| 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). |
