On Thu, Sep 04, 2014 at 12:32:12PM -0400, Xingbo Wu wrote: > After running a 16-thread sync-random-write test against qcow2, It is > observed that QCOW2 seems to be serializing all its metadata-related writes. > If qcow2 is designed to do this,* then what is the concern?* What would go > wrong if this ordering is relaxed?
How do you know that serializing part of the write request is a significant bottleneck? Please post your benchmark results with raw, qed, and qcow2 handling 1-, 8-, and 16-threads of I/O (or whatever similar benchmarks you have run). The bottleneck may actually be something else, so please share your benchmark configuration and results. > By providing less features, raw-file and QED scales well on parallel I/O > workload. I believe qcow2 does this with clear reasons. Thanks! QED serializes allocating writes, see qed_aio_write_alloc(). In qcow2 the BdrvQcowState->lock is held across metadata updates. The important pieces here are: * qcow2_co_writev() only releases the lock around data writes (including COW). * qcow2_co_flush_to_os() holds the lock around metadata updates Stefan
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