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Improving MetaData Performance of the Ext4 Journaling Device

Dec 30, 2009, 15:32 (0 Talkback[s])
(Other stories by Jeffrey B. Layton)

"There is always a relentless pursuit of more performance from our storage systems. This includes more performance from hardware (faster disks, SSD's), network (bigger pipes, larger MTU's), operating systems (caching, IO schedulers), and file systems. There are many levers than can be moved to improve performance but this article will look at one particular piece - the file system journal device. In particular, the metadata performance of ext4 will be considered as the journal is moved to different devices.

"Journaling for File Systems

"Sometimes bad things such as power failures happen to systems. Power interruptions or failures can cause a file system to become corrupt very quickly because an IO operation is interrupted and not completed. Consequently, the file system has to be checked (fsck) which means the entire file system has to be checked (walked) to find and correct any problems. As file systems grew the amount of time it takes to walk the file system greatly increased. For example, the author remembers performing an fsck on a 1TB file system in 2002-2003 that took several days. Having the system down for this amount of time is very painful.

"One way to help improve fsck times is to use a journaled file system. Rather than IO operations happening directly to the file system, the operations are added to the journal (typically a log) in the order they are supposed to happen. Then the file system grabs the operation from the head of the journal and completes it, erasing the operation from the journal only after the operation is finished and the file system is satisfied that the operation is complete."

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