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Re: Does myisampack affect the query cache?
Posted by: Philip Smolen
Date: September 06, 2010 04:26PM

Rick James Wrote:
-------------------------------------------------------
> I would expect the QC to be _temporarily_ flushed.
> Strange that it seems to be permanent.
It's not clearing the cache. I could handle that. It's refusing to use the cache for certain tables. Other tables work just fine.

>
> After stopping and restarting mysqld, does the QC
> start up again?
No change. I just did it again to be sure. It still will not cache any queries on the tables I described previously.

>
> How often do you need to run myisampack? Maybe
> too often?
Completely irrelevant:
a) This is not a temporary problem.
b) Routine maintenance is always scheduled for appropriate times.
c) The original plan was to do this on a different machine and then copy to to several machines after it was done. (To help track down this problem, I got rid of that step.)

I just now ran some additional tests and found something interesting: I copied these files to a different server. The query cache works as expected on the other server. The server that works has a slightly older version of mysql:
mysql --version
mysql Ver 14.12 Distrib 5.0.22, for redhat-linux-gnu (x86_64) using readline 5.0

These machines are using almost identical versions of my.cnf. If there are any relevant settings, I can look for those. I wasn't aware of any settings which could avoid specific tables, while working on others.

Just to be thorough, I ran some more tests. Using cp to duplicate the tables on the original server did not change anything. The query cache would not work any better with the copies of the tables than with the originals. I checked the log file on this server and did not see anything interesting.

Next I tried using myisampack on the second server, the one with slightly older software which doesn't have any trouble with its query cache. That also worked as it should.

Finally, I copied the newly packed files from the older server to the original server. I gave the table a new name to make sure there was no confusion. The original server refused to cache that table, too.

So it's doesn't matter where the file is packed. It always fails on the 5.0.51a server and always works on the 5.0.22 server.

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