Why is launchd(8) writing to MySQL data files?

I'm running a query in MySQL that's updating a table with about 20 M rows. While I watch the system with iosnoop, fs_usage, top and I see that launchd is writing to the table's MyISAM data file as well as mysqld.

According to top, CPU activity alternates between launchd and mysqld. When launchd开发者_开发问答 is active, mysqld is relatively inactive visa versa. And these periods of activity correspond to the launchd and mysqld write access to the MyISAM data file as reported by iosnoop and top.

This strikes me as very odd. Any idea why launchd is involved in writing MySQL data files?

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ADDITIONAL INFO

If I stop mysqld with launchctl unload and start it again without using launchd, i.e. with mysqld_safe & I see the same weird behavior reported in iosnoop, fs_usage, and top.

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It probably has something to do with launchd being mysqld's parent process. I don't think launched itself will write to MyIsam tables.