Re: [Qemu-block] [Qemu-devel] [PATCH 10/16] docs: block replication's description

[Eric Blake]

On 09/02/2015 02:51 AM, Wen Congyang wrote:
> Signed-off-by: Wen Congyang <address@hidden>
> Signed-off-by: Yang Hongyang <address@hidden>
> Signed-off-by: zhanghailiang <address@hidden>
> Signed-off-by: Gonglei <address@hidden>
> ---
>  docs/block-replication.txt | 183 
> +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 183 insertions(+)
>  create mode 100644 docs/block-replication.txt
> 


> +
> +    1) Primary write requests will be copied and forwarded to Secondary
> +       QEMU.
> +    2) Before Primary write requests are written to Secondary disk, the
> +       original sector content will be read from Secondary disk and
> +       buffered in the Disk buffer, but it will not overwrite the existing
> +       sector content(it could be from either "Secondary Write Requests" or

space before '(' in English sentences.

> +       previous COW of "Primary Write Requests") in the Disk buffer.
> +    3) Primary write requests will be written to Secondary disk.
> +    4) Secondary write requests will be buffered in the Disk buffer and it
> +       will overwrite the existing sector content in the buffer.
> +
> +== Architecture ==

> +                3 NBD  ------->  3 NBD                                       
>         |
> +                client    ||     server                                      
>     2 filter
> +                          ||        ^                                        
>         ^
> +--------.                 ||        |                                        
>         |
> +Primary |                 ||  Secondary disk <--------- hidden-disk 5 
> <--------- active-disk 4
> +--------'                 ||        |          backing        ^       backing
> +                          ||        |                         |
> +                          ||        |                         |
> +                          ||        '-------------------------'
> +                          ||           drive-backup sync=none
> +

> +
> +4) The disk on the secondary is represented by a custom block device
> +(called active-disk). It should be an empty disk, and the format should
> +support bdrv_make_empty() and backing file.

s/be an empty disk/start as an empty disk/

> +
> +5) The hidden-disk is created automatically. It buffers the original content
> +that is modified by the primary VM. It should also be an empty disk, and

s/be/start as/

> +the driver supports bdrv_make_empty() and backing file.

Missing mention that a drive-backup job is run to allow hidden-disk to
buffer any state that would otherwise be lost by the speculative
write-through of the NBD server into the secondary disk.

> +
> +== Failure Handling ==
> +There are 6 internal errors when block replication is running:
> +1. I/O error on primary disk
> +2. Forwarding primary write requests failed
> +3. Backup failed
> +4. I/O error on secondary disk
> +5. I/O error on active disk
> +6. Making active disk or hidden disk empty failed
> +In case 1 and 5, we just report the error to the disk layer. In case 2, 3,
> +4 and 6, we just report block replication's error to FT/HA manager(which

space before '('

> +decides when to do a new checkpoint, when to do failover).
> +There is one internal error when doing failover:
> +1. Commiting the data in active disk/hidden disk to secondary disk failed

s/Commiting/Committing/

> +We just to report this error to FT/HA manager.
> +
> +== New block driver interface ==

> +
> +== Usage ==
> +Primary:
> +  -drive if=xxx,driver=quorum,read-pattern=fifo,id=colo1,vote-threshold=1\
> +         children.0.file.filename=1.raw,\
> +         children.0.driver=raw,\
> +
> +  Run qmp command in primary qemu:
> +    child_add disk1 child.driver=replication,child.mode=primary,\
> +              child.file.host=xxx,child.file.port=xxx,\
> +              child.file.driver=nbd,child.ignore-errors=on

My comments earlier in this series mean this step should be two QMP
commands: the first is blockdev-add to create an unassociated BDS, the
second to then add that BDS into the quorum.

> +  Note:
> +  1. There should be only one NBD Client for each primary disk.
> +  2. host is the secondary physical machine's hostname or IP
> +  3. Each disk must have its own export name.
> +  4. It is all a single argument to -drive and child_add, and you should
> +     ignore the leading whitespace.
> +  5. The qmp command line must be run after running qmp command line in
> +     secondary qemu.
> +
> +Secondary:
> +  -drive if=none,driver=raw,file=1.raw,id=colo1 \
> +  -drive if=xxx,driver=replication,mode=secondary,\
> +         file.file.filename=active_disk.qcow2,\
> +         file.driver=qcow2,\
> +         file.backing.file.filename=hidden_disk.qcow2,\
> +         file.backing.driver=qcow2,\
> +         file.backing.allow-write-backing-file=on,\
> +         file.backing.backing.backing_reference=colo1\
> +
> +  Then run qmp command in secondary qemu:
> +    nbd-server-start host:port
> +    nbd-server-add -w colo1
> +
> +  Note:
> +  1. The export name in secondary QEMU command line is the secondary
> +     disk's id.
> +  2. The export name for the same disk must be the same
> +  3. The qmp command nbd-server-start and nbd-server-add must be run
> +     before running the qmp command migrate on primary QEMU
> +  4. Don't use nbd-server-start's other options
> +  5. Active disk, hidden disk and nbd target's length should be the
> +     same.
> +  6. It is better to put active disk and hidden disk in ramdisk.
> +  7. It is all a single argument to -drive, and you should ignore
> +     the leading whitespace.

Missing: document the steps taken during failover (that is, how do I
promote a Secondary into a new Primary, and then attach a new Secondary
to that point).  In particular, I suspect there may be differences
between whether you want to roll back to the state of the last
checkpoint (in hidden_disk) or just go with the current state of the
Secondary (in Active); either way, it probably involves doing an active
commit of the state you want into Secondary, then the formation of a new
quorum to start handing replication data off through a new NBD client
connection.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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