dm-zoned.rst (8453B)
1======== 2dm-zoned 3======== 4 5The dm-zoned device mapper target exposes a zoned block device (ZBC and 6ZAC compliant devices) as a regular block device without any write 7pattern constraints. In effect, it implements a drive-managed zoned 8block device which hides from the user (a file system or an application 9doing raw block device accesses) the sequential write constraints of 10host-managed zoned block devices and can mitigate the potential 11device-side performance degradation due to excessive random writes on 12host-aware zoned block devices. 13 14For a more detailed description of the zoned block device models and 15their constraints see (for SCSI devices): 16 17https://www.t10.org/drafts.htm#ZBC_Family 18 19and (for ATA devices): 20 21http://www.t13.org/Documents/UploadedDocuments/docs2015/di537r05-Zoned_Device_ATA_Command_Set_ZAC.pdf 22 23The dm-zoned implementation is simple and minimizes system overhead (CPU 24and memory usage as well as storage capacity loss). For a 10TB 25host-managed disk with 256 MB zones, dm-zoned memory usage per disk 26instance is at most 4.5 MB and as little as 5 zones will be used 27internally for storing metadata and performing reclaim operations. 28 29dm-zoned target devices are formatted and checked using the dmzadm 30utility available at: 31 32https://github.com/hgst/dm-zoned-tools 33 34Algorithm 35========= 36 37dm-zoned implements an on-disk buffering scheme to handle non-sequential 38write accesses to the sequential zones of a zoned block device. 39Conventional zones are used for caching as well as for storing internal 40metadata. It can also use a regular block device together with the zoned 41block device; in that case the regular block device will be split logically 42in zones with the same size as the zoned block device. These zones will be 43placed in front of the zones from the zoned block device and will be handled 44just like conventional zones. 45 46The zones of the device(s) are separated into 2 types: 47 481) Metadata zones: these are conventional zones used to store metadata. 49Metadata zones are not reported as useable capacity to the user. 50 512) Data zones: all remaining zones, the vast majority of which will be 52sequential zones used exclusively to store user data. The conventional 53zones of the device may be used also for buffering user random writes. 54Data in these zones may be directly mapped to the conventional zone, but 55later moved to a sequential zone so that the conventional zone can be 56reused for buffering incoming random writes. 57 58dm-zoned exposes a logical device with a sector size of 4096 bytes, 59irrespective of the physical sector size of the backend zoned block 60device being used. This allows reducing the amount of metadata needed to 61manage valid blocks (blocks written). 62 63The on-disk metadata format is as follows: 64 651) The first block of the first conventional zone found contains the 66super block which describes the on disk amount and position of metadata 67blocks. 68 692) Following the super block, a set of blocks is used to describe the 70mapping of the logical device blocks. The mapping is done per chunk of 71blocks, with the chunk size equal to the zoned block device size. The 72mapping table is indexed by chunk number and each mapping entry 73indicates the zone number of the device storing the chunk of data. Each 74mapping entry may also indicate if the zone number of a conventional 75zone used to buffer random modification to the data zone. 76 773) A set of blocks used to store bitmaps indicating the validity of 78blocks in the data zones follows the mapping table. A valid block is 79defined as a block that was written and not discarded. For a buffered 80data chunk, a block is always valid only in the data zone mapping the 81chunk or in the buffer zone of the chunk. 82 83For a logical chunk mapped to a conventional zone, all write operations 84are processed by directly writing to the zone. If the mapping zone is a 85sequential zone, the write operation is processed directly only if the 86write offset within the logical chunk is equal to the write pointer 87offset within of the sequential data zone (i.e. the write operation is 88aligned on the zone write pointer). Otherwise, write operations are 89processed indirectly using a buffer zone. In that case, an unused 90conventional zone is allocated and assigned to the chunk being 91accessed. Writing a block to the buffer zone of a chunk will 92automatically invalidate the same block in the sequential zone mapping 93the chunk. If all blocks of the sequential zone become invalid, the zone 94is freed and the chunk buffer zone becomes the primary zone mapping the 95chunk, resulting in native random write performance similar to a regular 96block device. 97 98Read operations are processed according to the block validity 99information provided by the bitmaps. Valid blocks are read either from 100the sequential zone mapping a chunk, or if the chunk is buffered, from 101the buffer zone assigned. If the accessed chunk has no mapping, or the 102accessed blocks are invalid, the read buffer is zeroed and the read 103operation terminated. 104 105After some time, the limited number of conventional zones available may 106be exhausted (all used to map chunks or buffer sequential zones) and 107unaligned writes to unbuffered chunks become impossible. To avoid this 108situation, a reclaim process regularly scans used conventional zones and 109tries to reclaim the least recently used zones by copying the valid 110blocks of the buffer zone to a free sequential zone. Once the copy 111completes, the chunk mapping is updated to point to the sequential zone 112and the buffer zone freed for reuse. 113 114Metadata Protection 115=================== 116 117To protect metadata against corruption in case of sudden power loss or 118system crash, 2 sets of metadata zones are used. One set, the primary 119set, is used as the main metadata region, while the secondary set is 120used as a staging area. Modified metadata is first written to the 121secondary set and validated by updating the super block in the secondary 122set, a generation counter is used to indicate that this set contains the 123newest metadata. Once this operation completes, in place of metadata 124block updates can be done in the primary metadata set. This ensures that 125one of the set is always consistent (all modifications committed or none 126at all). Flush operations are used as a commit point. Upon reception of 127a flush request, metadata modification activity is temporarily blocked 128(for both incoming BIO processing and reclaim process) and all dirty 129metadata blocks are staged and updated. Normal operation is then 130resumed. Flushing metadata thus only temporarily delays write and 131discard requests. Read requests can be processed concurrently while 132metadata flush is being executed. 133 134If a regular device is used in conjunction with the zoned block device, 135a third set of metadata (without the zone bitmaps) is written to the 136start of the zoned block device. This metadata has a generation counter of 137'0' and will never be updated during normal operation; it just serves for 138identification purposes. The first and second copy of the metadata 139are located at the start of the regular block device. 140 141Usage 142===== 143 144A zoned block device must first be formatted using the dmzadm tool. This 145will analyze the device zone configuration, determine where to place the 146metadata sets on the device and initialize the metadata sets. 147 148Ex:: 149 150 dmzadm --format /dev/sdxx 151 152 153If two drives are to be used, both devices must be specified, with the 154regular block device as the first device. 155 156Ex:: 157 158 dmzadm --format /dev/sdxx /dev/sdyy 159 160 161Formatted device(s) can be started with the dmzadm utility, too.: 162 163Ex:: 164 165 dmzadm --start /dev/sdxx /dev/sdyy 166 167 168Information about the internal layout and current usage of the zones can 169be obtained with the 'status' callback from dmsetup: 170 171Ex:: 172 173 dmsetup status /dev/dm-X 174 175will return a line 176 177 0 <size> zoned <nr_zones> zones <nr_unmap_rnd>/<nr_rnd> random <nr_unmap_seq>/<nr_seq> sequential 178 179where <nr_zones> is the total number of zones, <nr_unmap_rnd> is the number 180of unmapped (ie free) random zones, <nr_rnd> the total number of zones, 181<nr_unmap_seq> the number of unmapped sequential zones, and <nr_seq> the 182total number of sequential zones. 183 184Normally the reclaim process will be started once there are less than 50 185percent free random zones. In order to start the reclaim process manually 186even before reaching this threshold the 'dmsetup message' function can be 187used: 188 189Ex:: 190 191 dmsetup message /dev/dm-X 0 reclaim 192 193will start the reclaim process and random zones will be moved to sequential 194zones.