[Qemu-devel] Re: Caching modes

[Christoph Hellwig]

On Tue, Sep 21, 2010 at 10:13:01AM -0500, Anthony Liguori wrote:
> 1) make virtual WC guest controllable.  If a guest enables WC, &= 
> ~O_DSYNC.  If it disables WC, |= O_DSYNC.  Obviously, we can let a user 
> specify the virtual WC mode but it has to be changable during live 
> migration.

I have patches for that are almost ready to submit.

> 
> 2) only let the user choose between using and not using the host page 
> cache.  IOW, direct=on|off.  cache=XXX is deprecated.

Also done by that patch series.  That's exactly what I described to mail
roundtrips ago..

> My concern is ext4.  With a preallocated file and cache=none as 
> implemented today, performance is good even when barrier=1.  If we 
> enable O_DSYNC, performance will plummet.  Ultimately, this is an ext4 
> problem, not a QEMU problem.

For Linux or Windows guests WCE=0 is not a particularly good default
given that they can deal with the write caches, and mirrors the
situation with consumer SATA disk.  For for older Unix guests you'll
need to be able to persistently disable the write cache.

To make things more confusing the default ATA/SATA way to tune the
volatile write cache setting is not persistent - e.g. if you disable it
using hdparm it will come up enabled again. 

> 2) User does not have enterprise storage, but has an image on ext4 with 
> barrier=1.  User explicitly disables WC in guest because they don't know 
> what they're doing.
> 
> For (2), again it's probably the user doing the wrong thing because if 
> they don't have enterprise storage, then they shouldn't care about a 
> virtual WC.  Practically though, I've seen a lot of this with users.

This setting is just fine, especially if using O_DIRECT.  The guest
sends cache flush requests often enough to not make it a problem.  If
you do not use O_DIRECT in that scenario which will cache a lot more
data in theory - but any filesystem aware of cache flushes will flush
them frequent enough to not make it a problem.  It is a real problem
however when using ext3 in it's default setting in the guest which
doesn't use barrier.  But that's a bug in ext3 and nothing but
petitioning it's maintainer to fix it will help you there.