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From: | Avi Kivity |
Subject: | Re: [Qemu-devel] [PATCH] qcow2: Order concurrent AIO requests on the same unallocated cluster |
Date: | Tue, 01 Sep 2009 14:24:46 +0300 |
User-agent: | Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.1) Gecko/20090814 Fedora/3.0-2.6.b3.fc11 Thunderbird/3.0b3 |
On 09/01/2009 01:50 PM, Kevin Wolf wrote:
Can't this cause an even/odd pattern where all even-numbered requests run first, then all the odd-numbered requests? (0 goes to disk, 1 depends on it, 2 depends on 1, which isn't allocating now, so it goes to disk, 3 depends on 2, ...)I guess it can happen in theory, not sure if it matters in practice.
We should check then.
You are worried about image fragmentation? I think we could do something about it with a cleverer cluster allocation.
Not only image fragmentation - the odd requests will require RMW.
However, I don't think it's an argument against this patch. What currently happens isn't much better: Allocate n clusters, free n-1. Almost as good in producing fragmentation.
The patch introduces complexity so it makes working towards a non-fragmenting solution harder. I'm not saying it could be simplified, it's a side effect of using a state machine design.
Do you have performance numbers?No really detailed numbers. Installation time for RHEL on qcow2/virtio went down from 34 min to 19 min, though.
That's very impressive. cache=none or cache=wt? -- error compiling committee.c: too many arguments to function
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