[Gluster-users] Replica 3 - how to replace failed node (peer)

[Martin Toth]

Hi Karthik,

> On Thu, Apr 11, 2019 at 12:43 PM Martin Toth <snowmailer at gmail.com <mailto:snowmailer at gmail.com>> wrote:
> Hi Karthik,
> 
> more over, I would like to ask if there are some recommended settings/parameters for SHD in order to achieve good or fair I/O while volume will be healed when I will replace Brick (this should trigger healing process). 
> If I understand you concern correctly, you need to get fair I/O performance for clients while healing takes place as part of  the replace brick operation. For this you can turn off the "data-self-heal" and "metadata-self-heal" options until the heal completes on the new brick.

This is exactly what I mean. I am running VM disks on remaining 2 (out of 3 - one failed as mentioned) nodes and I need to ensure there will be fair I/O performance available on these two nodes while replace brick operation will heal volume.
I will not run any VMs on node where replace brick operation will be running. So if I understand correctly, when I will set :

# gluster volume set <volname> cluster.data-self-heal off
# gluster volume set <volname> cluster.metadata-self-heal off

this will tell Gluster clients (libgfapi and FUSE mount) not to read from node “where replace brick operation” is in place but from remaing two healthy nodes. Is this correct ? Thanks for clarification.

> Turning off client side healing doesn't compromise data integrity and consistency. During the read request from client, pending xattr is evaluated for replica copies and read is only served from correct copy. During writes, IO will continue on both the replicas, SHD will take care of healing files.
> After replacing the brick, we strongly recommend you to consider upgrading your gluster to one of the maintained versions. We have many stability related fixes there, which can handle some critical issues and corner cases which you could hit during these kind of scenarios.

This will be first priority in infrastructure after fixing this cluster back to fully functional replica3. I will upgrade to 3.12.x and then to version 5 or 6.

BR, 
Martin

> Regards,
> Karthik
> I had some problems in past when healing was triggered, VM disks became unresponsive because healing took most of I/O. My volume containing only big files with VM disks.
> 
> Thanks for suggestions.
> BR, 
> Martin
> 
>> On 10 Apr 2019, at 12:38, Martin Toth <snowmailer at gmail.com <mailto:snowmailer at gmail.com>> wrote:
>> 
>> Thanks, this looks ok to me, I will reset brick because I don't have any data anymore on failed node so I can use same path / brick name.
>> 
>> Is reseting brick dangerous command? Should I be worried about some possible failure that will impact remaining two nodes? I am running really old 3.7.6 but stable version.
>> 
>> Thanks,
>> BR!
>> 
>> Martin
>>  
>> 
>>> On 10 Apr 2019, at 12:20, Karthik Subrahmanya <ksubrahm at redhat.com <mailto:ksubrahm at redhat.com>> wrote:
>>> 
>>> Hi Martin,
>>> 
>>> After you add the new disks and creating raid array, you can run the following command to replace the old brick with new one:
>>> 
>>> - If you are going to use a different name to the new brick you can run
>>> gluster volume replace-brick <volname> <old-brick> <new-brick> commit force
>>> 
>>> - If you are planning to use the same name for the new brick as well then you can use
>>> gluster volume reset-brick <volname> <old-brick> <new-brick> commit force
>>> Here old-brick & new-brick's hostname &  path should be same.
>>> 
>>> After replacing the brick, make sure the brick comes online using volume status.
>>> Heal should automatically start, you can check the heal status to see all the files gets replicated to the newly added brick. If it does not start automatically, you can manually start that by running gluster volume heal <volname>.
>>> 
>>> HTH,
>>> Karthik
>>> 
>>> On Wed, Apr 10, 2019 at 3:13 PM Martin Toth <snowmailer at gmail.com <mailto:snowmailer at gmail.com>> wrote:
>>> Hi all,
>>> 
>>> I am running replica 3 gluster with 3 bricks. One of my servers failed - all disks are showing errors and raid is in fault state.
>>> 
>>> Type: Replicate
>>> Volume ID: 41d5c283-3a74-4af8-a55d-924447bfa59a
>>> Status: Started
>>> Number of Bricks: 1 x 3 = 3
>>> Transport-type: tcp
>>> Bricks:
>>> Brick1: node1.san:/tank/gluster/gv0imagestore/brick1
>>> Brick2: node2.san:/tank/gluster/gv0imagestore/brick1 <— this brick is down
>>> Brick3: node3.san:/tank/gluster/gv0imagestore/brick1
>>> 
>>> So one of my bricks is totally failed (node2). It went down and all data are lost (failed raid on node2). Now I am running only two bricks on 2 servers out from 3.
>>> This is really critical problem for us, we can lost all data. I want to add new disks to node2, create new raid array on them and try to replace failed brick on this node. 
>>> 
>>> What is the procedure of replacing Brick2 on node2, can someone advice? I can’t find anything relevant in documentation.
>>> 
>>> Thanks in advance,
>>> Martin
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