[mythtv-users] NFS and remote backend

Jason spuppet at comcast.net
Mon Jan 14 18:14:25 UTC 2008

On Mon, 14 Jan 2008 12:49:12 -0500, Patrick Ouellette <> wrote:
>>>> Given that I'm seeing the stuttering without NFS, I'm not sure it's
> the
>>>> source of the problems.  Nor would I imagine moving to Samba/CIFS
> would
>>>> help.  Maybe it's some sort of file system problem on the backend
>>>> (locking,
>>>> etc?).  I'm using ext3 on the media partition.
>>>> Aside from doing the work to figure out that streaming from the
> backend
>>>> helped, I haven't done much to suss out the reason for this.  I
> accepted
>>>> the
>>>> situation it was in and the relevant requirement to hit pause at the
>>>> start
>>>> of live tv.
>>> Could this be a case of MySql not keeping up?  Database writes slowing
>>> down the availability of the inprogress recording?
>>> Pat
>> Hi Pat,
>> I have to admit I'm not familiar with how MySQL is used during
> recording.
>> What is being added to the database for an in-progress and how many
> inserts
>> are likely to happen per unit of time?
> It is my understanding that the database is updated with the location of
> each keyframe to enable seeking  (this is the seek table).  Selection of
> keyframes depends on the video content and encoding method, so the best
> answer I can give for number of inserts per unit time is "lots."

Ah.  That does sound like quite a bit.  :)

>> Aside from that, what's a simple way to benchmark the MySQL performance
> in
>> a way that would give a result that's relevant to Myth's usage of it?
>> There's probably a billion things I could test MySQL for, but for this
>> situation, only a couple performance indicators would matter.  My
>> expectation would be that it's not a hardware performance limit:  The
> disk
>> the mysql is on is fast and it's on the same machine as the backend.
> I was able to improve my system's performance by changing some MySql
> settings.  I don't have any benchmark results to back up this claim
> other than the video no longer stuttered when watching an in-progress
> recording (AMD64 3000+, nforce4 chipset, S/PDIF audio out, 512MB RAM, 2
> SATA 150 drives in a RAID 0 array) with the database on the same
> physical disk, but a different partition.
> Is the disk the database is on also one that you record to?  I've heard
> lots of people solved stuttering issues by moving the database to a
> disk that was not being used for recording.
> I don't have any benchmarks for MySql off hand, and my Myth system is
> currently down (failed motherboard).  I suspect Google may be your
> friend.
> Pat

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.


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