Hi,

On one of our test boxen here, weve experienced a corrupted file during
database recovery after box power outage. The specific error message is

PANIC: invalid page header in block 6 of relation "17792"

At this point I fired up a hex dumper to inspect the file, and the last
block in the file (which that error refers to) was clearly garbage

This was on postgres 7.4. The system in question is using ReiserFS, and some
journal transactions were replayed on the same boot as the failed postgres
recovery. I beleive this is significant (see below)

By using postgres single-user database server and zero_damaged_pages option
I manged to get the database up again. There were a LOT of relations with
this problem !

It may be significant that this is an index (primary key) for a relation.
ALL of the files with problems were either indexes or primary keys!

I do NOT believe this was a hardware error. What I think happened is:
- postgres extended some indexes
- reiserfs journalled the metadata
- new file contents got buffered by the kernel in memory
- XLog stuff gets fsync()'d
- Power cycle
- reiserfs replayed metadata journal, extended the files
Probably makes the last blocks in each file invalid!
- postgres attempts to recover from its log, and bumps into the (now
garbage) blocks

I'll see if I can get some time to reproduce this reliably

Guy Thornley

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Guy Thornley <guy (AT) esphion (DOT) com> writes:
Hmmm ... that is mildly interesting, but it doesn't rise to the level of
warning bells in my head. At least not yet. Were the indexes involved
all on the same table, or different tables? If the former, it could
just be that that was the last set of changes to be flushed out after an
update of that table. If they were on different tables then it's a more
surprising coincidence. Could happen anyway I suppose --- index pages
are likely to be more heavily accessed than heap pages, and thus less
likely to get flushed out of the buffer cache.

regards, tom lane

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I played around a bit yesterday with an INSERT'ing shell script and a reset
button... I can now, with reasonable confidence, say was pure coincidence
they were all index files. I had the junk pages in normal heap files as well
as index files on several occasions while testing


Guy

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