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'? 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. 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 > _______________________________________________ Gluster-users mailing list Gluster-users@xxxxxxxxxxx http://supercolony.gluster.org/mailman/listinfo/gluster-users
