An ioctl(2) had an invalid argument, or a requested block size
              was invalid.

       EIO    The requested operation could not be completed.

       ENOMEM The byte count in read(2) is smaller than the next physical
              block on the tape.  (Before 2.2.18 and 2.4.0 the extra bytes
              have been silently ignored.)

       ENOSPC A write operation could not be completed because the tape
              reached end-of-medium.

       ENOSYS Unknown ioctl(2).

       ENXIO  During opening, the tape device does not exist.

              An attempt was made to read or write a variable-length block
              that is larger than the driver's internal buffer.

       EROFS  Open is attempted with O_WRONLY or O_RDWR when the tape in the
              drive is write-protected.

FILES         top

              the auto-rewind SCSI tape devices

              the nonrewind SCSI tape devices

NOTES         top

       1.  When exchanging data between systems, both systems have to agree
           on the physical tape block size.  The parameters of a drive after
           startup are often not the ones most operating systems use with
           these devices.  Most systems use drives in variable-block mode if
           the drive supports that mode.  This applies to most modern
           drives, including DATs, 8mm helical scan drives, DLTs, etc.  It
           may be advisable to use these drives in variable-block mode also
           in Linux (i.e., use MTSETBLK or MTSETDEFBLK at system startup to
           set the mode), at least when exchanging data with a foreign
           system.  The drawback of this is that a fairly large tape block
           size has to be used to get acceptable data transfer rates on the
           SCSI bus.

       2.  Many programs (e.g., tar(1)) allow the user to specify the
           blocking factor on the command line.  Note that this determines
           the physical block size on tape only in variable-block mode.

       3.  In order to use SCSI tape drives, the basic SCSI driver, a SCSI-
           adapter driver and the SCSI tape driver must be either configured
           into the kernel or loaded as modules.  If the SCSI-tape driver is
           not present, the drive is recognized but the tape support
           described in this page is not available.

       4.  The driver writes error messages to the console/log.  The SENSE
           codes written into some messages are automatically translated to
           text if verbose SCSI messages are enabled in kernel

       5.  The driver's internal buffering allows good throughput in fixed-
           block mode also with small read(2) and write(2) byte counts.
           With direct transfers this is not possible and may cause a
           surprise when moving to the 2.6 kernel.  The solution is to tell
           the software to use larger transfers (often telling it to use
           larger blocks).  If this is not possible, direct transfers can be

SEE ALSO         top


       The file drivers/scsi/ or Documentation/scsi/st.txt (kernel
       >= 2.6) in the Linux kernel source tree contains the most recent
       information about the driver and its configuration possibilities

COLOPHON         top

       This page is part of release 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2020-04-11                            ST(4)

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