[Gluster-users] Initial sync

Andreas Hollaus Andreas.Hollaus at ericsson.com
Wed Nov 12 10:32:22 UTC 2014


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?


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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