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Re: [Qemu-devel] [PATCH] virtio-spec: document block CMD and FLUSH
From: |
Christoph Hellwig |
Subject: |
Re: [Qemu-devel] [PATCH] virtio-spec: document block CMD and FLUSH |
Date: |
Tue, 4 May 2010 20:56:14 +0200 |
User-agent: |
Mutt/1.3.28i |
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. Qemu only implements FLUSH anyway.
- Re: [Qemu-devel] [PATCH] virtio-spec: document block CMD and FLUSH,
Christoph Hellwig <=