RE: ext3

[Kalum / Grendel <kalum delrom ro>]

On Mon, 30 Sep 2002,  Mark Knecht commented thusly,

> Grendel,
>    Thanks for the pointers. Both reiserfs and XFS look interesting to me,
> although there are two issues that I'm not clear about vs. the simple
> solution we have with ext2 - PCI bus overhead and latency.
>
>    If ext3, for instance, is just ext2 with journaling, then what percentage
> overhead do I have on the PCI bus running ext3? How many journal reads and
> writes take place vs. data reads and writes? Also, since the journaling is
> on the same drive, this causes the drive head to seek to some other
> location, which then means it has to seek back to get data. What sort of
> overhead does that entail?

Actually I dont know the answer to your questions. This is probably
answered by the file system designers themselves, as the issues are
complex.

What I can do tell you is that theorectically the perfect journalling file
system should be slower than the perfect non journalling file system
because of the overhead invovled in maintaining the journal as well as the
other issues you mention above.

But the problem is that ext2 (a non journaling system) is not well
designed, thus although reiserfs and XFS are both journalling systems they
both outperform ext2 (and thus ext3 too) despite beign burdened by having
to maintain a journal.

>    I know absolutely nothing about reiserfs or XFS, but I'd certainly want
> to better understand those issues. The 'large' file sizes on the page you
> pointed at below are really less than one second of audio at 96KHz. All
> indications are that these file systems would be an improvement, if they
> don't have some sort of latency issue. That can be tested using programs
> we're running on the low latency kernels.
>
>    Thanks for the pointers. I'm going to set up a partition of each type and
> try to check them all out over the next month or so.

Please do so, and tell of your results as it would be interesting. Both
reiserfs and XFS are very good file systems IMHO, and if you wish to know
further intricacies I suppose the authours of the file systems (Hans
reiser for reiserfs, and the linux XFS team for XFS) should be able to
help you.

Best Wishes,
Grendel


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