Multiple skiplist bugs found, patches attached

[Bron Gondwana]

On Tue, 13 Nov 2007 09:12:18 +0100 (CET), "Simon Matter" <simon.matter at invoca.ch> said:
> > On Mon, Nov 12, 2007 at 12:34:34AM +1100, Bron Gondwana wrote:
> >> Anyway - here it is.  A "recovery()" that copes if the logstart
> >> parameter in the database header is wrong.  No, I don't have a
> >> clue how that happened unless lseek() lied.  Maybe it sometimes
> >> lies, I don't know.  I'll be writing a test case for that soon
> >> too!
> >
> > I have some more suspicions now, but I wrote it all up in the
> > patch header, so here's the bugfixes only patch, a "robustness"
> > extras patch and the tool I used for testing.
> >
> > Ken, I know you've done some other work on the file changing
> > types.  I'd like to be even more agressive and convert just
> > about everything to bit32 and also rename some variables, but
> > I restricted myself in this to only fixing the most ugly case:
> > offset = htonl(offset).
> >
> > These patches are all against 2.3.10 (in this order), and may
> > need some fuzz fixing to apply against your latest CVS thanks
> > to those changes - sorry I haven't done that, but it's getting
> > on 1am for me, and I've just finished doing a lot of testing
> > and paring these down to simple and clear patches that don't
> > touch more than they need to fix the issues.
> >
> 
> > cyrus-skiplist-bugfixes-2.3.10.diff:
> >
> 
> > cyrus-skiplist-robustify-2.3.10.diff:
> >
> 
> Hi Bron,
> 
> I didn't have much troubles with skiplist over the years and it has been
> a
> blessing since moving away from BDB. But I did have a few issues with
> broken skiplist files so your patches are very welcome. I have included
> the patches in my private rpm packages to try how they work. Do you
> recommend both for general consumption?

They've been running for 24 hours on all our production systems with
no ill effects :)

Seriously - yes, I do.  They are quite short, and they're the culmination
of about 3 days of pretty heavy work over the weekend and Monday after we
lost a mailboxes.db on our busiest store to one of these bugs (my wife and
kids were getting ready to kill me for neglecting them towards the end,
I'm sure!)  I build multiple different patches and tested them over that
time.  I also wrote a Perl module that can read skiplist files natively
and tested some things with that as well.

These couple of patches I have posted are the best bits of those distilled
down into the simplest and clearest small set of changes.  They've been hit
pretty hard with the hammer scripts.


I've also got another patch which I'll attach here that I wrote today which
re-tunes the "how often to checkpoint" calculation.  I want our mailboxes.db
files especially to checkpoint more frequently, as that will make them
less "seeky" - which will help with cachelines at least.  We have enough
memory (and always plenty free) that I'm sure every page is hot in cache
within a few minutes.

The seekyness is mainly an issue with clients doing "LIST", which our web
interface does at login, so we want it to be as quick as possible.

As for seen files - well, they tend to be small and frequently updated,
so they'll just checkpoint about 4 times as often now.  Will save a tiny
bit of disk space but more interestingly reduce the memory footprint to
keep them all in cache.


Bron.
-- 
  Bron Gondwana
  brong at fastmail.fm

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