On Wed, May 4, 2016 at 4:02 PM, Alex Ignatov <a.igna...@postgrespro.ru>

> On 03.05.2016 2:17, Tom Lane wrote:
>> Alex Ignatov <a.igna...@postgrespro.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

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