admin-guide/device-mapper/dm-zoned.rst

2 dm-zoned
5 The dm-zoned device mapper target exposes a zoned block device (ZBC and
6 ZAC compliant devices) as a regular block device without any write
7 pattern constraints. In effect, it implements a drive-managed zoned
8 block device which hides from the user (a file system or an application
9 doing raw block device accesses) the sequential write constraints of
10 host-managed zoned block devices and can mitigate the potential
11 device-side performance degradation due to excessive random writes on
12 host-aware zoned block devices.
14 For a more detailed description of the zoned block device models and
21 http://www.t13.org/Documents/UploadedDocuments/docs2015/di537r05-Zoned_Device_ATA_Command_Set_ZAC.p…
23 The dm-zoned implementation is simple and minimizes system overhead (CPU
25 host-managed disk with 256 MB zones, dm-zoned memory usage per disk
29 dm-zoned target devices are formatted and checked using the dmzadm
32 https://github.com/hgst/dm-zoned-tools
37 dm-zoned implements an on-disk buffering scheme to handle non-sequential
38 write accesses to the sequential zones of a zoned block device.
40 metadata. It can also use a regular block device together with the zoned
41 block device; in that case the regular block device will be split logically
42 in zones with the same size as the zoned block device. These zones will be
43 placed in front of the zones from the zoned block device and will be handled
58 dm-zoned exposes a logical device with a sector size of 4096 bytes,
59 irrespective of the physical sector size of the backend zoned block
63 The on-disk metadata format is as follows:
65 1) The first block of the first conventional zone found contains the
66 super block which describes the on disk amount and position of metadata
69 2) Following the super block, a set of blocks is used to describe the
71 blocks, with the chunk size equal to the zoned block device size. The
72 mapping table is indexed by chunk number and each mapping entry
73 indicates the zone number of the device storing the chunk of data. Each
74 mapping entry may also indicate if the zone number of a conventional
78 blocks in the data zones follows the mapping table. A valid block is
79 defined as a block that was written and not discarded. For a buffered
80 data chunk, a block is always valid only in the data zone mapping the
91 accessed. Writing a block to the buffer zone of a chunk will
92 automatically invalidate the same block in the sequential zone mapping
96 block device.
98 Read operations are processed according to the block validity
105 After some time, the limited number of conventional zones available may
121 secondary set and validated by updating the super block in the secondary
124 block updates can be done in the primary metadata set. This ensures that
134 If a regular device is used in conjunction with the zoned block device,
136 start of the zoned block device. This metadata has a generation counter of
139 are located at the start of the regular block device.
144 A zoned block device must first be formatted using the dmzadm tool. This
150 	dmzadm --format /dev/sdxx
154 regular block device as the first device.
158 	dmzadm --format /dev/sdxx /dev/sdyy
165 	dmzadm --start /dev/sdxx /dev/sdyy
173 	dmsetup status /dev/dm-X
179 where <nr_zones> is the total number of zones, <nr_unmap_rnd> is the number
180 of 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
182 total number of sequential zones.
191 	dmsetup message /dev/dm-X 0 reclaim