195 lines
8.3 KiB
ReStructuredText
195 lines
8.3 KiB
ReStructuredText
========
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dm-zoned
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========
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The dm-zoned device mapper target exposes a zoned block device (ZBC and
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ZAC compliant devices) as a regular block device without any write
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pattern constraints. In effect, it implements a drive-managed zoned
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block device which hides from the user (a file system or an application
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doing raw block device accesses) the sequential write constraints of
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host-managed zoned block devices and can mitigate the potential
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device-side performance degradation due to excessive random writes on
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host-aware zoned block devices.
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For a more detailed description of the zoned block device models and
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their constraints see (for SCSI devices):
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https://www.t10.org/drafts.htm#ZBC_Family
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and (for ATA devices):
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http://www.t13.org/Documents/UploadedDocuments/docs2015/di537r05-Zoned_Device_ATA_Command_Set_ZAC.pdf
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The dm-zoned implementation is simple and minimizes system overhead (CPU
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and memory usage as well as storage capacity loss). For a 10TB
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host-managed disk with 256 MB zones, dm-zoned memory usage per disk
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instance is at most 4.5 MB and as little as 5 zones will be used
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internally for storing metadata and performing reclaim operations.
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dm-zoned target devices are formatted and checked using the dmzadm
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utility available at:
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https://github.com/hgst/dm-zoned-tools
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Algorithm
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=========
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dm-zoned implements an on-disk buffering scheme to handle non-sequential
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write accesses to the sequential zones of a zoned block device.
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Conventional zones are used for caching as well as for storing internal
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metadata. It can also use a regular block device together with the zoned
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block device; in that case the regular block device will be split logically
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in zones with the same size as the zoned block device. These zones will be
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placed in front of the zones from the zoned block device and will be handled
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just like conventional zones.
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The zones of the device(s) are separated into 2 types:
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1) Metadata zones: these are conventional zones used to store metadata.
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Metadata zones are not reported as useable capacity to the user.
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2) Data zones: all remaining zones, the vast majority of which will be
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sequential zones used exclusively to store user data. The conventional
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zones of the device may be used also for buffering user random writes.
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Data in these zones may be directly mapped to the conventional zone, but
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later moved to a sequential zone so that the conventional zone can be
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reused for buffering incoming random writes.
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dm-zoned exposes a logical device with a sector size of 4096 bytes,
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irrespective of the physical sector size of the backend zoned block
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device being used. This allows reducing the amount of metadata needed to
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manage valid blocks (blocks written).
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The on-disk metadata format is as follows:
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1) The first block of the first conventional zone found contains the
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super block which describes the on disk amount and position of metadata
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blocks.
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2) Following the super block, a set of blocks is used to describe the
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mapping of the logical device blocks. The mapping is done per chunk of
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blocks, with the chunk size equal to the zoned block device size. The
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mapping table is indexed by chunk number and each mapping entry
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indicates the zone number of the device storing the chunk of data. Each
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mapping entry may also indicate if the zone number of a conventional
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zone used to buffer random modification to the data zone.
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3) A set of blocks used to store bitmaps indicating the validity of
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blocks in the data zones follows the mapping table. A valid block is
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defined as a block that was written and not discarded. For a buffered
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data chunk, a block is always valid only in the data zone mapping the
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chunk or in the buffer zone of the chunk.
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For a logical chunk mapped to a conventional zone, all write operations
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are processed by directly writing to the zone. If the mapping zone is a
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sequential zone, the write operation is processed directly only if the
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write offset within the logical chunk is equal to the write pointer
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offset within of the sequential data zone (i.e. the write operation is
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aligned on the zone write pointer). Otherwise, write operations are
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processed indirectly using a buffer zone. In that case, an unused
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conventional zone is allocated and assigned to the chunk being
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accessed. Writing a block to the buffer zone of a chunk will
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automatically invalidate the same block in the sequential zone mapping
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the chunk. If all blocks of the sequential zone become invalid, the zone
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is freed and the chunk buffer zone becomes the primary zone mapping the
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chunk, resulting in native random write performance similar to a regular
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block device.
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Read operations are processed according to the block validity
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information provided by the bitmaps. Valid blocks are read either from
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the sequential zone mapping a chunk, or if the chunk is buffered, from
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the buffer zone assigned. If the accessed chunk has no mapping, or the
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accessed blocks are invalid, the read buffer is zeroed and the read
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operation terminated.
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After some time, the limited number of conventional zones available may
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be exhausted (all used to map chunks or buffer sequential zones) and
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unaligned writes to unbuffered chunks become impossible. To avoid this
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situation, a reclaim process regularly scans used conventional zones and
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tries to reclaim the least recently used zones by copying the valid
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blocks of the buffer zone to a free sequential zone. Once the copy
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completes, the chunk mapping is updated to point to the sequential zone
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and the buffer zone freed for reuse.
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Metadata Protection
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===================
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To protect metadata against corruption in case of sudden power loss or
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system crash, 2 sets of metadata zones are used. One set, the primary
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set, is used as the main metadata region, while the secondary set is
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used as a staging area. Modified metadata is first written to the
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secondary set and validated by updating the super block in the secondary
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set, a generation counter is used to indicate that this set contains the
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newest metadata. Once this operation completes, in place of metadata
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block updates can be done in the primary metadata set. This ensures that
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one of the set is always consistent (all modifications committed or none
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at all). Flush operations are used as a commit point. Upon reception of
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a flush request, metadata modification activity is temporarily blocked
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(for both incoming BIO processing and reclaim process) and all dirty
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metadata blocks are staged and updated. Normal operation is then
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resumed. Flushing metadata thus only temporarily delays write and
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discard requests. Read requests can be processed concurrently while
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metadata flush is being executed.
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If a regular device is used in conjunction with the zoned block device,
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a third set of metadata (without the zone bitmaps) is written to the
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start of the zoned block device. This metadata has a generation counter of
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'0' and will never be updated during normal operation; it just serves for
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identification purposes. The first and second copy of the metadata
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are located at the start of the regular block device.
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Usage
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=====
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A zoned block device must first be formatted using the dmzadm tool. This
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will analyze the device zone configuration, determine where to place the
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metadata sets on the device and initialize the metadata sets.
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Ex::
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dmzadm --format /dev/sdxx
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If two drives are to be used, both devices must be specified, with the
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regular block device as the first device.
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Ex::
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dmzadm --format /dev/sdxx /dev/sdyy
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Formatted device(s) can be started with the dmzadm utility, too.:
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Ex::
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dmzadm --start /dev/sdxx /dev/sdyy
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Information about the internal layout and current usage of the zones can
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be obtained with the 'status' callback from dmsetup:
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Ex::
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dmsetup status /dev/dm-X
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will return a line
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0 <size> zoned <nr_zones> zones <nr_unmap_rnd>/<nr_rnd> random <nr_unmap_seq>/<nr_seq> sequential
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where <nr_zones> is the total number of zones, <nr_unmap_rnd> is the number
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of unmapped (ie free) random zones, <nr_rnd> the total number of zones,
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<nr_unmap_seq> the number of unmapped sequential zones, and <nr_seq> the
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total number of sequential zones.
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Normally the reclaim process will be started once there are less than 50
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percent free random zones. In order to start the reclaim process manually
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even before reaching this threshold the 'dmsetup message' function can be
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used:
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Ex::
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dmsetup message /dev/dm-X 0 reclaim
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will start the reclaim process and random zones will be moved to sequential
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zones.
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