On 12/12/2015 11:20 PM, Andres Freund wrote:

On 2015-12-12 22:14:13 +0100, Tomas Vondra wrote:
this is the second improvement proposed in the thread [1] about ext4 data
loss issue. It adds another field to control file, tracking the last known
WAL segment. This does not eliminate the data loss, just the silent part of
it when the last segment gets lost (due to forgetting the rename, deleting
it by mistake or whatever). The patch makes sure the cluster refuses to
start if that happens.

Uh, that's fairly expensive. In many cases it'll significantly
increase the number of fsyncs.

It should do exactly 1 additional fsync per WAL segment. Or do you think otherwise?

> I've a bit of a hard time believing this'll be worthwhile.

The trouble is protections like this only seem worthwhile after the fact, when something happens. I think it's reasonable protection against issues similar to the one I reported ~2 weeks ago. YMMV.

> Additionally this doesn't seem to take WAL replay into account?

I think the comparison in StartupXLOG needs to be less strict, to allow cases when we actually replay more WAL segments. Is that what you mean?


Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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