qemu/docs/block-replication.txt

21 executes till the next checkpoint. To support disk contents checkpoint,
22 the modified disk contents in the Secondary VM must be buffered, and are
24 effort during a vmstate checkpoint, the disk modification operations of
25 the Primary disk are asynchronously forwarded to the Secondary node.
51     2) Before Primary write requests are written to Secondary disk, the
52        original sector content will be read from Secondary disk and
56     3) Primary write requests will be written to Secondary disk.
71      disk         ^                                                                   7 Quorum
77 Primary |                 ||  Secondary disk <--------- hidden-disk 5 <--------- active-disk 4
84 1) The disk on the primary is represented by a block device with two
85 children, providing replication between a primary disk and the host that
87 to make the primary always read from the local disk instead of going through
93 3) The secondary disk receives writes from the primary VM through QEMU's
96 4) The disk on the secondary is represented by a custom block device
97 (called active-disk). It should start as an empty disk, and the format
100 5) The hidden-disk is created automatically. It buffers the original content
101 that is modified by the primary VM. It should also start as an empty disk,
104 6) The blockdev-backup job (sync=none) is run to allow hidden-disk to buffer
106 of the NBD server into the secondary disk. So before block replication,
107 the primary disk and secondary disk should contain the same data.
115 1. I/O error on primary disk
118 4. I/O error on secondary disk
119 5. I/O error on active disk
120 6. Making active disk or hidden disk empty failed
122 In case 1 and 5, we just report the error to the disk layer. In case 2, 3,
171   1. There should be only one NBD Client for each primary disk.
173   3. Each disk must have its own export name.
212      disk's id.
213   2. The export name for the same disk must be the same
216   4. Active disk, hidden disk and nbd target's length should be the
218   5. It is better to put active disk and hidden disk in ramdisk.
247 1. Shared disk