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From: | Anthony Liguori |
Subject: | [Qemu-devel] Re: [RFC] Disk integrity in QEMU |
Date: | Mon, 13 Oct 2008 13:43:14 -0500 |
User-agent: | Thunderbird 2.0.0.17 (X11/20080925) |
Ryan Harper wrote:
* Anthony Liguori <address@hidden> [2008-10-09 12:00]:Read performance should be unaffected by using O_DSYNC. O_DIRECT will significantly reduce read performance. I think we should use O_DSYNC by default and I have sent out a patch that contains that. We will follow up with benchmarks to demonstrate this.
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.
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.
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.
Regards, Anthony Liguori
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