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Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format |
Date: |
Mon, 13 Sep 2010 12:41:02 +0200 |
User-agent: |
Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.12) Gecko/20100907 Fedora/3.0.7-1.fc12 Thunderbird/3.0.7 |
Am 10.09.2010 21:33, schrieb Anthony Liguori:
> On 09/10/2010 12:42 PM, Kevin Wolf wrote:
>>> It bounces all buffers still and I still think it's synchronous
>>> (although Kevin would know better).
>>>
>> Yes, it does bounce the buffers, though I'm looking into this anyway
>> because you raised concerns about unbounded allocations. (And it has
>> been on my todo list for a while, but there were always more urgent things)
>>
>> What's synchronous in qcow2 is metadata access and COW. The guest
>> requests themselves are handled asynchronously.
>
> I think we should differentiate between serialized requests and
> synchronous requests because I got a bit sloppy in my nomenclature.
>
> Metadata reads and writes along with COW operations are synchronous in
> qcow2 IIUC. The data read/write is asynchronous.
>
> Metadata is cached and if a cache hit is taken, then a full request can
> be asynchronous.
>
> It's not clear to me if qcow2 can handle parallel requests though
> assuming that both requests don't require a cache miss for meta data.
Without a cache miss, qcow2_aio_readv/writev is a lookup in an array
(the L2 table) and then directly translated into a raw_aio_readv/writev.
So I don't see any reason why it shouldn't handle requests in parallel.
> Does any meta data cache miss (read or write) cause a stall/flush of the
> request queue?
Metadata reads mean that we do a synchronous bdrv_pread and block the
caller for this time. I think the caller is either the device model or
the processing of AIO callbacks. So I guess effectively this is what you
called a stall.
Kevin
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, (continued)
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, Christoph Hellwig, 2010/09/10
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, Avi Kivity, 2010/09/10
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, Kevin Wolf, 2010/09/10
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, Anthony Liguori, 2010/09/10
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, Avi Kivity, 2010/09/10
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, Anthony Liguori, 2010/09/10
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, Avi Kivity, 2010/09/10
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, Anthony Liguori, 2010/09/10
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, Kevin Wolf, 2010/09/10
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, Anthony Liguori, 2010/09/10
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format,
Kevin Wolf <=
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, Avi Kivity, 2010/09/12
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, Anthony Liguori, 2010/09/12
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, Avi Kivity, 2010/09/12
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, Anthony Liguori, 2010/09/12
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, Avi Kivity, 2010/09/12
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, Anthony Liguori, 2010/09/12
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, Avi Kivity, 2010/09/13
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, Kevin Wolf, 2010/09/13
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, Avi Kivity, 2010/09/13
- Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format, Kevin Wolf, 2010/09/13