Re: [Qemu-devel] [Qemu-block] [PATCH for-2.6 2/2] block/gluster: prevent data loss after i/o error

[Pranith Kumar Karampuri]

+Raghavendra G who implemented this option in write-behind, to this 
upstream patch review discussion

Pranith
On 04/06/2016 06:50 PM, Kevin Wolf wrote:
> Am 06.04.2016 um 15:10 hat Jeff Cody geschrieben:
> > On Wed, Apr 06, 2016 at 01:51:59PM +0200, Kevin Wolf wrote:
> > > Am 06.04.2016 um 13:41 hat Kevin Wolf geschrieben:
> > > > Am 06.04.2016 um 13:19 hat Ric Wheeler geschrieben:
> > > > > We had a thread discussing this not on the upstream list.
> > > > >
> > > > > My summary of the thread is that I don't understand why gluster
> > > > > should drop cached data after a failed fsync() for any open file.
> > > > It certainly shouldn't, but it does by default. :-)
> > > >
> > > > Have a look at commit 3fcead2d in glusterfs.git, which at least
> > > > introduces an option to get usable behaviour:
> > > >
> > > >      { .key = {"resync-failed-syncs-after-fsync"},
> > > >        .type = GF_OPTION_TYPE_BOOL,
> > > >        .default_value = "off",
> > > >        .description = "If sync of \"cached-writes issued before fsync\" "
> > > >                       "(to backend) fails, this option configures whether "
> > > >                       "to retry syncing them after fsync or forget them. "
> > > >                       "If set to on, cached-writes are retried "
> > > >                       "till a \"flush\" fop (or a successful sync) on sync "
> > > >                       "failures. "
> > > >                       "fsync itself is failed irrespective of the value of "
> > > >                       "this option. ",
> > > >      },
> > > >
> > > > As you can see, the default is still to drop cached data, and this is
> > > > with the file still opened. qemu needs to make sure that this option is
> > > > set, and if Jeff's comment in the code below is right, there is no way
> > > > currently to make sure that the option isn't silently ignored.
> > > >
> > > > Can we get some function that sets an option and fails if the option is
> > > > unknown? Or one that queries the state after setting an option, so we
> > > > can check whether we succeeded in switching to the mode we need?
> > > >
> > > > > For closed files, I think it might still happen but this is the same
> > > > > as any file system (and unlikely to be the case for qemu?).
> > > > Our problem is only with open images. Dropping caches for files that
> > > > qemu doesn't use any more is fine as far as I'm concerned.
> > > >
> > > > Note that our usage can involve cases where we reopen a file with
> > > > different flags, i.e. first open a second file descriptor, then close
> > > > the first one. The image was never completely closed here and we would
> > > > still want the cache to preserve our data in such cases.
> > > Hm, actually, maybe we should just call bdrv_flush() before reopening an
> > > image, and if an error is returned, we abort the reopen. It's far from
> > > being a hot path, so the overhead of a flush shouldn't matter, and it
> > > seems we're taking an unnecessary risk without doing this.
> > [I seemed to have been dropped from the cc]
> >
> > Are you talking about doing a bdrv_flush() on the new descriptor (i.e.
> > reop_s->glfs)?  Because otherwise, we already do this in
> > bdrv_reopen_prepare() on the original fd.  It happens right before the call
> > to drv->bdrv_reopen_prepare():
> >
> >
> > 2020     ret = bdrv_flush(reopen_state->bs);
> > 2021     if (ret) {
> > 2022         error_setg_errno(errp, -ret, "Error flushing drive");
> > 2023         goto error;
> > 2024     }
> > 2025
> > 2026     if (drv->bdrv_reopen_prepare) {
> > 2027         ret = drv->bdrv_reopen_prepare(reopen_state, queue, &local_err);
> Ah, thanks. Yes, this is what I meant. I expected it somewhere close to
> the bdrv_drain_all() call, so I missed the call you quoted. So that's
> good news, at least this part of the problem doesn't exist then. :-)
>
> Kevin
>
> > > > > I will note that Linux in general had (still has I think?) the
> > > > > behavior that once the process closes a file (or exits), we lose
> > > > > context to return an error to. From that point on, any failed IO
