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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 (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 

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