Re: Page Checksums + Double Writes

Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> An obvious problem is that, if the abort rate is significantly
>> different from zero, and especially if the aborts are randomly
>> mixed in with commits rather than clustered together in small
>> portions of the XID space, the CLOG rollup data would become
>> useless.
> 
> Yeah, I'm afraid that with N large enough to provide useful
> acceleration, the cases where you'd actually get a win would be
> too thin on the ground to make it worth the trouble.
 
Just to get a real-life data point, I check the pg_clog directory
for Milwaukee County Circuit Courts.  They have about 300 OLTP
users, plus replication feeds to the central servers.  Looking at
the now-present files, there are 19,104 blocks of 256 bytes (which
should support N of 1024, per Robert's example).  Of those, 12,644
(just over 66%) contain 256 bytes of hex 55.
 
"Last modified" dates on the files go back to the 4th of October, so
this represents roughly three months worth of real-life
transactions.
 
-Kevin