> > > > > from the page cache to the target disk will be dropped from cache.
> > > > > To hold things in the cache would lead it to fill with old data that
> > > > > is not really recoverable and we have no good way to know that the
> > > > > situation is repairable and how long that might take. Upstream
> > > > > kernel people have debated this, the behavior might be tweaked for
> > > > > certain types of errors.
> > > > That's fine, we just don't want the next fsync() to signal success when
> > > > in reality the cache has thrown away our data. As soon as we close the
> > > > image, there is no next fsync(), so you can do whatever you like.
> > > >
> > > > Kevin
> > > >
> > > > > On 04/06/2016 07:02 AM, Kevin Wolf wrote:
> > > > > > [ Adding some CCs ]
> > > > > >
> > > > > > Am 06.04.2016 um 05:29 hat Jeff Cody geschrieben:
> > > > > > > Upon receiving an I/O error after an fsync, by default gluster will
> > > > > > > dump its cache.  However, QEMU will retry the fsync, which is especially
> > > > > > > useful when encountering errors such as ENOSPC when using the werror=stop
> > > > > > > option.  When using caching with gluster, however, the last written data
> > > > > > > will be lost upon encountering ENOSPC.  Using the cache xlator option of
> > > > > > > 'resync-failed-syncs-after-fsync' should cause gluster to retain the
> > > > > > > cached data after a failed fsync, so that ENOSPC and other transient
> > > > > > > errors are recoverable.
> > > > > > >
> > > > > > > Signed-off-by: Jeff Cody <address@hidden>
> > > > > > > ---
> > > > > > >   block/gluster.c | 27 +++++++++++++++++++++++++++
> > > > > > >   configure       |  8 ++++++++
> > > > > > >   2 files changed, 35 insertions(+)
> > > > > > >
> > > > > > > diff --git a/block/gluster.c b/block/gluster.c
> > > > > > > index 30a827e..b1cf71b 100644
> > > > > > > --- a/block/gluster.c
> > > > > > > +++ b/block/gluster.c
> > > > > > > @@ -330,6 +330,23 @@ static int qemu_gluster_open(BlockDriverState *bs,  QDict 
> > > > > > > *options,
> > > > > > >           goto out;
> > > > > > >       }
> > > > > > > +#ifdef CONFIG_GLUSTERFS_XLATOR_OPT
> > > > > > > +    /* Without this, if fsync fails for a recoverable reason (for instance,
> > > > > > > +     * ENOSPC), gluster will dump its cache, preventing retries.  This means
> > > > > > > +     * almost certain data loss.  Not all gluster versions support the
> > > > > > > +     * 'resync-failed-syncs-after-fsync' key value, but there is no way to
> > > > > > > +     * discover during runtime if it is supported (this api returns success for
> > > > > > > +     * unknown key/value pairs) */
> > > > > > Honestly, this sucks. There is apparently no way to operate gluster so
> > > > > > we can safely recover after a failed fsync. "We hope everything is fine,
> > > > > > but depending on your gluster version, we may now corrupt your image"
> > > > > > isn't very good.
> > > > > >
> > > > > > We need to consider very carefully if this is good enough to go on after
> > > > > > an error. I'm currently leaning towards "no". That is, we should only
> > > > > > enable this after Gluster provides us a way to make sure that the option
> > > > > > is really set.
> > > > > >
> > > > > > > +    ret = glfs_set_xlator_option (s->glfs, "*-write-behind",
> > > > > > > +                                           "resync-failed-syncs-after-fsync",
> > > > > > > +                                           "on");
> > > > > > > +    if (ret < 0) {
> > > > > > > +        error_setg_errno(errp, errno, "Unable to set xlator key/value pair");
> > > > > > > +        ret = -errno;
> > > > > > > +        goto out;
> > > > > > > +    }
> > > > > > > +#endif
> > > > > > We also need to consider the case without CONFIG_GLUSTERFS_XLATOR_OPT.
> > > > > > In this case (as well as theoretically in the case that the option
> > > > > > didn't take effect - if only we could know about it), a failed
> > > > > > glfs_fsync_async() is fatal and we need to stop operating on the image,
> > > > > > i.e. set bs->drv = NULL like when we detect corruption in qcow2 images.
> > > > > > The guest will see a broken disk that fails all I/O requests, but that's
> > > > > > better than corrupting data.
> > > > > >
> > > > > > Kevin