[lustre-discuss] ZFS mdt

Hans Henrik Happe happe at nbi.dk
Mon Jan 27 12:54:10 PST 2020


Obviously it depends on the number of inodes (files,dirs etc) you have.
However, if the zfs pool uses ashift=12, which will be the default for
most SSDs and large HDDs, it can have quite an overhead for MDTs. It
takes a lot of bandwidth on MDTs if you do a lot of metedata operations.

You can check it by running 'zdb |grep ashift'.

We are using ashift=9 on SSDs for this reason, even though the SDDs
would prefer it differently.

Hans Henrik

On 27.01.2020 19.07, Nehring, Shane R [LAS] wrote:
> Hey all,
> We've been running a lustre volume for a few years now and it's been
> working quite well. We've been using ZFS as the backend storage and
> while that's been working well I've noticed that the space usage is
> a little weird on the mdt:
> NAME            PROPERTY           VALUE     SOURCE
> store/work-mdt  used               4.91T     -
> store/work-mdt  logicalused        960G      -
> store/work-mdt  referenced         4.91T     -
> store/work-mdt  logicalreferenced  960G      -
> store/work-mdt  compressratio      1.00x     -
> Just wondering if anyone else has noticed this kind of overhead.
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