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Re: [Qemu-devel] [PATCH] virtio-spec: document block CMD and FLUSH
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PATCH] virtio-spec: document block CMD and FLUSH |
Date: |
Tue, 4 May 2010 22:01:24 +0300 |
User-agent: |
Mutt/1.5.19 (2009-01-05) |
On Tue, May 04, 2010 at 08:56:14PM +0200, Christoph Hellwig wrote:
> On Tue, Apr 20, 2010 at 02:46:35AM +0100, Jamie Lokier wrote:
> > Does this mean that virtio-blk supports all three combinations?
> >
> > 1. FLUSH that isn't a barrier
> > 2. FLUSH that is also a barrier
> > 3. Barrier that is not a flush
> >
> > 1 is good for fsync-like operations;
> > 2 is good for journalling-like ordered operations.
> > 3 sounds like it doesn't mean a lot as the host cache provides no
> > guarantees and has no ordering facility that can be used.
>
> No. The Linux virtio_blk guest driver either supports data integrity
> by using FLUSH or can send down BARRIER requests which aren't much
> help at all.
It seems we use BARRIER when we get REQ_HARDBARRIER, right?
What does the REQ_HARDBARRIER flag in request mean and when is it set?
> Qemu only implements FLUSH anyway.