[Gluster-users] Initial sync

Kaushal M kshlmster at gmail.com
Wed Nov 12 12:00:31 UTC 2014

The glusterfs volume configuration lives in /var/lib/glusterd . You'll
need to backup/restore the complete directory to get your

/etc/glusterfs contains a simple volfile needed to start glusterd,
which would be installed by the glusterfs package in most cases.
You'll need this as well.


On Wed, Nov 12, 2014 at 4:02 PM, Andreas Hollaus
<Andreas.Hollaus at ericsson.com> wrote:
> Hi,
> As I previously described, my root file system is located in RAM so I'll lose the
> gluster volume definition(s) in case of a reboot. However, I would like to backup the
> required files to a mounted disk so that they can be restored to /etc after the
> reboot. Which files would I have to backup/restore to be able to run 'gluster volume
> start'  without first re-creating the volume?
> Regards
> Andreas
> On 11/05/14 12:23, Ravishankar N wrote:
>> On 11/05/2014 03:18 PM, Andreas Hollaus wrote:
>>> Hi,
>>> I'm curious about this 5 phase transaction scheme that is described in the document
>>> (lock, pre-op, op, post-op, unlock).
>>> Are these stage switches all triggered from the client or can the server do it
>>> without notifying the client, for instance switching from 'op' to 'post-op'?
>> All stages are performed by the AFR translator in the client graph, where it is
>> loaded, in the sequence you listed.
>>> Decreasing the counter for the local pending operations could be done without talking
>>> to the client, even though I realize a message has to sent to the other server(s),
>>> possibly through the client.
>>> The reason I ask is that I'm trying to estimate the risk of ending up in a split
>>> brain situation, or at least understand if our servers will 'accuse' each other
>>> temporarily during this 5 phase transaction under normal circumstances. If I
>>> understand who sends messages to who and I what order, I'll have a better chance to
>>> see if we require any solution to split brain situations. As I've experienced
>>> problems to setup the 'favorite-child' option, I want to know if it's required or
>>> not. In our use case, quorum is not a solution, but losing some data is acceptable as
>>> long as the bricks are in sync.
>> If a file is split-brained, AFR does not allow modifications  by clients on it
>> until the split-brain is resolved. The afr xattrs and heal mechanisms ensure that
>> the bricks are in sync, so worries on that front.
>> Thanks,
>> Ravi
>>> Regards
>>> Andreas
>>> On 10/31/14 15:37, Ravishankar N wrote:
>>>> On 10/30/2014 07:23 PM, Andreas Hollaus wrote:
>>>>> Hi,
>>>>> Thanks! Seems like an interesting document. Although I've read blogs about how
>>>>> extended attributes are used as a change log, this seams like a more comprehensive
>>>>> document.
>>>>> I won't write directly to any brick. That's the reason I first have to create a
>>>>> volume which consists of only one brick, until the other server is available, and
>>>>> then add that second brick. I don't want to delay the file system clients until the
>>>>> second server is available, hence the reason for add-brick.
>>>>> I guess that this procedure is only needed the first time the volume is configured,
>>>>> right? If any of these bricks would fail later on, the change log would keep
>>>>> track of
>>>>> all changes to the file system even though only one of the bricks is available(?).
>>>> Yes, if one one brick of a replica pair goes down, the other one keeps track of
>>>> file modifications by the client, and would sync it back to the first one when it
>>>> comes back up.
>>>>> After a restart, volume settings stored in the configuration file would be accepted
>>>>> even though not all servers were up and running yet at that time, wouldn't they?
>>>> glusterd running on all nodes ensures that the volume configurations stored on each
>>>> node are in sync.
>>>>> Speaking about configuration files. When are these copied to each server?
>>>>> If I create a volume which consists of two bricks, I guess that those servers will
>>>>> create the configuration files, independently of each other, from the information
>>>>> sent from the client (gluster volume create...).
