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

From: "Josh Berkus" <josh(at)agliodbs(dot)com>
> There's actually three potential failure cases here:
>
> - One Volume: WAL is on the same volume as PGDATA, and that volume is
> completely out of space.
>
> - XLog Partition: WAL is on its own partition/volume, and fills it up.
>
> - Archiving: archiving is failing or too slow, causing the disk to fill
> up with waiting log segments.

I think there is one more case. Is this correct?

- Failure of a disk containing data directory or tablespace
If checkpoint can't write buffers to disk because of disk failure,
checkpoint cannot complete, thus WAL files accumulate in pg_xlog/.
This means that one disk failure will lead to postgres shutdown.

> xLog Partition
> --------------
>
> As Heikki pointed, out, a full dedicated WAL drive is hard to fix once
> it gets full, since there's nothing you can safely delete to clear
> space, even enough for a checkpoint record.

This sounds very scary. Is it possible to complete recovery and start up
postmaster with either or both of the following modifications?

[Idea 1]
During recovery, force archiving a WAL file and delete/recycle it in
pg_xlog/ as soon as its contents are applied.

[Idea 2]
During recovery, when disk full is encountered at end-of-recovery
checkpoint, force archiving all unarchived WAL files and delete/recycle them
at that time.

Regards
MauMau