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Re: [Qemu-devel] Re: [RFC] Disk integrity in QEMU
From: |
Avi Kivity |
Subject: |
Re: [Qemu-devel] Re: [RFC] Disk integrity in QEMU |
Date: |
Tue, 14 Oct 2008 18:42:45 +0200 |
User-agent: |
Thunderbird 2.0.0.16 (X11/20080723) |
Anthony Liguori wrote:
>
> With 16k writes I think we hit a pathological case with the particular
> storage backend we're using since it has many disks and the volume is
> striped. Also the results a bit different when going through a file
> system verses a LVM partition (the later being the first data set).
> Presumably, this is because even with no flags, writes happen
> synchronously to a LVM partition.
>
With no flags, writes should hit the buffer cache (which is the page
cache's name when used to cache block devices).
> Also, cache=off seems to do pretty terribly when operating on an ext3
> file. I suspect this has to do with how ext3 implements O_DIRECT.
Is the file horribly fragmented? Otherwise ext3 O_DIRECT should be
quite good.
Maybe the mapping is not in the host cache and has to be brought in.
>
> However, the data demonstrates pretty nicely that O_DSYNC gives you
> native write speed, but accelerated read speed which I think we agree
> is the desirable behavior. cache=off never seems to outperform
> cache=wt which is another good argument for it being the default over
> cache=off.
Without copyless block I/O, there's no reason to expect cache=none to
outperform cache=writethrough. I expect the read performance to
evaporate with a random access pattern over a large disk (or even
sequential access, given enough running time).
--
Do not meddle in the internals of kernels, for they are subtle and quick to
panic.
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU, (continued)
Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Fabrice Bellard, 2008/10/10
Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Laurent Vivier, 2008/10/13
[Qemu-devel] Re: [RFC] Disk integrity in QEMU, Ryan Harper, 2008/10/13
- [Qemu-devel] Re: [RFC] Disk integrity in QEMU, Anthony Liguori, 2008/10/13
- Re: [Qemu-devel] Re: [RFC] Disk integrity in QEMU,
Avi Kivity <=
- Re: [Qemu-devel] Re: [RFC] Disk integrity in QEMU, Laurent Vivier, 2008/10/13
- Re: [Qemu-devel] Re: [RFC] Disk integrity in QEMU, Ryan Harper, 2008/10/13
- Re: [Qemu-devel] Re: [RFC] Disk integrity in QEMU, Laurent Vivier, 2008/10/13
- Re: [Qemu-devel] Re: [RFC] Disk integrity in QEMU, Ryan Harper, 2008/10/13
- Re: [Qemu-devel] Re: [RFC] Disk integrity in QEMU, Laurent Vivier, 2008/10/15
- Re: [Qemu-devel] Re: [RFC] Disk integrity in QEMU, Laurent Vivier, 2008/10/16
- Re: [Qemu-devel] Re: [RFC] Disk integrity in QEMU, Anthony Liguori, 2008/10/16
- Re: [Qemu-devel] Re: [RFC] Disk integrity in QEMU, Laurent Vivier, 2008/10/16
- Re: [Qemu-devel] Re: [RFC] Disk integrity in QEMU, Avi Kivity, 2008/10/17
- Re: [Qemu-devel] Re: [RFC] Disk integrity in QEMU, Laurent Vivier, 2008/10/17