306 lines
10 KiB
ReStructuredText
306 lines
10 KiB
ReStructuredText
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dm-dust
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=======
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This target emulates the behavior of bad sectors at arbitrary
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locations, and the ability to enable the emulation of the failures
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at an arbitrary time.
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This target behaves similarly to a linear target. At a given time,
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the user can send a message to the target to start failing read
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requests on specific blocks (to emulate the behavior of a hard disk
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drive with bad sectors).
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When the failure behavior is enabled (i.e.: when the output of
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"dmsetup status" displays "fail_read_on_bad_block"), reads of blocks
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in the "bad block list" will fail with EIO ("Input/output error").
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Writes of blocks in the "bad block list will result in the following:
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1. Remove the block from the "bad block list".
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2. Successfully complete the write.
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This emulates the "remapped sector" behavior of a drive with bad
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sectors.
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Normally, a drive that is encountering bad sectors will most likely
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encounter more bad sectors, at an unknown time or location.
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With dm-dust, the user can use the "addbadblock" and "removebadblock"
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messages to add arbitrary bad blocks at new locations, and the
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"enable" and "disable" messages to modulate the state of whether the
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configured "bad blocks" will be treated as bad, or bypassed.
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This allows the pre-writing of test data and metadata prior to
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simulating a "failure" event where bad sectors start to appear.
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Table parameters
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----------------
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<device_path> <offset> <blksz>
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Mandatory parameters:
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<device_path>:
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Path to the block device.
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<offset>:
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Offset to data area from start of device_path
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<blksz>:
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Block size in bytes
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(minimum 512, maximum 1073741824, must be a power of 2)
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Usage instructions
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------------------
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First, find the size (in 512-byte sectors) of the device to be used::
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$ sudo blockdev --getsz /dev/vdb1
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33552384
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Create the dm-dust device:
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(For a device with a block size of 512 bytes)
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::
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$ sudo dmsetup create dust1 --table '0 33552384 dust /dev/vdb1 0 512'
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(For a device with a block size of 4096 bytes)
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::
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$ sudo dmsetup create dust1 --table '0 33552384 dust /dev/vdb1 0 4096'
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Check the status of the read behavior ("bypass" indicates that all I/O
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will be passed through to the underlying device; "verbose" indicates that
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bad block additions, removals, and remaps will be verbosely logged)::
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$ sudo dmsetup status dust1
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0 33552384 dust 252:17 bypass verbose
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$ sudo dd if=/dev/mapper/dust1 of=/dev/null bs=512 count=128 iflag=direct
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128+0 records in
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128+0 records out
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$ sudo dd if=/dev/zero of=/dev/mapper/dust1 bs=512 count=128 oflag=direct
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128+0 records in
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128+0 records out
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Adding and removing bad blocks
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------------------------------
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At any time (i.e.: whether the device has the "bad block" emulation
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enabled or disabled), bad blocks may be added or removed from the
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device via the "addbadblock" and "removebadblock" messages::
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$ sudo dmsetup message dust1 0 addbadblock 60
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kernel: device-mapper: dust: badblock added at block 60
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$ sudo dmsetup message dust1 0 addbadblock 67
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kernel: device-mapper: dust: badblock added at block 67
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$ sudo dmsetup message dust1 0 addbadblock 72
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kernel: device-mapper: dust: badblock added at block 72
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These bad blocks will be stored in the "bad block list".
