Re: Rebalancing after adding larger bricks

Jackie Tung <jackie@xxxxxxxx> · Tue, 11 Oct 2016 10:02:16 -0700

Joe,

Thanks for that, that was educational.  Gluster docs claim that since 3.7, DHT hash ranges are weighted based on brick sizes by default:

$ gluster volume get <vol cluster.weighted-rebalance
Option                                  Value                                   
------                                  -----                                   
cluster.weighted-rebalance              on       

When running rebalance with force, I see this in the rebalance log:

...
[2016-10-11 16:38:37.655144] I [MSGID: 109045] [dht-selfheal.c:1751:dht_fix_layout_of_directory] 0-cronut-dht: subvolume 10 (cronut-replicate-10): 5721127 chunks
[2016-10-11 16:38:37.655154] I [MSGID: 109045] [dht-selfheal.c:1751:dht_fix_layout_of_directory] 0-cronut-dht: subvolume 11 (cronut-replicate-11): 7628846 chunks
…

subvolume >=11 are 8TB, subvolume <= 10 is are 6TB.

Do you think it is possible to even out usage on all bricks by % utilized now?  This would be the case if gluster rebalanced simply by what the scaled DHT says, including all required data migrations?

It would be preferable for us to avoid having to depend on cluster.min-free-disk to manage overflow later on - as this introduces one extra read of the link followed by the actual IOP.

Thanks,
Jackie

On Oct 10, 2016, at 11:13 AM, Joe Julian <joe@xxxxxxxxxxxxxxxx> wrote:

  I've written an example of how gluster's dht works on my blog at
      https://joejulian.name/blog/dht-misses-are-expensive/ which might
      make it clear why the end result is not what you expected.
By setting cluster.min-free-disk (defaults to 10%) you can, at
      least, ensure that your new bricks are utilized as needed to
      prevent over filling your smaller bricks.

    On 10/10/2016 10:13 AM, Jackie Tung
      wrote:

      Hi,

      We have a 2 node, distributed replicated setup (11
        bricks on each node).  Each of these bricks are 6TB in size.

      node_A:/brick1 replicates node_B:/brick1
      node_A:/brick2 replicates node_B:/brick2
      node_A:/brick3 replicates node_B:/brick3
      …
      …
      node_A:/brick11 replicates node_B:/brick11

      We recently added 5 more bricks to make it 16 bricks
        on each node in total.  Each of these new bricks are 8TB in
        size.

      We completed a full rebalance operation (status says
        “completed”).

      However the end result is somewhat unexpected:

        /dev/sdl1
              7.3T 2.2T 5.2T 29%
        /dev/sdk1
              7.3T 2.0T 5.3T 28%
        /dev/sdj1
              7.3T 2.0T 5.3T 28%
        /dev/sdn1
              7.3T 2.2T 5.2T 30%
        /dev/sdp1
              7.3T 2.2T 5.2T 30%
        /dev/sdc1 5.5T
            2.3T 3.2T 42%
        /dev/sdf1 5.5T
            2.3T 3.2T 43%
        /dev/sdo1 5.5T
            2.3T 3.2T 42%
        /dev/sda1 5.5T
            2.3T 3.2T 43%
        /dev/sdi1 5.5T
            2.3T 3.2T 42%
        /dev/sdh1 5.5T
            2.3T 3.2T 43%
        /dev/sde1 5.5T
            2.3T 3.2T 42%
        /dev/sdb1 5.5T
            2.3T 3.2T 42%
        /dev/sdm1 5.5T
            2.3T 3.2T 42%
        /dev/sdg1 5.5T
            2.3T 3.2T 42%
        /dev/sdd1 5.5T
            2.3T 3.2T 42%

      The df output in bold are the new
        8TB drives.
      Was I wrong to expect the % usage to be roughly
        equal?  Is there some parameter I need to tweak to make
        rebalance account for disk sizes properly?

      I’m using Gluster 3.8 on Ubuntu.

      Thanks,
      Jackie

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