Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.
1 dm-zoned 2 ======== 3 4 The dm-zoned device mapper target exposes a zoned block device (ZBC and 5 ZAC compliant devices) as a regular block device without any write 6 pattern constraints. In effect, it implements a drive-managed zoned 7 block device which hides from the user (a file system or an application 8 doing raw block device accesses) the sequential write constraints of 9 host-managed zoned block devices and can mitigate the potential 10 device-side performance degradation due to excessive random writes on 11 host-aware zoned block devices. 12 13 For a more detailed description of the zoned block device models and 14 their constraints see (for SCSI devices): 15 16 http://www.t10.org/drafts.htm#ZBC_Family 17 18 and (for ATA devices): 19 20 http://www.t13.org/Documents/UploadedDocuments/docs2015/di537r05-Zoned_Device_ATA_Command_Set_ZAC.pdf 21 22 The dm-zoned implementation is simple and minimizes system overhead (CPU 23 and memory usage as well as storage capacity loss). For a 10TB 24 host-managed disk with 256 MB zones, dm-zoned memory usage per disk 25 instance is at most 4.5 MB and as little as 5 zones will be used 26 internally for storing metadata and performaing reclaim operations. 27 28 dm-zoned target devices are formatted and checked using the dmzadm 29 utility available at: 30 31 https://github.com/hgst/dm-zoned-tools 32 33 Algorithm 34 ========= 35 36 dm-zoned implements an on-disk buffering scheme to handle non-sequential 37 write accesses to the sequential zones of a zoned block device. 38 Conventional zones are used for caching as well as for storing internal 39 metadata. 40 41 The zones of the device are separated into 2 types: 42 43 1) Metadata zones: these are conventional zones used to store metadata. 44 Metadata zones are not reported as useable capacity to the user. 45 46 2) Data zones: all remaining zones, the vast majority of which will be 47 sequential zones used exclusively to store user data. The conventional 48 zones of the device may be used also for buffering user random writes. 49 Data in these zones may be directly mapped to the conventional zone, but 50 later moved to a sequential zone so that the conventional zone can be 51 reused for buffering incoming random writes. 52 53 dm-zoned exposes a logical device with a sector size of 4096 bytes, 54 irrespective of the physical sector size of the backend zoned block 55 device being used. This allows reducing the amount of metadata needed to 56 manage valid blocks (blocks written). 57 58 The on-disk metadata format is as follows: 59 60 1) The first block of the first conventional zone found contains the 61 super block which describes the on disk amount and position of metadata 62 blocks. 63 64 2) Following the super block, a set of blocks is used to describe the 65 mapping of the logical device blocks. The mapping is done per chunk of 66 blocks, with the chunk size equal to the zoned block device size. The 67 mapping table is indexed by chunk number and each mapping entry 68 indicates the zone number of the device storing the chunk of data. Each 69 mapping entry may also indicate if the zone number of a conventional 70 zone used to buffer random modification to the data zone. 71 72 3) A set of blocks used to store bitmaps indicating the validity of 73 blocks in the data zones follows the mapping table. A valid block is 74 defined as a block that was written and not discarded. For a buffered 75 data chunk, a block is always valid only in the data zone mapping the 76 chunk or in the buffer zone of the chunk. 77 78 For a logical chunk mapped to a conventional zone, all write operations 79 are processed by directly writing to the zone. If the mapping zone is a 80 sequential zone, the write operation is processed directly only if the 81 write offset within the logical chunk is equal to the write pointer 82 offset within of the sequential data zone (i.e. the write operation is 83 aligned on the zone write pointer). Otherwise, write operations are 84 processed indirectly using a buffer zone. In that case, an unused 85 conventional zone is allocated and assigned to the chunk being 86 accessed. Writing a block to the buffer zone of a chunk will 87 automatically invalidate the same block in the sequential zone mapping 88 the chunk. If all blocks of the sequential zone become invalid, the zone 89 is freed and the chunk buffer zone becomes the primary zone mapping the 90 chunk, resulting in native random write performance similar to a regular 91 block device. 92 93 Read operations are processed according to the block validity 94 information provided by the bitmaps. Valid blocks are read either from 95 the sequential zone mapping a chunk, or if the chunk is buffered, from 96 the buffer zone assigned. If the accessed chunk has no mapping, or the 97 accessed blocks are invalid, the read buffer is zeroed and the read 98 operation terminated. 99 100 After some time, the limited number of convnetional zones available may 101 be exhausted (all used to map chunks or buffer sequential zones) and 102 unaligned writes to unbuffered chunks become impossible. To avoid this 103 situation, a reclaim process regularly scans used conventional zones and 104 tries to reclaim the least recently used zones by copying the valid 105 blocks of the buffer zone to a free sequential zone. Once the copy 106 completes, the chunk mapping is updated to point to the sequential zone 107 and the buffer zone freed for reuse. 108 109 Metadata Protection 110 =================== 111 112 To protect metadata against corruption in case of sudden power loss or 113 system crash, 2 sets of metadata zones are used. One set, the primary 114 set, is used as the main metadata region, while the secondary set is 115 used as a staging area. Modified metadata is first written to the 116 secondary set and validated by updating the super block in the secondary 117 set, a generation counter is used to indicate that this set contains the 118 newest metadata. Once this operation completes, in place of metadata 119 block updates can be done in the primary metadata set. This ensures that 120 one of the set is always consistent (all modifications committed or none 121 at all). Flush operations are used as a commit point. Upon reception of 122 a flush request, metadata modification activity is temporarily blocked 123 (for both incoming BIO processing and reclaim process) and all dirty 124 metadata blocks are staged and updated. Normal operation is then 125 resumed. Flushing metadata thus only temporarily delays write and 126 discard requests. Read requests can be processed concurrently while 127 metadata flush is being executed. 128 129 Usage 130 ===== 131 132 A zoned block device must first be formatted using the dmzadm tool. This 133 will analyze the device zone configuration, determine where to place the 134 metadata sets on the device and initialize the metadata sets. 135 136 Ex: 137 138 dmzadm --format /dev/sdxx 139 140 For a formatted device, the target can be created normally with the 141 dmsetup utility. The only parameter that dm-zoned requires is the 142 underlying zoned block device name. Ex: 143 144 echo "0 `blockdev --getsize ${dev}` zoned ${dev}" | dmsetup create dmz-`basename ${dev}`