Re: [PATCH] qemu: use statically allocate 512 byte buffer in the stack for sector in bdrv_commit

["Javier Guerra" <javier-796Irmz5ZkZBDgjK7y7TUQ@public.gmane.org>]

On 1/7/08, Laurent Vivier <Laurent.Vivier@bull.net> wrote:
> Le lundi 07 janvier 2008 à 11:03 -0500, Javier Guerra a écrit :
> > hopefully, it would now work with "-cache=off", don't you think?
>
> Well, I don't think the problem is at the host level but at the guest
> level, because both instances of qemu share the host cache and thus
> first instance should see changes made by the second instance (and
> vice-versa).

that's what a cluster filesystem is designed to cope with (and in fact expects)

> There are also some caches at qemu level to emulate DMA, for instance in
> hw/ide.c it is MAX_MULT_SECTORS (16) which is 8 kB buffer, perhaps your
> problem is here but "cache=off" doesn't remove this.
> Did you try to change MAX_MULT_SECTORS to 1 ?

nope, don't know enough of qemu internals...

but if those caches can be flushed from the guest, the filesystem
should do that when writing its metadata

> What do you call a "cluster filesystem" ?

GFS, OCFS2, etc.   that is, filesystems that are designed to run on
more than one host with shared block storage.  usually that means
FibreChannel or iSCSI, but on VMs, a common backing could work too (at
least in theory).

Xen manages it, at least on paravirtualized guests; hadn't tried on
HVM guests. maybe the IDE-like emulation is too poor an interface to
handle it.

-- 
Javier
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