[Gluster-users] Recovering from remove-brick where shards did not rebalance

Wed Sep 8 16:11:35 UTC 2021

Hi Xavi, 

I am working with a distributred-replicated volume. What I've been doing is copying the shards from each node to their own "recovery" directory, discarding shards that are 0 bytes, then comparing the remainder and combining unique shards into a common directory. Then I'd build a sorted list so the shards are sorted numerically adding the "main file" to the top of the list and then have cat run through the list. I had one pair of shards that diff told me were not equal, but their byte size was equivalent. In that case, I'm not sure which is the "correct" shard, but I'd note that and just pick one with the intention of circling back if cat'ing things together didn't work out...which so far I haven't had any luck. 

How can I identify if a shard is not full size? I haven't checked every single shard, but they seem to be 64 MB in size. Would that mean I need to make sure all but the last shard is 64 MB? I suspect this might be my issue. 

Also, is shard 0 what would appear as the actual file (so largefile.raw or whatever)? It seems in my scenario these files are ~48 MB. I assume that means I need to extend it to 64 MB? 

This is all great information. Thanks! 

~ Anthony 

> From: "Xavi Hernandez" <jahernan at redhat.com>
> To: "anthony" <anthony at vofr.net>
> Cc: "gluster-users" <gluster-users at gluster.org>
> Sent: Wednesday, September 8, 2021 1:57:51 AM
> Subject: Re: [Gluster-users] Recovering from remove-brick where shards did not
> rebalance

> Hi Anthony,

> On Tue, Sep 7, 2021 at 8:20 PM Anthony Hoppe < [ mailto:anthony at vofr.net |
> anthony at vofr.net ] > wrote:

>> I am currently playing with concatenating main file + shards together. Is it
>> safe to assume that a shard with the same ID and sequence number
>> (5da7d7b9-7ff3-48d2-8dcd-4939364bda1f.242 for example) is identical across
>> bricks? That is, I can copy all the shards into a single location overwriting
>> and/or discarding duplicates, then concatenate them together in order? Or is it
>> a more complex?

> Assuming it's a replicated volume, a given shard should appear on all bricks of
> the same replicated subvolume. If there were no pending heals, they should all
> have the same contents (however you can easily check that by running an md5sum
> (or similar) on each file).

> On distributed-replicated volumes it's possible to have the same shard on two
> different subvolumes. In this case one of the subvolumes contains the real
> file, and the other a special 0-bytes file with mode '---------T'. You need to
> take the real file and ignore the second one.

> Shards may be smaller than the shard size. In this case you should extend the
> shard to the shard size before concatenating it with the rest of the shards
> (for example using "truncate -s"). The last shard may be smaller. It doesn't
> need to be extended.

> Once you have all the shards, you can concatenate them. Note that the first
> shard of a file (or shard 0) is not inside the .shard directory. You must take
> it from the location where the file is normally seen.

> Regards,

> Xavi

>>> From: "anthony" < [ mailto:anthony at vofr.net | anthony at vofr.net ] >
>>> To: "gluster-users" < [ mailto:gluster-users at gluster.org |
>>> gluster-users at gluster.org ] >
>>> Sent: Tuesday, September 7, 2021 10:18:07 AM
>>> Subject: Re: [Gluster-users] Recovering from remove-brick where shards did not
>>> rebalance

>>> I've been playing with re-adding the bricks and here is some interesting
>>> behavior.

>>> When I try to force add the bricks to the volume while it's running, I get
>>> complaints about one of the bricks already being a member of a volume. If I
>>> stop the volume, I can then force-add the bricks. However, the volume won't
>>> start without force. Once the volume is force started, all of the bricks remain
>>> offline.

>>> I feel like I'm close...but not quite there...

>>>> From: "anthony" < [ mailto:anthony at vofr.net | anthony at vofr.net ] >
>>>> To: "Strahil Nikolov" < [ mailto:hunter86_bg at yahoo.com | hunter86_bg at yahoo.com ]
>>>> >
>>>> Cc: "gluster-users" < [ mailto:gluster-users at gluster.org |
>>>> gluster-users at gluster.org ] >
>>>> Sent: Tuesday, September 7, 2021 7:45:44 AM
>>>> Subject: Re: [Gluster-users] Recovering from remove-brick where shards did not
>>>> rebalance

>>>> I was contemplating these options, actually, but not finding anything in my
>>>> research showing someone had tried either before gave me pause.

>>>> One thing I wasn't sure about when doing a force add-brick was if gluster would
>>>> wipe the existing data from the added bricks. Sounds like that may not be the
>>>> case?

>>>> With regards to concatenating the main file + shards, how would I go about
>>>> identifying the shards that pair with the main file? I see the shards have
>>>> sequence numbers, but I'm not sure how to match the identifier to the main
>>>> file.

>>>> Thanks!!

>>>>> From: "Strahil Nikolov" < [ mailto:hunter86_bg at yahoo.com | hunter86_bg at yahoo.com
>>>>> ] >
>>>>> To: "anthony" < [ mailto:anthony at vofr.net | anthony at vofr.net ] >,
>>>>> "gluster-users" < [ mailto:gluster-users at gluster.org |
>>>>> gluster-users at gluster.org ] >
>>>>> Sent: Tuesday, September 7, 2021 6:02:36 AM
>>>>> Subject: Re: [Gluster-users] Recovering from remove-brick where shards did not
>>>>> rebalance

>>>>> The data should be recoverable by concatenating the main file with all shards.
>>>>> Then you can copy the data back via the FUSE mount point.

>>>>> I think that some users reported that add-brick with the force option allows to
>>>>> 'undo' the situation and 're-add' the data, but I have never tried that and I
>>>>> cannot guarantee that it will even work.

>>>>> The simplest way is to recover from a recent backup , but sometimes this leads
>>>>> to a data loss.

>>>>> Best Regards,
>>>>> Strahil Nikolov

>>>>>> On Tue, Sep 7, 2021 at 9:29, Anthony Hoppe
>>>>>> < [ mailto:anthony at vofr.net | anthony at vofr.net ] > wrote:
>>>>>> Hello,

>>>>>> I did a bad thing and did a remove-brick on a set of bricks in a
>>>>>> distributed-replicate volume where rebalancing did not successfully rebalance
>>>>>> all files. In sleuthing around the various bricks on the 3 node pool, it
>>>>>> appears that a number of the files within the volume may have been stored as
>>>>>> shards. With that, I'm unsure how to proceed with recovery.

>>>>>> Is it possible to re-add the removed bricks somehow and then do a heal? Or is
>>>>>> there a way to recover data from shards somehow?

>>>>>> Thanks!
>>>>>> ________

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