Lustre 1.6 Operations Manual

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On the journal:
ext3 journaling is asynchronous unless a handle specifically requests a
synchronous operation. Filesystem-modifying operations on the MDS that make
up a single file create operation are:
■ Allocate inode (inode bitmap, group descriptor, new inode)
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Create directory entry (directory block, parent inode for timestamps)
Update lov_objids file (Lustre file)
■ Update last_rcvd file (Lustre file)
For a single inode, each of the above items dirties a single block in the journal (7
blocks = 28 KB in total). When many new files are created at one time, dirty
blocks are merged in the journal, because each block needs to be dirtied only
once per transaction (5s or 1/4 of full journal, whichever occurs earlier). For 1,000
files created in a single directory, this works out to 516 KB, if they are all created
within the same transaction.
In 2.6 kernels it is possible to tune the ext3 journal commit interval with "-o
commit={seconds}". This may be desirable for performance testing.
ext3 code reserves a lot more blocks (about 70) for worst-case scenarios (e.g.,
growing a directory which also results in a split of the directory index, quota
updates, adding new indirect blocks for each of the Lustre files modified). These
are returned to the journal when the transaction is complete; most are returned
unused.
To avoid spurious journal commits due to these temporary reservations, calculate
the journal size based on this formula (assuming a default of 32 MDS threads):
70 blocks/thread * 32 threads * 4 KB/block * 4 = 35840 KB
Appendix D Lustre Knowledge Base D-35