On 09/14/2010 07:08 PM, Anthony Liguori wrote:
Yes, I hit this too. So without this patch, it does serialize all
allocating writes?
Yes, but my patch is not enough as it turns out.
When dealing with O_DIRECT, we have to handle RMW on our own which
means we need to serialize access to the same sector.
The way we're planning on addressing this in the short term is to
break the single allocator queue into a per-L2 table queue. So
writes to the same L2 would be serialized but writes to different L2s
would not be serialized.
So at least I read the code correctly.
The next step (also addressed in the qcow2 performance plan) is to
batch writes to L2. You'd actually expect to have many concurrent
allocating writes to one L2. The first is sent to disk, but the
following ones just mark the L2 dirty. When the write returns, it
sees it's still dirty and goes back to disk again.