Lustre 1.6 Operations Manual

21.3 Options to Format MDT and OST
Filesystems
The backing filesystems on an MDT and OSTs are independent of one another, so the
formatting parameters for them should not be same. The size of the MDT backing
filesystem depends solely on how many inodes you want in the entire Lustre
filesystem. This is not related to the size of the aggregate OST space.
21.3.1 Planning for Inodes
Each time you create a file on a Lustre filesystem, it consumes one inode on the MDT
and one inode for each OST object that the file is striped over. Normally, it is based
on the default stripe count option -c; but this may change on a per-file basis. In
ext3/ldiskfs filesystems, inodes are pre-allocated, so creating a new file does not
consume any of the free blocks. However, this also means that the format-time
options should be conservative, as it is not possible to increase the number of inodes
after the filesystem is formatted. If there is a shortage of inodes or space on the OSTs,
it is possible to add OSTs to the filesystem.
To be on the safe side, plan for 4 KB per inode on the MDT (the default). For the
OST, the amount of space taken by each object depends entirely upon the usage
pattern of the users/applications running on the system. Lustre, by necessity,
defaults to a very conservative estimate for the object size (16 KB per object). You can
almost always increase this value for filesystem installations. Many Lustre
filesystems have average file sizes over 1 MB per object.
21.3.2 Calculating MDT Size
When calculating the MDT size, the only important factor is the average size of files
to be stored in the filesystem. If the average file size is, for example, 5 MB and you
have 100 TB of usable OST space, then you need at least (100 TB * 1024 GB/TB * 1024
MB/GB / 5 MB/inode) = 20 million inodes. Sun recommends that you have twice
the minimum (40 million inodes in this example). At the default 4 KB per inode, this
works out to only 160 GB of space for the MDT.
Conversely, if you have a very small average file size (4 KB for example), Lustre is
not very efficient. This is because you consume as much space on the MDT as on the
OSTs. This is not a very common configuration for Lustre.
21-4 Lustre 1.6 Operations Manual • September 2008