Theodore Tso wrote:
Ah, but as soon as the repacker thread runs continuously, then you
lose all or most of the claimed advantage of "wandering logs".
Specifically, the claim of the "wandering log" is that you don't have
to write your data twice --- once to the log, and once to the final
location on disk (whereas with ext3 you end up having to do double
writes). But if the repacker is running continuously, you end up
doing double writes anyway, as the repacker moves things from a
location that is convenient for the log, to a location which is
efficient for reading. Worse yet, if the repacker is moving disk
blocks or objects which are no longer in cache, it may end up having
to read objects in before writing them to a final location on disk.
So instead of a write-write overhead, you end up with a
write-read-write overhead.
There's no reason to repack *all* of the data. Many workloads write and
delete whole files, so file data should be contiguous. The repacker
would only need to move metadata and small files.
--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.
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