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While the device is in "bypass" mode, reads and writes will succeed::
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$ sudo dmsetup status dust1
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0 33552384 dust 252:17 bypass
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Enabling block read failures
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----------------------------
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To enable the "fail read on bad block" behavior, send the "enable" message::
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$ sudo dmsetup message dust1 0 enable
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kernel: device-mapper: dust: enabling read failures on bad sectors
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$ sudo dmsetup status dust1
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0 33552384 dust 252:17 fail_read_on_bad_block
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With the device in "fail read on bad block" mode, attempting to read a
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block will encounter an "Input/output error"::
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$ sudo dd if=/dev/mapper/dust1 of=/dev/null bs=512 count=1 skip=67 iflag=direct
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dd: error reading '/dev/mapper/dust1': Input/output error
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0+0 records in
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0+0 records out
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0 bytes copied, 0.00040651 s, 0.0 kB/s
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...and writing to the bad blocks will remove the blocks from the list,
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therefore emulating the "remap" behavior of hard disk drives::
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$ sudo dd if=/dev/zero of=/dev/mapper/dust1 bs=512 count=128 oflag=direct
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128+0 records in
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128+0 records out
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kernel: device-mapper: dust: block 60 removed from badblocklist by write
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kernel: device-mapper: dust: block 67 removed from badblocklist by write
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kernel: device-mapper: dust: block 72 removed from badblocklist by write
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kernel: device-mapper: dust: block 87 removed from badblocklist by write
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Bad block add/remove error handling
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-----------------------------------
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Attempting to add a bad block that already exists in the list will
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result in an "Invalid argument" error, as well as a helpful message::
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$ sudo dmsetup message dust1 0 addbadblock 88
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device-mapper: message ioctl on dust1 failed: Invalid argument
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kernel: device-mapper: dust: block 88 already in badblocklist
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Attempting to remove a bad block that doesn't exist in the list will
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result in an "Invalid argument" error, as well as a helpful message::
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$ sudo dmsetup message dust1 0 removebadblock 87
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device-mapper: message ioctl on dust1 failed: Invalid argument
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kernel: device-mapper: dust: block 87 not found in badblocklist
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Counting the number of bad blocks in the bad block list
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-------------------------------------------------------
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To count the number of bad blocks configured in the device, run the
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following message command::
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$ sudo dmsetup message dust1 0 countbadblocks
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A message will print with the number of bad blocks currently
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configured on the device::
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countbadblocks: 895 badblock(s) found
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Querying for specific bad blocks
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--------------------------------
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To find out if a specific block is in the bad block list, run the
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following message command::
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$ sudo dmsetup message dust1 0 queryblock 72
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The following message will print if the block is in the list::
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dust_query_block: block 72 found in badblocklist
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The following message will print if the block is not in the list::
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dust_query_block: block 72 not found in badblocklist
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The "queryblock" message command will work in both the "enabled"
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and "disabled" modes, allowing the verification of whether a block
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will be treated as "bad" without having to issue I/O to the device,
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or having to "enable" the bad block emulation.
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Clearing the bad block list
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---------------------------
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To clear the bad block list (without needing to individually run
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a "removebadblock" message command for every block), run the
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following message command::
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$ sudo dmsetup message dust1 0 clearbadblocks
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After clearing the bad block list, the following message will appear::
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dust_clear_badblocks: badblocks cleared
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If there were no bad blocks to clear, the following message will
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appear::
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dust_clear_badblocks: no badblocks found
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Listing the bad block list
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--------------------------
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To list all bad blocks in the bad block list (using an example device
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with blocks 1 and 2 in the bad block list), run the following message
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command::
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$ sudo dmsetup message dust1 0 listbadblocks
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1
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2
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If there are no bad blocks in the bad block list, the command will
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execute with no output::
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$ sudo dmsetup message dust1 0 listbadblocks
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Message commands list
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---------------------
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Below is a list of the messages that can be sent to a dust device:
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Operations on blocks (requires a <blknum> argument)::
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addbadblock <blknum>
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queryblock <blknum>
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removebadblock <blknum>
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...where <blknum> is a block number within range of the device
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(corresponding to the block size of the device.)
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Single argument message commands::
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countbadblocks
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clearbadblocks
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listbadblocks
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disable
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enable
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quiet
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Device removal
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--------------
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When finished, remove the device via the "dmsetup remove" command::
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$ sudo dmsetup remove dust1
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Quiet mode
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----------
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On test runs with many bad blocks, it may be desirable to avoid
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excessive logging (from bad blocks added, removed, or "remapped").
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This can be done by enabling "quiet mode" via the following message::
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$ sudo dmsetup message dust1 0 quiet
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This will suppress log messages from add / remove / removed by write
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operations. Log messages from "countbadblocks" or "queryblock"
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message commands will still print in quiet mode.
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The status of quiet mode can be seen by running "dmsetup status"::
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$ sudo dmsetup status dust1
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0 33552384 dust 252:17 fail_read_on_bad_block quiet
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To disable quiet mode, send the "quiet" message again::
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$ sudo dmsetup message dust1 0 quiet
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$ sudo dmsetup status dust1
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0 33552384 dust 252:17 fail_read_on_bad_block verbose
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(The presence of "verbose" indicates normal logging.)
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"Why not...?"
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-------------
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scsi_debug has a "medium error" mode that can fail reads on one
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specified sector (sector 0x1234, hardcoded in the source code), but
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it uses RAM for the persistent storage, which drastically decreases
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the potential device size.
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dm-flakey fails all I/O from all block locations at a specified time
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frequency, and not a given point in time.
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When a bad sector occurs on a hard disk drive, reads to that sector
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are failed by the device, usually resulting in an error code of EIO
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("I/O error") or ENODATA ("No data available"). However, a write to
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the sector may succeed, and result in the sector becoming readable
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after the device controller no longer experiences errors reading the
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sector (or after a reallocation of the sector). However, there may
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be bad sectors that occur on the device in the future, in a different,
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unpredictable location.
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This target seeks to provide a device that can exhibit the behavior
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of a bad sector at a known sector location, at a known time, based
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on a large storage device (at least tens of gigabytes, not occupying
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system memory).
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