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diff --git a/Documentation/admin-guide/device-mapper/dm-dust.rst b/Documentation/admin-guide/device-mapper/dm-dust.rst new file mode 100644 index 000000000..e35ec8cd2 --- /dev/null +++ b/Documentation/admin-guide/device-mapper/dm-dust.rst @@ -0,0 +1,305 @@ +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; "verbose" indicates that +bad block additions, removals, and remaps will be verbosely logged):: + + $ sudo dmsetup status dust1 + 0 33552384 dust 252:17 bypass verbose + + $ 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:: + + 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:: + + dust_query_block: block 72 found in badblocklist + +The following message will print if the block is not in the list:: + + dust_query_block: 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:: + + dust_clear_badblocks: badblocks cleared + +If there were no bad blocks to clear, the following message will +appear:: + + dust_clear_badblocks: no badblocks found + +Listing the bad block list +-------------------------- + +To list all bad blocks in the bad block list (using an example device +with blocks 1 and 2 in the bad block list), run the following message +command:: + + $ sudo dmsetup message dust1 0 listbadblocks + 1 + 2 + +If there are no bad blocks in the bad block list, the command will +execute with no output:: + + $ sudo dmsetup message dust1 0 listbadblocks + +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 + listbadblocks + 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). |