Log-structured file system: Difference between revisions

Revision as of 09:02, 8 May 2011

Revision as of 22:10, 9 June 2011

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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. However many flash based devices cancannot onlyrewrite writepart of a complete block at a time, and they must first perform a (slow) erase cycle of each block before being able to re-write, so by putting all the writes in one block, this can help performance as opposed to writes scattered into various blocks, each one of which must be copied into a buffer, erased, and written back.

== See also ==