Re: Is pg_control file crashsafe?

On Wed, May 4, 2016 at 4:02 PM, Alex Ignatov <a(dot)ignatov(at)postgrespro(dot)ru>
wrote:

>
>
> 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.

The actual data written is always sizeof(ControlFileData) which should be
less than one sector. I think it is only possible that we get a torn write
for pg_control, if while writing + fsyncing, the filesystem maps that data
to different sectors.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com