Log-structured file system: Difference between revisions

Revision as of 18:02, 19 June 2009

Revision as of 19:25, 20 January 2010

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* The [[design rationale]] for log-structured file systems assumes that most reads will be optimized away by ever-enlarging memory caches. This assumption does not always hold:

** On magnetic media—where seeks are relatively expensive—the log structure may actually make reads much slower, since it [[fragmentation (computer)#External_fragmentation|fragments]] files that conventional file systems normally keep contiguous with in-place writes.

** On flash memory—where seek times are usually negligible—the log structure may not confer a worthwhile performance gain because write fragmentation has much less of an impact on write throughput{{Fact|date=November 2008}}. However many flash based devices can only write a complete block at a time, becauseand they must first perform a (slow) erase cycle of each block before being able to write, so by putting all the writes in one block, this can help performance vsas opposed to writes scattered into various blocks, each one of which must be copied into a buffer, erased, and written back.

== References ==