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

[Andrea Arcangeli <andrea@qumranet.com>]

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On Sun, Oct 12, 2008 at 01:02:57PM -0500, Anthony Liguori wrote:
> You get correctness by using O_DSYNC.  cache=off should disable the use of 
> the page cache everywhere.

The parent shared image is generally readonly (assuming no cluster fs
or shared database storage). So O_DSYNC on the parent will be a noop
but it's ok if you like it as a default.

By default having cache enabled on the parent makes sense to me
(O_DSYNC doesn't disable the cache like O_DIRECT does, reads are
cached). Because the qemu command line is qcow2 internals agnostic
(you can't specify which parent/child image to use, that's left to
qemu-img to set on the qcow2 metadata) I guess the O_DIRECT/O_DSYNC
behavior on the parent image should also be left to qemu-img. Assuming
there's any reserved bitflag left in the qcow2 metadata to use to
specify those bits.

I also attached the results of my o_direct measurements. O_DIRECT
seems very optimal already after the fixes to qcow2 to avoid
submitting aio_read/write only large as a qcow2 cluster size. I was
initially fooled because I didn't reduce the ram on the host to the
guest size + less than the min filesize of iozone, after that O_DIRECT
wins. All tests were run with the emulated ide driver, which is the
one that soldice is using right now with non-linux guest. The
aio-thread patch can't make any difference with ide as verified here.

I also tried to enlarge the max dma in the ide driver to 512k (it's
limited to 128k) but I couldn't measure any benefit. 128k large DMA on
host seems enough to reach platter speed.

I also tried with dma disabled on the guest ide driver, and that
destroys the O_DIRECT performance because then the commands are too
small to reach platter speed. The host IDE driver needs something
>=64k to reach platter speed.

In short I think except for the boot-time O_DIRECT is a must and
things like this are why MAP_SHARED isn't nearly as good as O_DIRECT
for certain cases, as it won't waste any cpu in the VM pagetable
manglings and msyncing. So the parent image is the only one where it
makes sense to allow caching to speed up the boot time and application
startup on the shared executables.

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