Trond,
--On 21 January 2013 15:50:48 +0000 "Myklebust, Trond"
<Trond.Myklebust@xxxxxxxxxx> wrote:
I don't think QEMU is actually using O_DIRECT unless I set cache=none
on the drive. That causes a different interesting failure which isn't
my focus just now!
Then your reference to Ian's bug is a red herring.
If the application is using buffered writes, then the data is
immediately copied from userspace to the page cache. Once the copy to
the page cache is done, userspace can do whatever it wants with the
original buffer, because only the page cache pages are used in the RPC
calls.
aio doesn't change any of this...
So, just to be clear, if a process is using NFS and AIO with O_DSYNC
(but not O_DIRECT) - which is I think what QEMU is meant to be doing -
then it should *never* be zero copy (even if writes happen to be
appropriately aligned). Is that correct? If so, I can strace the
process and see exactly what flags it is using.
--
Alex Bligh
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