[mythtv-users] NFS and remote backend

Jason spuppet at comcast.net
Tue Jan 22 15:58:21 UTC 2008

> My MySQL datastore is on a different physical disk / controller
> (/ possibly even bus) than the RAID array used for storage.
> I've previously added some settings to mysql to improve the read
> performance (query cache, etc), but nothing for writes.
> I'll Google around for some things related to improving write
> performance in MySQL.  Given that your system is down, if you have
> any recollection regarding what the settings were called, I would
> appreciate it.  Even a few keywords to look for might help guide
> my success with Google.

I've made some progress here.  I ended up making some changes to
the MySQL configuration that didn't seem to help at all.  For the
record, there don't seem to be a whole lot of write-focused
settings in MySQL that I could dig up.  Most seemed to be focused
on read performance settings.

After coming up empty there, I decided that maybe the issue was
with writing to the disk at the same time another thread wanted
to read that data.  That made sense in my mind because the
stuttering was worst when watching at real time with the
recording.  Stuttering was almost non-existant when watching 10
or more seconds behind real time.

Since I've got a 3Ware RAID controller driving my array, I found
the following knowledgebase entry for optimizing performance in a
Linux 2.6 kernel:


Based on that info, I set max_sectors_kb to 64, nr_requests to
512, the deadline elevator, dirty_background_ratio to 20 and
dirty_ratio to 60.

Once I had done that, I tried out live TV on a selection of channels:

    No stutter.

Well, almost.  I get occasional stutter, but it's entirely
tolerable now.

I did notice, though, that in the backend logs that I was seeing the "TFW,
Error: Write -- IOBOUND" errors indicating buffers in the backend
were filling up.

After some investigation of this problem (see [1] and [2] below),
I decided that I had the right IO scheduler (deadline) since
reports indicated that CFQ didn't improve things much.  (I'd
rather not go against the recommendation from 3Ware.)

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