Re: Is pg_control file crashsafe?

On 03.05.2016 2:17, Tom Lane wrote:
> Alex Ignatov <a(dot)ignatov(at)postgrespro(dot)ru> writes:
>> I think that rename can help a little bit. At least on some FS it is
>> atomic operation.
>
> Writing a single sector ought to be atomic too. I'm very skeptical that
> it'll be an improvement to just move the risk from one filesystem
> operation to another; especially not to one where there's not even a
> terribly portable way to request fsync.
>
> regards, tom lane
>
>
pg_control is 8k long(i think it is legth of one page in default PG
compile settings).
I also think that 8k recording can be atomic. Even if recording of one
sector is atomic nobody can say about what sector from 8k record of
pg_control should be written first. It can be last sector or say sector
number 10 from 16. That why i mentioned renaming from tmp file to
pg_control. Renaming in FS usually is atomic operation. And after power
loss we have either old version of pg_control or new version of it. But
not torn pg_control file.

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