[mythtv-users] SUPER slow I/O on storage drive

[Christopher Meredith]

On Tue, Nov 9, 2010 at 9:40 AM, Travis Tabbal <travis at tabbal.net> wrote:

>
>
> On Tue, Nov 9, 2010 at 8:36 AM, Christopher Meredith <chmeredith at gmail.com
> > wrote:
>
>>
>> So the filesystem isn't the problem and it looks like the drive isn't the
>> problem. I can't imagine the software would be the problem because VLC,
>> XBMC, Xine, and mplayer all have buffering problems while playing my these
>> recordings also. And some of the recordings are ATSC, some are from the
>> HD-PVR. These are just data dumps from the devices; there's no processing by
>> the backend.
>>
>> I've got to be missing something. What should I check next?
>>
>
>
> Controller port? How is the speed on the local machine with a disk
> benchmark? What else is running on that machine and drive?  If the local
> speed test is OK, how about a network speed test without the drive?
>

The only disk benchmark I've run is 'hdparm -Tt /dev/sdb', from which I get
these results:

/dev/sdb:
 Timing cached reads:   2620 MB in  2.00 seconds = 1309.43 MB/sec
 Timing buffered disk reads:  182 MB in  3.01 seconds =  60.41 MB/sec

This is comparable to other disks on the system that are working fine. The
drive in question is a dedicated DVR recordings drive. It's a 160GB SATA
drive with one big ext4 partition.

This is a combined FE/BE (using VDPAU) so the network isn't the problem. I
have made sure there aren't any CPU-intensive actions occurring, such as
commercial flagging. During the times that the buffering problems persist,
the most intensive process is mythfrontend and it never goes about 10% CUP
usage.

I don't know if this is relevant or related, but a few months back, shortly
after a trunk upgrade, I noticed that LiveTV would occasionally suffer an
extremely brief pause. This would happen every few seconds or so. I found
that pressing pause until there was about a 10-second buffer would fix the
problem. Thinking back on it, I'm not sure there was a good reason I should
have had to do that, and now I'm wondering whether it's related to this
problem. The behavior I'm seeing now is like that brief pause, only longer
and more often.
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