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diff --git a/Documentation/device-mapper/dm-zoned.txt b/Documentation/device-mapper/dm-zoned.txt deleted file mode 100644 index 736fcc78d193..000000000000 --- a/Documentation/device-mapper/dm-zoned.txt +++ /dev/null @@ -1,144 +0,0 @@ -dm-zoned -======== - -The dm-zoned device mapper target exposes a zoned block device (ZBC and -ZAC compliant devices) as a regular block device without any write -pattern constraints. In effect, it implements a drive-managed zoned -block device which hides from the user (a file system or an application -doing raw block device accesses) the sequential write constraints of -host-managed zoned block devices and can mitigate the potential -device-side performance degradation due to excessive random writes on -host-aware zoned block devices. - -For a more detailed description of the zoned block device models and -their constraints see (for SCSI devices): - -http://www.t10.org/drafts.htm#ZBC_Family - -and (for ATA devices): - -http://www.t13.org/Documents/UploadedDocuments/docs2015/di537r05-Zoned_Device_ATA_Command_Set_ZAC.pdf - -The dm-zoned implementation is simple and minimizes system overhead (CPU -and memory usage as well as storage capacity loss). For a 10TB -host-managed disk with 256 MB zones, dm-zoned memory usage per disk -instance is at most 4.5 MB and as little as 5 zones will be used -internally for storing metadata and performaing reclaim operations. - -dm-zoned target devices are formatted and checked using the dmzadm -utility available at: - -https://github.com/hgst/dm-zoned-tools - -Algorithm -========= - -dm-zoned implements an on-disk buffering scheme to handle non-sequential -write accesses to the sequential zones of a zoned block device. -Conventional zones are used for caching as well as for storing internal -metadata. - -The zones of the device are separated into 2 types: - -1) Metadata zones: these are conventional zones used to store metadata. -Metadata zones are not reported as useable capacity to the user. - -2) Data zones: all remaining zones, the vast majority of which will be -sequential zones used exclusively to store user data. The conventional -zones of the device may be used also for buffering user random writes. -Data in these zones may be directly mapped to the conventional zone, but -later moved to a sequential zone so that the conventional zone can be -reused for buffering incoming random writes. - -dm-zoned exposes a logical device with a sector size of 4096 bytes, -irrespective of the physical sector size of the backend zoned block -device being used. This allows reducing the amount of metadata needed to -manage valid blocks (blocks written). - -The on-disk metadata format is as follows: - -1) The first block of the first conventional zone found contains the -super block which describes the on disk amount and position of metadata -blocks. - -2) Following the super block, a set of blocks is used to describe the -mapping of the logical device blocks. The mapping is done per chunk of -blocks, with the chunk size equal to the zoned block device size. The -mapping table is indexed by chunk number and each mapping entry -indicates the zone number of the device storing the chunk of data. Each -mapping entry may also indicate if the zone number of a conventional -zone used to buffer random modification to the data zone. - -3) A set of blocks used to store bitmaps indicating the validity of -blocks in the data zones follows the mapping table. A valid block is -defined as a block that was written and not discarded. For a buffered -data chunk, a block is always valid only in the data zone mapping the -chunk or in the buffer zone of the chunk. - -For a logical chunk mapped to a conventional zone, all write operations -are processed by directly writing to the zone. If the mapping zone is a -sequential zone, the write operation is processed directly only if the -write offset within the logical chunk is equal to the write pointer -offset within of the sequential data zone (i.e. the write operation is -aligned on the zone write pointer). Otherwise, write operations are -processed indirectly using a buffer zone. In that case, an unused -conventional zone is allocated and assigned to the chunk being -accessed. Writing a block to the buffer zone of a chunk will -automatically invalidate the same block in the sequential zone mapping -the chunk. If all blocks of the sequential zone become invalid, the zone -is freed and the chunk buffer zone becomes the primary zone mapping the -chunk, resulting in native random write performance similar to a regular -block device. - -Read operations are processed according to the block validity -information provided by the bitmaps. Valid blocks are read either from -the sequential zone mapping a chunk, or if the chunk is buffered, from -the buffer zone assigned. If the accessed chunk has no mapping, or the -accessed blocks are invalid, the read buffer is zeroed and the read -operation terminated. - -After some time, the limited number of convnetional zones available may -be exhausted (all used to map chunks or buffer sequential zones) and -unaligned writes to unbuffered chunks become impossible. To avoid this -situation, a reclaim process regularly scans used conventional zones and -tries to reclaim the least recently used zones by copying the valid -blocks of the buffer zone to a free sequential zone. Once the copy -completes, the chunk mapping is updated to point to the sequential zone -and the buffer zone freed for reuse. - -Metadata Protection -=================== - -To protect metadata against corruption in case of sudden power loss or -system crash, 2 sets of metadata zones are used. One set, the primary -set, is used as the main metadata region, while the secondary set is -used as a staging area. Modified metadata is first written to the -secondary set and validated by updating the super block in the secondary -set, a generation counter is used to indicate that this set contains the -newest metadata. Once this operation completes, in place of metadata -block updates can be done in the primary metadata set. This ensures that -one of the set is always consistent (all modifications committed or none -at all). Flush operations are used as a commit point. Upon reception of -a flush request, metadata modification activity is temporarily blocked -(for both incoming BIO processing and reclaim process) and all dirty -metadata blocks are staged and updated. Normal operation is then -resumed. Flushing metadata thus only temporarily delays write and -discard requests. Read requests can be processed concurrently while -metadata flush is being executed. - -Usage -===== - -A zoned block device must first be formatted using the dmzadm tool. This -will analyze the device zone configuration, determine where to place the -metadata sets on the device and initialize the metadata sets. - -Ex: - -dmzadm --format /dev/sdxx - -For a formatted device, the target can be created normally with the -dmsetup utility. The only parameter that dm-zoned requires is the -underlying zoned block device name. Ex: - -echo "0 `blockdev --getsize ${dev}` zoned ${dev}" | dmsetup create dmz-`basename ${dev}` |