[Gluster-users] How reliable is XFS under Gluster?

[Paul Robert Marino]

XFS is fine Ive been using it on various distros in production for
over a decade now and I've rarely had any problems with it and when I
have they have been trivial to fix which is something I honestly cant
say about ext3 or ext4.

Usually when there is a power failure during a write if the
transaction wasn't completely committed to the disk it is rolled back
via the journal.the one exception to this is when you have a battery
backed cache where the battery discharges before power is restored, or
a very cheap consumer grade disk which uses its cache for writes and
lies about the sync state.
in either of these scenarios any file system will have problems.

Out of any of the filesystems Ive worked with in general XFS handles
the battery discharge senario the cleanest and is the easiest to
recover.
if you have the second scenario with the cheap disks with a cache that
lies nothing will help you not even a fsync because the hardware lies.
Also the subject of fsync is a little more complicated than most
people think there are several kinds of fsync and each behaves
differently on different filesystems. PostgreSQL has documentation
about it here http://www.postgresql.org/docs/9.1/static/runtime-config-wal.html
looks at wal_sync_method if you would like to have a better about how
fsync works without getting too deep into the subject.

By the way most apps don't need to do fsyncs and it would bring your
system to a crawl if they all did so take people saying
all programs should fsync with a grain of salt.

In most cases when these problems come up its really that they didn't
set the right IO scheduler for what the server does. For example CFQ
which is the EL default can leave your write in ram cache for quite a
while before sending it to disk in an attempt to optimize your IO;
however the deadline scheduler will attempt to optimize your IO but
will predictably sync it to disk after a period of time regardless of
whether it was able to fully optimize it or not. Also there is noop
which does no optimization at all and leaves every thing to the
hardware, this is common and recommended for VM's and there is some
argument to use it with high end raid controllers for things like
financial data where you need to absolutely ensure the write happen
ASAP because there may be fines or other large penalties if you loose
any data.



On Sat, Dec 7, 2013 at 3:04 AM, Franco Broi <Franco.Broi at iongeo.com> wrote:
> Been using ZFS for about 9 months and am about to add as other 400TB, no
> issues so far.
>
> On 7 Dec 2013 04:23, Brian Foster <bfoster at redhat.com> wrote:
> On 12/06/2013 01:57 PM, Kal Black wrote:
>> Hello,
>> I am in the point of picking up a FS for new brick nodes. I was used to
>> like and use ext4 until now but I recently red for an issue introduced by
>> a
>> patch in ext4 that breaks the distributed translator. In the same time, it
>> looks like the recommended FS for a brick is no longer ext4 but XFS which
>> apparently will also be the default FS in the upcoming RedHat7. On the
>> other hand, XFS is being known as a file system that can be easily
>> corrupted (zeroing files) in case of a power failure. Supporters of the
>> file system claim that this should never happen if an application has been
>> properly coded (properly committing/fsync-ing data to storage) and the
>> storage itself has been properly configured (disk cash disabled on
>> individual disks and battery backed cache used on the controllers). My
>> question is, should I be worried about losing data in a power failure or
>> similar scenarios (or any) using GlusterFS and XFS? Are there best
>> practices for setting up a Gluster brick + XFS? Has the ext4 issue been
>> reliably fixed? (my understanding is that this will be impossible unless
>> ext4 isn't being modified to allow popper work with Gluster)
>>
>
> Hi Kal,
>
> You are correct in that Red Hat recommends using XFS for gluster bricks.
> I'm sure there are plenty of ext4 (and other fs) users as well, so other
> users should chime in as far as real experiences with various brick
> filesystems goes. Also, I believe the dht/ext issue has been resolved
> for some time now.
>
> With regard to "XFS zeroing files on power failure," I'd suggest you
> check out the following blog post:
>
> http://sandeen.net/wordpress/computers/xfs-does-not-null-files-and-requires-no-flux/
>
> My cursory understanding is that there were apparently situations where
> the inode size of a recently extended file would be written to the log
> before the actual extending data is written to disk, thus creating a
> crash window where the updated size would be seen, but not the actual
> data. In other words, this isn't a "zeroing files" behavior in as much
> as it is an ordering issue with logging the inode size. This is probably
> why you've encountered references to fsync(), because with the fix your
> data is still likely lost (unless/until you've run an fsync to flush to
> disk), you just shouldn't see the extended inode size unless the actual
> data made it to disk.
>
> Also note that this was fixed in 2007. ;)
>
> Brian
>
>> Best regards
>>
>>
>>
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>
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