[lustre-discuss] changing inode size on MDT

[Andreas Dilger]

There are several confusing/misleading comments on this thread that need to be clarified...

On Oct 2, 2019, at 13:45, Hebenstreit, Michael <michael.hebenstreit at intel.com<mailto:michael.hebenstreit at intel.com>> wrote:

http://wiki.lustre.org/Lustre_Tuning#Number_of_Inodes_for_MDS

Note that I've updated this page to reflect current defaults.  The Lustre Operations Manual has a much better description of these parameters.


and I'd like to use --mkfsoptions='-i 1024' to have more inodes in the MDT. We already run out of inodes on that FS (probably due to an ZFS bug in early IEEL version) - so I'd like to increase #inodes if possible.

The "-i 1024" option (bytes-per-inode ratio) is only needed for ldiskfs since it statically allocates the inodes at mkfs time, it is not relevant for ZFS since ZFS dynamically allocates inodes and blocks as needed.

On Oct 2, 2019, at 14:00, Colin Faber <cfaber at gmail.com<mailto:cfaber at gmail.com>> wrote:
With 1K inodes you won't have space to accommodate new features, IIRC the current minimal limit on modern lustre is 2K now. If you're running out of MDT space you might consider DNE and multiple MDT's to accommodate that larger name space.

To clarify, since Lustre 2.10 any new ldiskfs MDT will allocate 1024 bytes for the inode itself (-I 1024).  That allows enough space *within* the inode to efficiently store xattrs for more complex layouts (PFL, FLR, DoM).  If xattrs do not fit inside the inode itself then they will be stored in an external 4KB inode block.

The MDT is formatted with a bytes-per-inode *ratio* of 2.5KB, which means (approximately) one inode will be created for every 2.5kB of the total MDT size.  That 2.5KB of space includes the 1KB for the inode itself, plus space for a directory entry (or multiple if hard-linked), extra xattrs, the journal (up to 4GB for large MDTs), Lustre recovery logs, ChangeLogs, etc.  Each directory inode will have at least one 4KB block allocated.

So, it is _possible_ to reduce the inode *ratio* below 2.5KB if you know what you are doing (e.g. 2KB/inode or 1.5KB/inode, this can be an arbitrary number of bytes, it doesn't have to be an even multiple of anything) but it definitely isn't possible to have 1KB inode size and 1KB per inode ratio, as there wouldn't be *any* space left for directories, log files, journal, etc.

Cheers, Andreas
--
Andreas Dilger
Principal Lustre Architect
Whamcloud






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