[Gluster-users] Replica 2 to replica 3

[Strahil Nikolov]

On April 9, 2020 5:16:47 PM GMT+03:00, Valerio Luccio <valerio.luccio at nyu.edu> wrote:
>Hi,
>
>I'm afraid I still need some help.
>
>When I originally set up my gluster (about 3 years ago), I set it up as
>
>Distributed-Replicated without specifying a replica count and I believe
>
>that defaulted to a replica 2. I have 4 servers with 3 RAIDs attached
>to 
>each server. This was my result:
>
>    Number of Bricks: 6 x 2 = 12
>    Transport-type: tcp
>    Bricks:
>    Brick1: hydra1:/gluster1/data
>    Brick2: hydra1:/gluster2/data
>    Brick3: hydra1:/gluster3/data
>    Brick4: hydra2:/gluster1/data
>    Brick5: hydra2:/gluster2/data
>    Brick6: hydra2:/gluster3/data
>    Brick7: hydra3:/gluster1/data
>    Brick8: hydra3:/gluster2/data
>    Brick9: hydra3:/gluster3/data
>    Brick10: hydra4:/gluster1/data
>    Brick11: hydra4:/gluster2/data
>    Brick12: hydra4:/gluster3/data
>
>If I understand this correctly I have 6 sub-volumes with Brick2 replica
>
>of Brick1, Brick4 of Brick3, etc. Correct ?
>
>I realize now that it would probably have been better to specify a 
>different order, but now I cannot change it.
>
>Now I want to store oVirt images on the Gluster and it requires either 
>replica 1 or replica 3. I need to be able to reuse the bricks I have
>and 
>was planning to remove some bricks, initialize them and add them back
>as 
>a replica 3.
>
>Am I supposed to remove 6 bricks, one from each sub-volume ? Will that 
>work ? Will I lose storage space ? Can I just remove a brick from each 
>server and use those for the replica 3 ?
>
>Thanks for all the help.

Hi Valerio,

You can find a small system and make  it an arbiter ,  so you will end up with 'replica 3  arbiter 1' .

The arbiter  storage calculation is that you need  4K for each file in the volume.If you don't know them  the general rule of thumb is your arbiter brick be 1/1024 the size of the data brick.

Keep in mind that it will be better  if your  arbiter has an SSD (for example mounted on /gluster) from which you can create 6 directories.The arbiter stores  only metadata and the SSD random access performance will be the optimal approach.

Something like:
arbiter:/gluster/data1
arbiter:/gluster/data2
arbiter:/gluster/data3
arbiter:/gluster/data4
arbiter:/gluster/data5
arbiter:/gluster/data6


Of course , proper testing on a test volume is a good approach before  implementing on prod.

Best Regards,
Strahil Nikolov