Re: Shrinking a device - performance?

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

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On Mon, Mar 27, 2017 at 01:17:26PM +0200, Christian Theune wrote:
> Hi,
> 
> I’m currently shrinking a device and it seems that the performance of shrink is abysmal. I intended to shrink a ~22TiB filesystem down to 20TiB. This is still using LVM underneath so that I can’t just remove a device from the filesystem but have to use the resize command.
> 
> Label: 'backy'  uuid: 3d0b7511-4901-4554-96d4-e6f9627ea9a4
>         Total devices 1 FS bytes used 18.21TiB
>         devid    1 size 20.00TiB used 20.71TiB path /dev/mapper/vgsys-backy
> 
> This has been running since last Thursday, so roughly 3.5days now. The “used” number in devid1 has moved about 1TiB in this time. The filesystem is seeing regular usage (read and write) and when I’m suspending any application traffic I see about 1GiB of movement every now and then. Maybe once every 30 seconds or so.
> 
> Does this sound fishy or normal to you?

   On my hardware (consumer HDDs and SATA, RAID-1 over 6 devices), it
takes about a minute to move 1 GiB of data. At that rate, it would
take 1000 minutes (or about 16 hours) to move 1 TiB of data.

   However, there are cases where some items of data can take *much*
longer to move. The biggest of these is when you have lots of
snapshots. When that happens, some (but not all) of the metadata can
take a very long time. In my case, with a couple of hundred snapshots,
some metadata chunks take 4+ hours to move.

   Hugo.

-- 
Hugo Mills             | Great films about cricket: Silly Point Break
hugo@... carfax.org.uk |
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