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

Tung-Han Hsieh thhsieh at twcp1.phys.ntu.edu.tw
Mon Apr 26 00:29:29 PDT 2021

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.



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