Re: Solaris ZFS on Linux [Was: Re: the " 'official' point of view"expressedby kernelnewbies.org regarding reiser4 inclusion]

[Avi Kivity]

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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