From 3b1a94c88b798d4f3bd1a5b61f5c8fb9d987c242 Mon Sep 17 00:00:00 2001 From: Damien Le Moal Date: Wed, 7 Jun 2017 15:55:39 +0900 Subject: 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 Reviewed-by: Hannes Reinecke Reviewed-by: Bart Van Assche [Mike Snitzer partly refactored Damien's original work to cleanup the code] Signed-off-by: Mike Snitzer --- Documentation/device-mapper/dm-zoned.txt | 144 +++++++++++++++++++++++++++++++ 1 file changed, 144 insertions(+) create mode 100644 Documentation/device-mapper/dm-zoned.txt (limited to 'Documentation') diff --git a/Documentation/device-mapper/dm-zoned.txt b/Documentation/device-mapper/dm-zoned.txt new file mode 100644 index 000000000000..736fcc78d193 --- /dev/null +++ b/Documentation/device-mapper/dm-zoned.txt @@ -0,0 +1,144 @@ +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}` -- cgit v1.2.3