[lustre-discuss] Use of lazystatfs

[Mike Mosley]

Hello everyone,

We have drained some of our OSS/OSTs and plan to deactivate them soon.  The
process ahead leads us to a couple of questions that we hope somebody can
advise us on.

Scenario
We have fully drained the target OSTs using * 'lfs find'* to identify all
files located on the targets and then feeding the list to '*lfs migrate*. '
A final scan shows there are no files left on the targets.

Questions
1) Running '*lfs df -h'* still shows some space being used even though we
have drained all of the data.   Is that normal?  i.e.

UUID                       bytes        Used   Available Use% Mounted on

hydra-OST0010_UUID         84.7T      583.8M       80.5T   1%
/dfs/hydra[OST:16]

hydra-OST0011_UUID         84.7T      581.4M       80.5T   1%
/dfs/hydra[OST:17]

hydra-OST0012_UUID         84.7T      581.7M       80.5T   1%
/dfs/hydra[OST:18]

hydra-OST0013_UUID         84.7T      582.4M       80.5T   1%
/dfs/hydra[OST:19]

hydra-OST0014_UUID         84.7T      584.1M       80.5T   1%
/dfs/hydra[OST:20]

hydra-OST0015_UUID         84.7T      583.4M       80.5T   1%
/dfs/hydra[OST:21]

hydra-OST0016_UUID         84.7T      583.6M       80.5T   1%
/dfs/hydra[OST:22]

hydra-OST0017_UUID         84.7T      581.8M       80.5T   1%
/dfs/hydra[OST:23]

hydra-OST0018_UUID         84.7T      582.6M       80.5T   1%
/dfs/hydra[OST:24]

hydra-OST0019_UUID         84.7T      582.7M       80.5T   1%
/dfs/hydra[OST:25]

hydra-OST001a_UUID         84.7T      580.0M       80.5T   1%
/dfs/hydra[OST:26]

hydra-OST001b_UUID         84.7T      580.4M       80.5T   1%
/dfs/hydra[OST:27]

hydra-OST001c_UUID         84.7T      582.1M       80.5T   1%
/dfs/hydra[OST:28]

hydra-OST001d_UUID         84.7T      583.2M       80.5T   1%
/dfs/hydra[OST:29]

hydra-OST001e_UUID         84.7T      583.7M       80.5T   1%
/dfs/hydra[OST:30]

hydra-OST001f_UUID         84.7T      587.7M       80.5T   1%
/dfs/hydra[OST:31]


2) According to some comments, prior to deactivating the OSS/OSTs, we
should add the *'lazystatfs'* option to all of our client mounts so that
they do not hang once we deactivate some of the OSTs.   Is that correct?
If so, why would you not just always have that option set?    What are the
ramifications of doing it well in advance of the OST deactivations?

Thanks in advance for feedback,

Mike
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