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Re: [Qemu-devel] [PATCH COLO v3 01/14] docs: block replication's descrip


From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH COLO v3 01/14] docs: block replication's description
Date: Thu, 23 Apr 2015 12:05:00 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0


On 23/04/2015 11:14, Wen Congyang wrote:
> The bs->file->driver should support backing file, and use backing reference
> already.
> 
> What about the primary side? We should control when to connect to NBD server,
> not in nbd_open().

My naive suggestion could be to add a BDRV_O_NO_CONNECT option to
bdrv_open and a separate bdrv_connect callback.  Open would fail if
BDRV_O_NO_CONNECT is specified and drv->bdrv_connect is NULL.

You would then need a way to have quorum pass BDRV_O_NO_CONNECT.

Perhaps quorum is not a great match after all, and it's better to add a
new "colo" driver similar to quorum but simpler and only using the read
policy that you need for colo.  The new driver would also know how to
use BDRV_O_NO_CONNECT.  In any case the amount of work needed would not
be too big.

Paolo



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