Re: [Lsf-pc] [LSF/MM TOPIC] I/O error handling and fsync()

[Kevin Wolf <kwolf@redhat.com>]

Am 23.01.2017 um 01:21 hat Theodore Ts'o geschrieben:
> On Sun, Jan 22, 2017 at 06:31:57PM -0500, Jeff Layton wrote:
> > 
> > Ahh, sorry if I wasn't clear.
> > 
> > I know Kevin posed this topic in the context of QEMU/KVM, and I figure
> > that running virt guests (themselves doing all sorts of workloads) is a
> > pretty common setup these days. That was what I meant by "use case"
> > here. Obviously there are many other workloads that could benefit from
> > (or be harmed by) changes in this area.
> > 
> > Still, I think that looking at QEMU/KVM as a "application" and
> > considering what we can do to help optimize that case could be helpful
> > here (and might also be helpful for other workloads).
> 
> Well, except for QEMU/KVM, Kevin has already confirmed that using
> Direct I/O is a completely viable solution.  (And I'll add it solves a
> bunch of other problems, including page cache efficiency....)

Yes, "don't ever use non-O_DIRECT in production" is probably workable as
a solution to the "state after failed fsync()" problem, as long as it is
consistently implemented throughout the stack. That is, if we use a
network protocol in QEMU (NFS, gluster, etc.), the server needs to use
O_DIRECT, too, if we don't want to get the same problem one level down
the stack. I'm not sure if that's possible with all of them, but if it
is, it's mostly just a matter of configuring them correctly.

However, if we look at the greater problem of hanging requests that came
up in the more recent emails of this thread, it is only moved rather
than solved. Chances are that already write() would hang now instead of
only fsync(), but we still have a hard time dealing with this.

Kevin

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