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Diffstat (limited to 'Documentation/device-mapper/dm-dust.txt')
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diff --git a/Documentation/device-mapper/dm-dust.txt b/Documentation/device-mapper/dm-dust.txt new file mode 100644 index 000000000000..954d402a1f6a --- /dev/null +++ b/Documentation/device-mapper/dm-dust.txt @@ -0,0 +1,272 @@ +dm-dust +======= + +This target emulates the behavior of bad sectors at arbitrary +locations, and the ability to enable the emulation of the failures +at an arbitrary time. + +This target behaves similarly to a linear target. At a given time, +the user can send a message to the target to start failing read +requests on specific blocks (to emulate the behavior of a hard disk +drive with bad sectors). + +When the failure behavior is enabled (i.e.: when the output of +"dmsetup status" displays "fail_read_on_bad_block"), reads of blocks +in the "bad block list" will fail with EIO ("Input/output error"). + +Writes of blocks in the "bad block list will result in the following: + +1. Remove the block from the "bad block list". +2. Successfully complete the write. + +This emulates the "remapped sector" behavior of a drive with bad +sectors. + +Normally, a drive that is encountering bad sectors will most likely +encounter more bad sectors, at an unknown time or location. +With dm-dust, the user can use the "addbadblock" and "removebadblock" +messages to add arbitrary bad blocks at new locations, and the +"enable" and "disable" messages to modulate the state of whether the +configured "bad blocks" will be treated as bad, or bypassed. +This allows the pre-writing of test data and metadata prior to +simulating a "failure" event where bad sectors start to appear. + +Table parameters: +----------------- +<device_path> <offset> <blksz> + +Mandatory parameters: + <device_path>: path to the block device. + <offset>: offset to data area from start of device_path + <blksz>: block size in bytes + (minimum 512, maximum 1073741824, must be a power of 2) + +Usage instructions: +------------------- + +First, find the size (in 512-byte sectors) of the device to be used: + +$ sudo blockdev --getsz /dev/vdb1 +33552384 + +Create the dm-dust device: +(For a device with a block size of 512 bytes) +$ sudo dmsetup create dust1 --table '0 33552384 dust /dev/vdb1 0 512' + +(For a device with a block size of 4096 bytes) +$ sudo dmsetup create dust1 --table '0 33552384 dust /dev/vdb1 0 4096' + +Check the status of the read behavior ("bypass" indicates that all I/O +will be passed through to the underlying device): +$ sudo dmsetup status dust1 +0 33552384 dust 252:17 bypass + +$ sudo dd if=/dev/mapper/dust1 of=/dev/null bs=512 count=128 iflag=direct +128+0 records in +128+0 records out + +$ sudo dd if=/dev/zero of=/dev/mapper/dust1 bs=512 count=128 oflag=direct +128+0 records in +128+0 records out + +Adding and removing bad blocks: +------------------------------- + +At any time (i.e.: whether the device has the "bad block" emulation +enabled or disabled), bad blocks may be added or removed from the +device via the "addbadblock" and "removebadblock" messages: + +$ sudo dmsetup message dust1 0 addbadblock 60 +kernel: device-mapper: dust: badblock added at block 60 + +$ sudo dmsetup message dust1 0 addbadblock 67 +kernel: device-mapper: dust: badblock added at block 67 + +$ sudo dmsetup message dust1 0 addbadblock 72 +kernel: device-mapper: dust: badblock added at block 72 + +These bad blocks will be stored in the "bad block list". +While the device is in "bypass" mode, reads and writes will succeed: + +$ sudo dmsetup status dust1 +0 33552384 dust 252:17 bypass + +Enabling block read failures: +----------------------------- + +To enable the "fail read on bad block" behavior, send the "enable" message: + +$ sudo dmsetup message dust1 0 enable +kernel: device-mapper: dust: enabling read failures on bad sectors + +$ sudo dmsetup status dust1 +0 33552384 dust 252:17 fail_read_on_bad_block + +With the device in "fail read on bad block" mode, attempting to read a +block will encounter an "Input/output error": + +$ sudo dd if=/dev/mapper/dust1 of=/dev/null bs=512 count=1 skip=67 iflag=direct +dd: error reading '/dev/mapper/dust1': Input/output error +0+0 records in +0+0 records out +0 bytes copied, 0.00040651 s, 0.0 kB/s + +...and writing to the bad blocks will remove the blocks from the list, +therefore emulating the "remap" behavior of hard disk drives: + +$ sudo dd if=/dev/zero of=/dev/mapper/dust1 bs=512 count=128 oflag=direct +128+0 records in +128+0 records out + +kernel: device-mapper: dust: block 60 removed from badblocklist by write +kernel: device-mapper: dust: block 67 removed from badblocklist by write +kernel: device-mapper: dust: block 72 removed from badblocklist by write +kernel: device-mapper: dust: block 87 removed from badblocklist by write + +Bad block add/remove error handling: +------------------------------------ + +Attempting to add a bad block that already exists in the list will +result in an "Invalid argument" error, as well as a helpful message: + +$ sudo dmsetup message dust1 0 addbadblock 88 +device-mapper: message ioctl on dust1 failed: Invalid argument +kernel: device-mapper: dust: block 88 already in badblocklist + +Attempting to remove a bad block that doesn't exist in the list will +result in an "Invalid argument" error, as well as a helpful message: + +$ sudo dmsetup message dust1 0 removebadblock 87 +device-mapper: message ioctl on dust1 failed: Invalid argument +kernel: device-mapper: dust: block 87 not found in badblocklist + +Counting the number of bad blocks in the bad block list: +-------------------------------------------------------- + +To count the number of bad blocks configured in the device, run the +following message command: + +$ sudo dmsetup message dust1 0 countbadblocks + +A message will print with the number of bad blocks currently +configured on the device: + +kernel: device-mapper: dust: countbadblocks: 895 badblock(s) found + +Querying for specific bad blocks: +--------------------------------- + +To find out if a specific block is in the bad block list, run the +following message command: + +$ sudo dmsetup message dust1 0 queryblock 72 + +The following message will print if the block is in the list: +device-mapper: dust: queryblock: block 72 found in badblocklist + +The following message will print if the block is in the list: +device-mapper: dust: queryblock: block 72 not found in badblocklist + +The "queryblock" message command will work in both the "enabled" +and "disabled" modes, allowing the verification of whether a block +will be treated as "bad" without having to issue I/O to the device, +or having to "enable" the bad block emulation. + +Clearing the bad block list: +---------------------------- + +To clear the bad block list (without needing to individually run +a "removebadblock" message command for every block), run the +following message command: + +$ sudo dmsetup message dust1 0 clearbadblocks + +After clearing the bad block list, the following message will appear: + +kernel: device-mapper: dust: clearbadblocks: badblocks cleared + +If there were no bad blocks to clear, the following message will +appear: + +kernel: device-mapper: dust: clearbadblocks: no badblocks found + +Message commands list: +---------------------- + +Below is a list of the messages that can be sent to a dust device: + +Operations on blocks (requires a <blknum> argument): + +addbadblock <blknum> +queryblock <blknum> +removebadblock <blknum> + +...where <blknum> is a block number within range of the device + (corresponding to the block size of the device.) + +Single argument message commands: + +countbadblocks +clearbadblocks +disable +enable +quiet + +Device removal: +--------------- + +When finished, remove the device via the "dmsetup remove" command: + +$ sudo dmsetup remove dust1 + +Quiet mode: +----------- + +On test runs with many bad blocks, it may be desirable to avoid +excessive logging (from bad blocks added, removed, or "remapped"). +This can be done by enabling "quiet mode" via the following message: + +$ sudo dmsetup message dust1 0 quiet + +This will suppress log messages from add / remove / removed by write +operations. Log messages from "countbadblocks" or "queryblock" +message commands will still print in quiet mode. + +The status of quiet mode can be seen by running "dmsetup status": + +$ sudo dmsetup status dust1 +0 33552384 dust 252:17 fail_read_on_bad_block quiet + +To disable quiet mode, send the "quiet" message again: + +$ sudo dmsetup message dust1 0 quiet + +$ sudo dmsetup status dust1 +0 33552384 dust 252:17 fail_read_on_bad_block verbose + +(The presence of "verbose" indicates normal logging.) + +"Why not...?" +------------- + +scsi_debug has a "medium error" mode that can fail reads on one +specified sector (sector 0x1234, hardcoded in the source code), but +it uses RAM for the persistent storage, which drastically decreases +the potential device size. + +dm-flakey fails all I/O from all block locations at a specified time +frequency, and not a given point in time. + +When a bad sector occurs on a hard disk drive, reads to that sector +are failed by the device, usually resulting in an error code of EIO +("I/O error") or ENODATA ("No data available"). However, a write to +the sector may succeed, and result in the sector becoming readable +after the device controller no longer experiences errors reading the +sector (or after a reallocation of the sector). However, there may +be bad sectors that occur on the device in the future, in a different, +unpredictable location. + +This target seeks to provide a device that can exhibit the behavior +of a bad sector at a known sector location, at a known time, based +on a large storage device (at least tens of gigabytes, not occupying +system memory). |