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Re: [Qemu-devel] [PATCH for-2.6 2/2] block/gluster: prevent data loss af

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH for-2.6 2/2] block/gluster: prevent data loss after i/o error
Date: Wed, 6 Apr 2016 13:41:40 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

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.

> 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.


> 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;
> >>      }
> >>+    /* 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

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