>>>> All volume config/management commands must be run from any of the servers that make
>>>> up the volume and not the client (unless both happen to be in the same machine). As
>>>> mentioned above, when any of the volume commands are run on any one server,
>>>> glusterd orchestrates the necessary action on all servers and keeps them in sync.
>>>>>    In case I later on add a brick, I guess that the settings have to be copied
>>>>> to the
>>>>> new brick after they have been modified on the first one, right (or will they be
>>>>> recreated on all servers from the information specified by the client, like in the
>>>>> previous case)?
>>>>> Will configuration files be copied in other situations as well, for instance in
>>>>> case
>>>>> one of the servers which is part of the volume for some reason would be missing
>>>>> those
>>>>> files? In my case, the root file system is recreated from an image at each
>>>>> reboot, so
>>>>> everything created in /etc will be lost. Will GlusterFS settings be restored
>>>>> from the
>>>>> other server automatically
>>>> No, it is expected that servers have persistent file-systems.  There are ways to
>>>> restore such bricks; see
>>>> http://gluster.org/community/documentation/index.php/Gluster_3.4:_Brick_Restoration_-_Replace_Crashed_Server
>>>> -Ravi
>>>>> or do I need to backup and restore those myself? Even
>>>>> though the brick doesn't know that it is part of a volume in case it lose the
>>>>> configuration files, both the other server(s) and the client(s) will probably
>>>>> recognize it as being part of the volume. I therefore believe that such a
>>>>> self-healing would actually be possible, even though it may not be implemented.
>>>>> Regards
>>>>> Andreas
>>>>>   On 10/30/14 05:21, Ravishankar N wrote:
>>>>>> On 10/28/2014 03:58 PM, Andreas Hollaus wrote:
>>>>>>> Hi,
>>>>>>> I'm curious about how GlusterFS manages to sync the bricks in the initial phase,
>>>>>>> when
>>>>>>> the volume is created or
>>>>>>> extended.
>>>>>>> I first create a volume consisting of only one brick, which clients will start to
>>>>>>> read and write.
>>>>>>> After a while I add a second brick to the volume to create a replicated volume.
>>>>>>> If this new brick is empty, I guess that files will be copied from the first
>>>>>>> brick to
>>>>>>> get the bricks in sync, right?
>>>>>>> However, if the second brick is not empty but rather contains a subset of the
>>>>>>> files
>>>>>>> on the first brick I don't see
>>>>>>> how GlusterFS will solve the problem of syncing the bricks.
>>>>>>> I guess that all files which lack extended attributes could be removed in this
>>>>>>> scenario, because they were created
>>>>>>> when the disk was not part of a GlusterFS volume. However, in case the brick was
>>>>>>> used
>>>>>>> in the volume previously,
>>>>>>> for instance before that server restarted, there will be extended attributes for
>>>>>>> the
>>>>>>> files on the second brick which
>>>>>>> weren't updated during the downtime (when the volume consisted of only one
>>>>>>> brick).
>>>>>>> There could be multiple
>>>>>>> changes to the files during this time. In this case I don't understand how the
>>>>>>> extended attributes could be used to
>>>>>>> determine which of the bricks contains the most recent file.
>>>>>>> Can anyone explain how this works? Is it only allowed to add empty bricks to a
>>>>>>> volume?
>>>>>> It is allowed to add only empty bricks to the volume. Writing directly to
>>>>>> bricks is
>>>>>> not supported. One needs to access the volume only from a mount point or using
>>>>>> libgfapi.
>>>>>> After adding a brick to increase the distribute count, you need to run the volume
>>>>>> rebalance command so that the some of the existing files are hashed (moved) to
>>>>>> this
>>>>>> newly added brick.
>>>>>> After adding a brick to increase the replica count, you need to run the volume
>>>>>> heal
>>>>>> full command to sync the files from the other replica into the newly added brick.
>>>>>> https://github.com/gluster/glusterfs/blob/master/doc/features/afr-v1.md will give
>>>>>> you an idea of how the replicate translator uses xattrs to keep files in sync.
>>>>>> HTH,
>>>>>> Ravi
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