Re: Compressing WAL - Mailing list pgsql-performance

Added to TODO:

    * Compress WAL entries [wal]

I have also added this email to TODO.detail.

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Simon Riggs wrote:
> On Sun, 2005-04-10 at 21:12 -0400, Bruce Momjian wrote:
> > Jim C. Nasby wrote:
> > > Maybe better for -hackers, but here it goes anyway...
> > >
> > > Has anyone looked at compressing WAL's before writing to disk? On a
> > > system generating a lot of WAL it seems there might be some gains to be
> > > had WAL data could be compressed before going to disk, since today's
> > > machines are generally more I/O bound than CPU bound. And unlike the
> > > base tables, you generally don't need to read the WAL, so you don't
> > > really need to worry about not being able to quickly scan through the
> > > data without decompressing it.
> >
> > I have never heard anyone talk about it, but it seems useful.  I think
> > compressing the page images written on first page modification since
> > checkpoint would be a big win.
>
> Well it was discussed 2-3 years ago as part of the PITR preamble. You
> may be surprised to read that over...
>
> A summary of thoughts to date on this are:
>
> xlog.c XLogInsert places backup blocks into the wal buffers before
> insertion, so is the right place to do this. It would be possible to do
> this before any LWlocks are taken, so would not not necessarily impair
> scalability.
>
> Currently XLogInsert is a severe CPU bottleneck around the CRC
> calculation, as identified recently by Tom. Digging further, the code
> used seems to cause processor stalls on Intel CPUs, possibly responsible
> for much of the CPU time. Discussions to move to a 32-bit CRC would also
> be effected by this because of the byte-by-byte nature of the algorithm,
> whatever the length of the generating polynomial. PostgreSQL's CRC
> algorithm is the fastest BSD code available. Until improvement is made
> there, I would not investigate compression further. Some input from
> hardware tuning specialists is required...
>
> The current LZW compression code uses a 4096 byte lookback size, so that
> would need to be modified to extend across a whole block. An
> alternative, suggested originally by Tom and rediscovered by me because
> I just don't read everybody's fine words in history, is to simply take
> out the freespace in the middle of every heap block that consists of
> zeros.
>
> Any solution in this area must take into account the variability of the
> size of freespace in database blocks. Some databases have mostly full
> blocks, others vary. There would also be considerable variation in
> compressability of blocks, especially since some blocks (e.g. TOAST) are
> likely to already be compressed. There'd need to be some testing done to
> see exactly the point where the costs of compression produce realisable
> benefits.
>
> So any solution must be able to cope with both compressed blocks and
> non-compressed blocks. My current thinking is that this could be
> achieved by using the spare fourth bit of the BkpBlocks portion of the
> XLog structure, so that either all included BkpBlocks are compressed or
> none of them are, and hope that allows benefit to shine through. Not
> thought about heap/index issues.
>
> It is possible that an XLogWriter process could be used to assist in the
> CRC and compression calculations also, an a similar process used to
> assist decompression for recovery, in time.
>
> I regret I do not currently have time to pursue further.
>
> Best Regards, Simon Riggs
>

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