Re: [PATCH] remus: support DRBD disk backends

[Shriram Rajagopalan <rshriram@cs.ubc.ca>]

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On Fri, May 20, 2011 at 9:23 PM, Shriram Rajagopalan <rshriram@cs.ubc.ca>wrote:

> On Fri, May 20, 2011 at 10:21 AM, Ian Jackson <Ian.Jackson@eu.citrix.com>wrote:
>
>> Shriram Rajagopalan writes ("[Xen-devel] [PATCH] remus: support DRBD disk
>> backends"):
>> > remus: support DRBD disk backends
>> >
>> > DRBD disk backends can be used instead of tapdisk backends for Remus.
>> > This requires a Remus style disk replication protocol (asynchronous
>> > replication with output buffering at backup), that is not available in
>> > standard DRBD code. A modified version that supports this new
>> replication
>> > protocol is available from git://aramis.nss.cs.ubc.ca/drbd-8.3-remus
>>
>> Normally, "drbd:" disk strings would be handled by
>> /etc/xen/scripts/block-drbd, I think ?  Why does remus need to do
>> something different ?
>>
>> Yep. So, remus does use that. But DRBD does not have the replication
> protocol
> that remus needs. Its asynchronous/synchronous replication does not
> fit Remus'
> requirement's of a checkpoint based replication, wherein, the backup has to
> "buffer"
> the disk writes in memory and release them only when the checkpoint is
> being made.
>
> So, the DRBD version supplied in the link above supports a new protocol D,
> that does
> just that. Now, in order to send the tell to the backup that it can "flush"
> the buffered
> disk writes to disk, some sort signalling has to be done from remus, after
> VM is
> suspended. And this patch does just that.
>
>> The block device handling in libxl is in a bit of a state of flux but
>> perhaps it would be useful to start thinking about whether remus could
>> use it ?
>>
>> Dont understand. I do realize that libxl style of block device handling is
> not stabilized,
> and it doesnt support DRBD at the moment. But I am working on remus patches
> for
> libxl support :). I ll start a separate discussion soon on that, but I
> wanted to get this
> one out of the way first.
>
> thanks
> shriram
>
>> Thanks,
>> Ian.
>>
>>
> Ian, is there anything else you would like to know before pulling in this
patch?

shriram

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