[mythtv-users] 0.27: Stalls and other niggles

Michael T. Dean mtdean at thirdcontact.com
Thu Jan 16 18:27:00 UTC 2014


On 01/16/2014 11:32 AM, Mike Thomas wrote:
> What I have noticed is mysqld almost always seems to be the process
> doing the most I/O at the moment of the stall, but is rarely doing any
> I/O at other times. It is almost as if mysqld is batching updates to
> its tables and logs.

Sounds like you're storing your MySQL data on a file system with 
barriers enabled, so mysqld writes data to disk and the file system 
barriers cause it to block until all data is physically written to disk 
platters (not just to disk cache)--which takes a long time.  During this 
time, MythTV (and everything else) is unable to access required data in 
MySQL.

The proper fix is /not/ just disabling barriers, instead making it so 
that data can be written safely--using barriers--fast enough to prevent 
impacts to system services.  There are many things you can do to fix 
this, each with its own advantages/disadvantages (and costs), but before 
going there, I'd recommend just remounting your MySQL-data-containing 
file system with barriers disabled to see if this is, in fact, the 
problem you need to fix.  If so, then come back to find the best fix for 
you.

Mike


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