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

[Avi Kivity]

 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)

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