Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format

[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.  
Does any meta data cache miss (read or write) cause a stall/flush of the 
request queue?

In QED, all operations are asynchronous including metadata cache misses 
and COW operations.

Multiple requests are handled in parallel with the exception of metadata 
writes.  A metadata write will stall all future metadata cache miss 
fulfillment but they remain asynchronous.  Requests that can be 
satisfied from the cache will be executed in parallel though.

In the future, we'll do something more sophisticated and allow multiple 
simultaneous metadata writes provided they aren't do the same L2/L1 
location.

Regards,

Anthony Liguori