dm zoned: drive-managed zoned block device target

The dm-zoned device mapper target provides transparent write access
to zoned block devices (ZBC and ZAC compliant block devices).
dm-zoned hides to the device user (a file system or an application
doing raw block device accesses) any constraint imposed on write
requests by the device, equivalent to a drive-managed zoned block
device model.

Write requests are processed using a combination of on-disk buffering
using the device conventional zones and direct in-place processing for
requests aligned to a zone sequential write pointer position.
A background reclaim process implemented using dm_kcopyd_copy ensures
that conventional zones are always available for executing unaligned
write requests. The reclaim process overhead is minimized by managing
buffer zones in a least-recently-written order and first targeting the
oldest buffer zones. Doing so, blocks under regular write access (such
as metadata blocks of a file system) remain stored in conventional
zones, resulting in no apparent overhead.

dm-zoned implementation focus on simplicity and on minimizing overhead
(CPU, memory and storage overhead). For a 14TB host-managed disk with
256 MB zones, dm-zoned memory usage per disk instance is at most about
3 MB and as little as 5 zones will be used internally for storing metadata
and performing buffer zone reclaim operations. This is achieved using
zone level indirection rather than a full block indirection system for
managing block movement between zones.

dm-zoned primary target is host-managed zoned block devices but it can
also be used with host-aware device models to mitigate potential
device-side performance degradation due to excessive random writing.

Zoned block devices can be formatted and checked for use with the dm-zoned
target using the dmzadm utility available at:

https://github.com/hgst/dm-zoned-tools

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
[Mike Snitzer partly refactored Damien's original work to cleanup the code]
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

1 files changed, 144 insertions, 0 deletions

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+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}`