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<div>Use mmap is much simpler than local backup image, and without</div>
<div>any influence to consistency. but it doesn't improve availability</div>
<div>while whole sheepdog cluster is die(such as some odd network</div>
<div>partition issue).</div>
<div><br>
</div>
<div>anyway, if no one againsts, I will try to implement it in the</div>
<div>near future (after we solved most stability issues)</div>
<div><br>
</div>
<div>Yibin Shen</div>
<br>
<div class="gmail_quote">On Sun, Oct 9, 2011 at 1:01 AM, MORITA Kazutaka <span dir="ltr">
<<a href="mailto:morita.kazutaka@lab.ntt.co.jp">morita.kazutaka@lab.ntt.co.jp</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex; border-left:1px #ccc solid; padding-left:1ex">
At Sat, 8 Oct 2011 11:12:00 +0800,<br>
<div class="im">Yibin Shen wrote:<br>
> we have talked about similar topic in the past weeks.<br>
><br>
> IMO, let sheepdog support local backup image will promote the availability effectively,<br>
> the local backup image is located in the machine where vm runs,<br>
> then writeback policy is adopted just like what qcow/qcow2's backup image do.<br>
><br>
> the storage size issue can be solved through decrease the number of copies.<br>
><br>
> benefits:<br>
> 1)network traffic will be reduced, which will also make corosync work more stable.<br>
> 2)even whole sheepdog cluster crashed, the local image can still provide service.<br>
><br>
> tradeoffs:<br>
> 1)stronge consistency is damaged for I/O is not triggered synchronously now.<br>
> to solve this problem, at least we need maintain a journal for local backup image.<br>
> 2)migration of vm will became much more complicated.<br>
><br>
> so, maybe this is only a transitionary solution before sheepdog archive industry available standard.<br>
<br>
</div>
Someone suggested a bit similar idea before. The idea is:<br>
<br>
- A VM use a local mmaped file as a disk cache of a Sheepdog volume.<br>
- Write requests are regarded as completed when the data are written<br>
to the mmap address (write-back).<br>
- If the VM sends a sync request, its response will be blocked until<br>
the data is replicated to multiple storage nodes.<br>
<br>
I think this improves a performance significantly, especially when<br>
network latency is high (e.g. WAN environment). The data consistency<br>
is not a problem if sync requests ensure that the data is replicated.<br>
<br>
To support this idea, we need to support a write-back mode in<br>
Sheepdog; we need to add a sync operation to Sheepdog protocols and<br>
implement bdrv_co_flush() in the Sheepdog QEMU block driver.<br>
<br>
<br>
Thanks,<br>
<br>
Kazutaka<br>
<br>
><br>
><br>
> Yibin Shen<br>
<div>
<div></div>
<div class="h5">> On Sat, Oct 8, 2011 at 3:28 AM, Mark Pace <<a href="mailto:pace@jolokianetworks.com">pace@jolokianetworks.com</a><mailto:<a href="mailto:pace@jolokianetworks.com">pace@jolokianetworks.com</a>>> wrote:<br>
><br>
> On 10/7/2011 1:20 AM, MORITA Kazutaka wrote:<br>
><br>
><br>
> - hide quoted text -<br>
><br>
> Other than the storage size issue, the backup node would be a<br>
> bottleneck if there are many VMs. The backup node requires a huge<br>
> amount of disk space and bandwidth, but if we could use such machine,<br>
> we wouldn't need a clustered storage system. However, on a small<br>
> cluster environment with a few nodes, the backup node idea looks good.<br>
> If someone sends a patch to support it, I'll accept it. Thanks,<br>
> Kazutaka<br>
><br>
><br>
> I'm not sure I agree. Shared storage is a single point of failure and<br>
> the reason we're looking to Sheepdog is the ability to survive a shared<br>
> storage outage. Pumping a shared storage box up to provide a backup<br>
> location is not nearly as expensive as creating a redundant/replicated<br>
> shared storage environment that is capable of serving virtual machines<br>
> properly. But, that being said, your absolutely correct in that machine<br>
> is going to be beefy in large environments, etc.<br>
><br>
> Now, I'm not a programmer, but would happily pay for someone to write it<br>
> -- if anyone is interested, please contact me.<br>
><br>
><br>
> Mark Pace<br>
--<br>
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</div>
</blockquote>
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