KernelTrap - LogFS

LogFS, A Scalable Flash Filesystem

Jeremy — Mon, 07 Apr 2008 22:13:09 +0000

Jörn Engel posted the sixth version of patches introducing his new LogFS filesystem for flash devices to the Linux kernel. He highlighted some areas of the code that need some more work, and cc'd the appropriate people for further review. Regarding LogFS itself, he noted that one of its big advantages compared to other solutions was improved mount time and reduced memory consumption compared to other solutions, "LogFS has an on-medium tree, fairly similar to Ext2 in structure, so mount times are O(1)." He went on to add that flash is becoming more and more common in standard PC hardware, explaining:

"Flash behaves significantly different to hard disks. In order to use flash, the current standard practice is to add an emulation layer and an old-fashioned hard disk filesystem. As can be expected, this is eating up some of the benefits flash can offer over hard disks. In principle it is possible to achieve better performance with a flash filesystem than with the current emulated approach. In practice our current flash filesystems are not even near that theoretical goal. LogFS in its current state is already closer."

Comparing UBIFS And LogFS

Jeremy — Mon, 31 Mar 2008 15:00:26 +0000

Following the recent announcement that UBIFS is nearly production ready, it was asked how UBIFS compares to LogFS. LogFS author Jörn Engel suggested, "both share similar design goals. Biggest difference is that ubifs works on top of ubi and depends on ubi support, while logfs works on plain mtd (or block devices) and does everything itself. Code size difference is huge. Ubi weighs some 11kloc, ubifs some 30, logfs some 8." He continued:

"Ubi scales linearly, as it does a large scan at init time. It is still reasonably fast, as it reads just a few bytes worth of header per block. Logfs mounts in O(1) but will currently become mindbogglingly slow when the filesystem nears 100% full and write are purely random. Not that any other flash filesystem would perform well under these conditions - it is the known worst case scenario."

Artem Bityutskiy replied, "I personally refuse to compare a finished FS with handles all the crucial flash features to a non-finished FS. It just makes no sense. LogFS was talked about back 2005 in Linux Kongress, but is not finished yet. Let's talk about it when it is production ready."