[mythtv-users] Freezing/Pixelating

[Mike Thomas]

On Mon, 21 Mar 2016 16:45:06 -0700
Gabriel Kuri <gkuri at ieee.org> wrote:
> While watching TV, the video on the remote frontend sometimes freezes
> or pixelates. This last time, I had to exit back to the main menu and
> go back in to the channel to continue watching because it just froze
> for way too long. Later, I looked at the logs on the backend and
> noticed this in the log about the same time it froze ...
> [snip]
> The video is on a RAID 5 array that is healthy and the SMART status
> of all 4 drives is good. So I don't believe I have a failing drive.

Dear Gabriel,

I will let others remark on the specific messages but I will respond to
the RAID 5 point.

RAID 5 is the slowest form of RAID for writes. To update a single disc
block (when it's not in cache) on a four disc RAID 5 array involves
four disc reads, a computation which can only start after all four
reads have completed, and then two disc writes. The writes will have
missed their time slots and thus must wait for the platters to rotate
into position.

Although single block I/O is not the bulk of what's going on on a myth
system (video files are big), inode updates are, and they are updated
very frequently.

As to a failed disc in a raid array, no you don't have one. You would
*really* know about it then, believe me.

The message from me is if you want to use RAID for data security, use
RAID 1. RAID 5 is fine for archive storage but not for active
filesystems. *If* you have a performance problem and *if* someone thinks
it is disc-related, I would point the finger at that. This comes up on
the list from time to time.

Mike.