On 01.05.2016 0:55, Bruce Momjian wrote:
On Thu, Apr 28, 2016 at 09:58:00PM +0000, Alex Ignatov wrote:
Hello everyone!
We have some issue with truncated pg_control file on Windows after power
My questions is :
1) Is pg_control protected from say , power crash or partial write?
2) How PG update pg_control? By writing in it or writing in some temp file and
after that rename it to pg_control to be atomic?
We write pg_controldata in one write() OS call:

     if (write(fd, buffer, PG_CONTROL_SIZE) != PG_CONTROL_SIZE)

3) Can PG have  multiple pg_control copy to be more fault tolerant?

PS During some experiments we found that at present time there is no any method
to do crash recovery with "restored" version of pg_control (based on some
manipulations with pg_resetxlog ).
  Only by using pg_resetxlog and setting it parameters to values taken from wal
file (pg_xlogdump)we can at least start PG and saw that PG state is at the
moment of last check point. But we have no real confidence that PG is in
consistent state(also docs on pg_resetxlogs told us about it too)
We have talked about improving the reliability of pg_control, but
failures are so rare we have never done anything to improve it.  I know
Tatsuo has talked about making pg_control more reliable, so I am CC'ing

Oh! Good. Thank you!
It is rare but as we saw now it is our reality too. One of our customers had this issue on previous week =)

I think that rename can help a little bit. At least on some FS it is atomic operation.

Alex Ignatov
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company

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