[Gluster-users] Initial sync

Wed Nov 5 11:23:18 UTC 2014

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
>