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

From: Aurelien Jarno
Subject: Re: [Qemu-devel] [RFC] Disk integrity in QEMU
Date: Mon, 13 Oct 2008 08:42:51 +0200
User-agent: Mozilla-Thunderbird (X11/20080724)

Mark Wagner a écrit :
> Anthony Liguori wrote:
>> Mark Wagner wrote:
>>> If you stopped and listened to yourself, you'd see that you are making 
>>> my point...
>>> AFAIK, QEMU is neither designed nor intended to be an Enterprise 
>>> Storage Array,
>>> I thought this group is designing a virtualization layer.  However, 
>>> the persistent
>>> argument is that since Enterprise Storage products will often 
>>> acknowledge a write
>>> before the data is actually on the disk, its OK for QEMU to do the same.
>> I think you're a little lost in this thread.  We're going to have QEMU 
>> only acknowledge writes when they complete.  I've already sent out a 
>> patch.  Just waiting a couple days to let everyone give their input.
> Actually, I'm just don't being clear enough in trying to point out that I
> don't think just setting a default value for "cache" goes far enough. My
> argument has nothing to do with the default value. It has to do with what the
> right thing to do is in specific situations regardless of the value of the
> cache setting.
> My point is that if a file is opened in the guest with the O_DIRECT (or 
> then QEMU *must* honor that regardless of whatever value the current value of
> "cache" is.
> So, if the system admin for the host decides to set cache=on and something
> in the guest opens a file with O_DIRECT, I feel that it is a violation
> of the system call for the host to cache the write in its local cache w/o
> sending it immediately to the storage subsystem. It must get an ACK from
> the storage subsystem before it can return to the guest in order to preserve
> the guarantee.
> So, if your proposed default value for the cache is in effect, then O_DSYNC
> should provide the write-thru required by the guests use of O_DIRECT on the
> writes.  However, if the default cache value is not used and its set to
> cache=on, and if the guest is using O_DIRECT or O_DSYNC, I feel there are
> issues that need to be addressed.

Everybody agrees that we should support data integrity *by default*. But
please admit that some persons have different needs than yours, and
actually *want* to lie to the guest. We should propose such and option,
with a *big warning*.

  .''`.  Aurelien Jarno             | GPG: 1024D/F1BCDB73
 : :' :  Debian developer           | Electrical Engineer
 `. `'   address@hidden         | address@hidden
   `-    people.debian.org/~aurel32 | www.aurel32.net

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