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SubjectRe: Soft-Updates for Linux ?
Date
Robert Redelmeier wrote:
> I'm wondering if there are any plans to bring Kirk McKusick's "Soft-Updates"
> to Linux in 2.5 ??? Kirk has recently modified his licence (friendlier).
> I could only find some noncomittal postings on LKML from 1997.
>
> S-U brings considerable benefits akin to JFS for crash protection to *BSD
> systems. Essentially, they are ordered disk writes that makes sure data
> gets on the disk before metadata is altered. They go through the buffering
> system, so performance isn't bad.

I analyzed the algorithm and concluded that it would indeed improve
filesystem robustness, but I'm not interested in an almost-reliable
solution if I can have a 100% reliable solution instead. That's just me
of course. Personally I'll use Ext3 (full-data version) for the next
phase of Tux2 development and I guess I will join Stephen's testing
corps because I tend to hit the reset button *a lot*. I don't care
about efficiency for this, I just care about losing my last edit, or
worse.

To be fair, when Soft Updates is working perfectly you will move from a
situation where you are constantly at risk of catstrophic filesystem
damage to one where you will just be losing track of some free blocks,
have some file lengths wrong, and some partly completed writes. That's
a huge improvement and I'd strongly recommend that anybody who didn't
have anything better should use it.

One thing to keep in mind in all of this is: nobody is testing the
reliability of their journalling or any other kind of filesystem just by
running it. To test these things you have to crash/interrupt the system
*a lot*. Otherwise, you are just fooling yourself and everybody else.
How many crashes does it take to find that one little window of
vulnerability that comes up every 10,000 crashes normally but suddenly
starts coming up every time just because your customer uses their system
a different way? You're doing the right thing by crash-testing it, now
instead of doing it 5 times do it 1,000 times. Here's one of my
favorite tests: unzip a kernel source tree and wait until the disk light
goes out. A second or so after it comes on again (kflushd) hit the
reset button.

[...]
> You boys and girls don't try this at home on Linux! The ext2 fsck is horrible
> after a powerfail, and I've lost superblocks and had to re-install :( .

I've never had it that bad but I've been dropped to single user shell
plenty of times and lost a few files. However I don't think it's fair
to compare Soft Updates to raw Ext2. One of them is trying to be a
failsafe filesystem and the other is just hoping to hell the power
doesn't fail.

--
Daniel
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