Re: [Qemu-devel] Re: [RFC] Disk integrity in QEMU

[Avi Kivity <avi@redhat.com>]

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.