Re: WAL Re-Writes

On Mon, Feb 8, 2016 at 8:16 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> On 2016-02-08 10:38:55 +0530, Amit Kapila wrote:
> > I think deciding it automatically without user require to configure it,
> > certainly has merits, but what about some cases where user can get
> > benefits by configuring themselves like the cases where we use
> > PG_O_DIRECT flag for WAL (with o_direct, it will by bypass OS
> > buffers and won't cause misaligned writes even for smaller chunk sizes
> > like 512 bytes or so). Some googling [1] reveals that other databases
> > also provides user with option to configure wal block/chunk size (as
> > BLOCKSIZE), although they seem to decide chunk size based on
> > disk-sector size.
>
> FWIW, you usually can't do that small writes with O_DIRECT. Usually it
> has to be 4KB (pagesize) sized, aligned (4kb again) writes. And on
> filesystems that do support doing such writes, they essentially fall
> back to doing buffered IO.
>

I have not observed this during the tests (observation is based on the
fact that whenever there is a use of OS buffer cache, writing in smaller
chunks (lesser than 4K) leads to reads and in-turn decrease the
performance). I don't see such an implication even in documentation.

> > An additional thought, which is not necessarily related to this patch
is,
> > if user chooses and or we decide to write in 512 bytes sized chunks,
> > which is usually a disk sector size, then can't we think of avoiding
> > CRC for each record for such cases, because each WAL write in
> > it-self will be atomic. While reading, if we process in wal-chunk-sized
> > units, then I think it should be possible to detect end-of-wal based
> > on data read.
>
> O_DIRECT doesn't give any useful guarantees to do something like the
> above. It doesn't have any ordering or durability implications. You
> still need to do fdatasyncs and such.
>

It doesn't need to, if we use o_sync flag which we always use whenever
we use O_DIRECT mode during WAL writes.

> Besides, with the new CRC implications, that doesn't really seem like
> such a large win anyway.
>

I haven't check this till now that how much big win we can get if we
can avoid CRC's and still provide same reliability, but I think it can
certainly save CPU instructions both during writes and replay and
performance must be better than current.

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