Re: Rebalancing raid1 after adding a device

[Hugo Mills <hugo@carfax.org.uk>]

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On Tue, Jun 18, 2019 at 02:50:34PM -0400, Austin S. Hemmelgarn wrote:
> On 2019-06-18 14:45, Hugo Mills wrote:
> >On Tue, Jun 18, 2019 at 08:26:32PM +0200, Stéphane Lesimple wrote:
> >>I've been a btrfs user for quite a number of years now, but it seems
> >>I need the wiseness of the btrfs gurus on this one!
> >>
> >>I have a 5-hdd btrfs raid1 setup with 4x3T+1x10T drives.
> >>A few days ago, I replaced one of the 3T by a new 10T, running btrfs
> >>replace and then resizing the FS to use all the available space of
> >>the new device.
> >>
> >>The filesystem was 90% full before I expanded it so, as expected,
> >>most of the space on the new device wasn't actually allocatable in
> >>raid1, as very few available space was available on the 4 other
> >>devs.
> >>
> >>Of course the solution is to run a balance, but as the filesystem is
> >>now quite big, I'd like to avoid running a full rebalance. This
> >>would be quite i/o intensive, would be running for several days, and
> >>putting and unecessary stress on the drives. This also seems
> >>excessive as in theory only some Tb would need to be moved: if I'm
> >>correct, only one of two block groups of a sufficient amount of
> >>chunks to be moved to the new device so that the sum of the amount
> >>of available space on the 4 preexisting devices would at least equal
> >>the available space on the new device, ~7Tb instead of moving ~22T.
> >>I don't need to have a perfectly balanced FS, I just want all the
> >>space to be allocatable.
> >>
> >>I tried using the -ddevid option but it only instructs btrfs to work
> >>on the block groups allocated on said device, as it happens, it
> >>tends to move data between the 4 preexisting devices and doesn't fix
> >>my problem. A full balance with -dlimit=100 did no better.
> >
> >    -dlimit=100 will only move 100 GiB of data (i.e. 200 GiB), so it'll
> >be a pretty limited change. You'll need to use a larger number than
> >that if you want it to have a significant visible effect.
> Last I checked, that's not how the limit filter works.  AFAIUI, it's
> an upper limit on how full a chunk can be to be considered for the
> balance operation.  So, balancing with only `-dlimit=100` should
> actually balance all data chunks (but only data chunks, because you
> haven't asked for metadata balancing).

   That's usage, not limit. limit is simply counting the number of
block groups to move.

   Hugo.

-- 
Hugo Mills             | Great films about cricket: Umpire of the Rising Sun
hugo@... carfax.org.uk |
http://carfax.org.uk/  |
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