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
