From: Alex Bligh <alex@alex.org.uk>
To: "Myklebust, Trond" <Trond.Myklebust@netapp.com>
Cc: linux-nfs@vger.kernel.org, ian.campbell@citrix.com,
Alex Bligh <alex@alex.org.uk>
Subject: Re: Fatal crash with NFS, AIO & tcp retransmit
Date: Mon, 21 Jan 2013 17:12:16 +0000 [thread overview]
Message-ID: <734E2E0455BD4515C657BA69@Ximines.local> (raw)
In-Reply-To: <4FA345DA4F4AE44899BD2B03EEEC2FA915C17543@SACEXCMBX04-PRD.hq.netapp.com>
Trond,
--On 21 January 2013 15:50:48 +0000 "Myklebust, Trond"
<Trond.Myklebust@netapp.com> 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
next prev parent reply other threads:[~2013-01-21 17:12 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-01-21 13:06 Fatal crash with NFS, AIO & tcp retransmit Alex Bligh
2013-01-21 14:38 ` Myklebust, Trond
2013-01-21 15:01 ` Alex Bligh
2013-01-21 15:10 ` Alex Bligh
2013-01-21 15:21 ` Ian Campbell
2013-01-25 13:45 ` Alex Bligh
2013-01-21 15:50 ` Myklebust, Trond
2013-01-21 15:54 ` Ian Campbell
2013-01-21 17:12 ` Alex Bligh [this message]
2013-01-21 17:20 ` Myklebust, Trond
2013-01-21 17:29 ` Alex Bligh
2013-01-23 15:22 ` Alex Bligh
2013-01-23 15:34 ` Myklebust, Trond
2013-01-23 17:33 ` Alex Bligh
2013-01-23 17:37 ` Myklebust, Trond
2013-01-23 17:42 ` Alex Bligh
2013-01-23 17:48 ` Peter Staubach
2013-01-23 18:13 ` Myklebust, Trond
2013-01-23 19:37 ` Alex Bligh
2013-01-24 10:42 ` Ian Campbell
2013-01-24 12:00 ` Alex Bligh
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