[lustre-discuss] How to reduce the size of an existing OST ?

Hans Henrik Happe happe at nbi.dk
Wed Apr 28 00:09:51 PDT 2021


One option is of cause to completely remove the ost as described in the
manual and then do the new setup.

Another option, would be to copy (zfs send/recv) the ost away from the
disks, then creating the new multi ost setup and copy it back. You might
need to migrate it data away from the ost first to make it fit the new

A third is to just have multiple osts on the same ZFS pool, but I guess
this is not a supported/tested case and there might even be reasons that
it will not work at all. At least you need to setup quotas to ensure
free space in Lustre isn't inflated.

Hans Henrik

On 26.04.2021 09.29, Tung-Han Hsieh wrote:
> Dear All,
> We have an existing OST with ZFS backend which occupies the full size
> (63 TB) of a large storage. Now this OST already has 56% full of data.
> The machine which holds this OST does not have other OSTs. But for a
> long time, we found that the loading of this machine is unreasonably
> high (usually > 50). But the other machines with several OSTs in the
> same Lustre file system have quite low loading (usually < 1.0). So we
> are thinking to create multiple OSTs in that high loading machine and
> see whether the loading could be reduced.
> Since it is quite difficult to find another storage to fully backup
> the target OST, so now the question is: Whether there is a safe way
> to reduce the size of the existing OST, such that we can create new
> OSTs on the remaining space of the same device ?
> I guess the procedures might be:
> 1. Shutdown the whole lustre file system (unmount all clients, OSTs,
>    and MDTs)
> 2. For the target OST, try to do defragment to move all data together,
>    in order to make a large continous space free.
> 3. Reduce the total size of that OST using tools something like the
>    resizefs (I don't know whether there is such a tool)
> 4. Reduce the total size of the backend ZFS dataset of that OST.
> All the above procedures were anology (and also from my imagination)
> to the idea of Linux LVM, which has a standard way to reduce a logical
> volume containing an ext4 file system. Just curious and want to discuss
> whether there is any possibility for a Lustre OST.
> Thanks for your attention.
> Cheers,
> T.H.Hsieh
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