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

[Ric Wheeler <rwheeler@redhat.com>]

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. 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?).

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.

Regards,

Ric


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 <jcody@redhat.com>
>> ---
>>   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