Re: [Qemu-devel] [PATCH 6/7] qed: Read/write support

[Stefan Hajnoczi]

On Mon, Sep 27, 2010 at 11:12 AM, Avi Kivity <address@hidden> wrote:
>  On 09/23/2010 05:41 PM, Stefan Hajnoczi wrote:
>>
>> This patch implements the read/write state machine.  Operations are
>> fully asynchronous and multiple operations may be active at any time.
>>
>> Allocating writes are serialized to make metadata updates consistent.
>> For example, two write requests to the same unallocated cluster should
>> only allocate the new cluster once and then update it, rather than
>> racing to allocate the cluster twice and losing on of the writes.  This
>> scheme can be made more fine-grained in the future so that independent
>> metadata updates can be active at the same time.
>>
>
> A comment re prefill-postfill - it seems to generate excessive I/O, for
> example an allocating write into the middle of a cluster will generate two
> reads (if a backing file exists) and three writes.  It would be simpler and
> more efficient to read the backing cluster, splice in the guest iovec, and
> issue a single write.  That is one read and two writes fewer.
>
> (assuming I read the code right)

Yes, you read it correctly.  I haven't tried out other write-out
approaches yet but it is something I've been thinking about.  For a
first version of QED the current approach works but we should be able
to get better performance with an improved write-out strategy.

Stefan