[Gluster-devel] GlusterFS Snapshot internals

[Paul Cuzner]

Hi Rajesh, 

Thanks for updating the design doc. It reads well. 

I have a number of questions that would help my understanding; 

Logging : The doc doesn't mention how the snapshot process is logged - 
- will snapshot use an existing log or a new log? 
- Will the log be specific to a volume, or will all snapshot activity be logged in a single file? 
- will the log be visible on all nodes, or just the originating node? 
- will the highlevel snapshot action be visible when looking from the other nodes either in the logs or at the cli? 

Restore : You mention that after a restore operation, the snapshot will be automatically deleted. 
- I don't believe this is a prudent thing to do. Here's an example, I've seen ALOT. Application has a programmatic error, leading to data 'corruption' - devs work on the program, storage guys roll the volume back. So far so good...devs provide the updated program, and away you go...BUT the issue is not resolved, so you need to roll back again to the same point in time. If you delete the snap automatically, you loose the restore point. Yes the admin could take another snap after the restore - but why add more work into a recovery process where people are already stressed out :) I'd recommend leaving the snapshot if possible, and let it age out naturally. 

Auto-delete : Is this a post phase of the snapshot create, so the successfully creation of a new snapshot will trigger the pruning of old versions? 

Snapshot Naming : The doc states the name is mandatory. 
- why not offer a default - volume_name_timestamp - instead of making the caller decide on a name. Having this as a default will also make the list under .snap more usable by default. 
- providing a sensible default will make it easier for end users for self service restore. More sensible defaults = more happy admins :) 

Quorum and snaprestore : the doc mentions that when a returning brick comes back, it will be snap'd before pending changes are applied. If I understand the use of quorum correctly, can you comment on the following scenario; 
- With a brick offline, we'll be tracking changes. Say after 1hr a snap is invoked because quorum is met 
- changes continue on the volume for another 15 minutes beyond the snap, when the offline brick comes back online. 
- at this point there are two point in times to bring the brick back to - the brick needs the changes up to the point of the snap, then a snap of the brick followed by the 'replay' of the additional changes to get back to the same point in time as the other replica's in the replica set. 
- of course, the brick could be offline for 24 or 48 hours due to a hardware fault - during which time multiple snapshots could have been made 
- it wasn't clear to me how this scenario is dealt with from the doc? 

barrier : two things are mentioned here - a buffer size and a timeout value. 
- from an admin's pespective, being able to specify the timeout (secs) is likely to be more workable - and will allow them to align this setting with any potential timeout setting within the application running against the gluster volume. I don't think most admins will know or want to know how to size the buffer properly. 

Hopefully the above makes sense. 

Cheers, 

Paul C 

----- Original Message -----

> From: "Rajesh Joseph" <rjoseph at redhat.com>
> To: "gluster-devel" <gluster-devel at nongnu.org>
> Sent: Wednesday, 2 April, 2014 3:55:28 AM
> Subject: [Gluster-devel] GlusterFS Snapshot internals

> Hi all,

> I have updated the GlusterFS snapshot forge wiki.

> https://forge.gluster.org/snapshot/pages/Home

> Please go through it and let me know if you have any questions or queries.

> Best Regards,
> Rajesh

> [PS]: Please ignore previous mail. Accidentally hit send before completing :)

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