On 2014-08-18 13:06:15 -0400, Robert Haas wrote: > On Mon, Aug 18, 2014 at 7:19 AM, Rahila Syed <rahilasye...@gmail.com> wrote: > >>According to the measurement result, the amount of WAL generated in > >>"Multiple Blocks in one run" than that in "Single Block in one run". > >>So ISTM that compression of multiple blocks at one run can improve > >>the compression ratio. Am I missing something? > > > > Sorry for using unclear terminology. WAL generated here means WAL that gets > > generated in each run without compression. > > So, the value WAL generated in the above measurement is uncompressed WAL > > generated to be specific. > > uncompressed WAL = compressed WAL + Bytes saved. > > > > Here, the measurements are done for a constant amount of time rather than > > fixed number of transactions. Hence amount of WAL generated does not > > correspond to compression ratios of each algo. Hence have calculated bytes > > saved in order to get accurate idea of the amount of compression in each > > scenario and for various algorithms. > > > > Compression ratio i.e Uncompressed WAL/compressed WAL in each of the above > > scenarios are as follows: > > > > Compression algo Multiple Blocks in one run Single Block in one run > > > > LZ4 1.21 1.27 > > > > Snappy 1.19 1.25 > > > > pglz 1.14 1.16 > > > > This shows compression ratios of both the scenarios Multiple blocks and > > single block are nearly same for this benchmark. > > I don't agree with that conclusion. The difference between 1.21 and > 1.27, or between 1.19 and 1.25, is quite significant. Even the > difference beyond 1.14 and 1.16 is not trivial. We should try to get > the larger benefit, if it is possible to do so without an unreasonable > effort.
Agreed. One more question: Do I see it right that multiple blocks compressed together compress *worse* than compressing individual blocks? If so, I have a rather hard time believing that the patch is sane. Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
