Re: Hard limit on WAL space used (because PANIC sucks)

On 2013-06-06 23:28:19 +0200, Christian Ullrich wrote:
> * Heikki Linnakangas wrote:
>
> >The current situation is that if you run out of disk space while writing
> >WAL, you get a PANIC, and the server shuts down. That's awful. We can
>
> >So we need to somehow stop new WAL insertions from happening, before
> >it's too late.
>
> >A naive idea is to check if there's enough preallocated WAL space, just
> >before inserting the WAL record. However, it's too late to check that in
>
> There is a database engine, Microsoft's "Jet Blue" aka the Extensible
> Storage Engine, that just keeps some preallocated log files around,
> specifically so it can get consistent and halt cleanly if it runs out of
> disk space.
>
> In other words, the idea is not to check over and over again that there is
> enough already-reserved WAL space, but to make sure there always is by
> having a preallocated segment that is never used outside a disk space
> emergency.

That's not a bad technique. I wonder how reliable it would be in
postgres. Do all filesystems allow a rename() to succeed if there isn't
actually any space left? E.g. on btrfs I wouldn't be sure. We need to
rename because WAL files need to be named after the LSN timelineid...

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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