Lustre 1.6 Operations Manual

21.3.3 Overriding Default Formatting Options
To override the default formatting options for any of the Lustre backing filesystems,
use the --mkfsoptions='backing fs options' argument to mkfs.lustre, to
pass formatting options to the backing mkfs. For all options to format backing ext3
and ldiskfs filesystems, see the mke2fs(8) man page; this section only discusses
several Lustre-specific options.
21.3.3.1 Number of Inodes for MDT
To override the inode ratio, use the option -i
(for example) --mkfsoptions=-i 4096 to create 1 inode per 4096 bytes of
filesystem space).
Note – Use this ratio to make sure that Extended Attributes (EAs) can fit on the
inode as well. Otherwise, you have to make an indirect allocation to hold the EAs,
which impacts performance owing to the additional seeks.
Alternately, if you are specifying some absolute number of inodes, use the -
N option. To avoid unintentional mistakes, do not specify the
-i option with an inode ratio below one inode per 1024 bytes. Use the -N option
instead.
By default, a 2 TB MDT has 512M inodes. Currently, the largest supported filesystem
size is 8 TB, which holds 2B inodes. With an MDT inode ratio of 1024 bytes per
inode, a 2 TB MDT holds 2B inodes and a 4 TB MDT holds 4B inodes (the maximum
number of inodes currently supported by ext3).
21.3.3.2 Inode Size for MDT
Lustre uses "large" inodes on backing filesystems to efficiently store Lustre metadata
with each file. On the MDT, each inode is at least 512 bytes in size (by default), while
on the OST each inode is 256 bytes in size. Lustre (or more specifically the backing
ext3 filesystem), also needs sufficient space for other metadata, like the journal (up
to 400 MB), bitmaps and directories. There are also a few regular files that Lustre
uses to maintain cluster consistency.
To specify a larger inode size, use the -I option. We do NOT
recommend specifying a smaller-than-default inode size, as this can lead to serious
performance problems; you cannot change this parameter after formatting the
filesystem. The inode ratio must always be larger than the inode size.
Chapter 21 Lustre Tuning 21-5