[mythtv-users] Freezing/Pixelating

[Mike Thomas]

On Tue, 22 Mar 2016 10:18:54 -0700
Gabriel Kuri <gkuri at ieee.org> wrote:
> I doubt it's a RAID 5 performance issue. The performance of the disk
> array for both reads and writes is pretty good. Write performance is
> around 320Mbps ...
> 
> # dd if=/dev/zero of=/export/test2.img bs=1G count=1 oflag=dsync
> 1+0 records in
> 1+0 records out
> 1073741824 bytes (1.1 GB) copied, 22.4376 s, 47.9 MB/s
> 
> and read performance is almost 800Mbps ...
> 
> # dd if=/export/record/1752_20160312073000.mpg of=/dev/null bs=8k
> 2027724+1 records in
> 2027724+1 records out
> 16611117128 bytes (17 GB) copied, 171.084 s, 97.1 MB/s
> 
> 
> So back to my original question, are those RingBuf errors indicating
> that the myth backend stopped receiving data from the HD Homerun
> Prime or the myth backend was unable to read data from the file and
> stream it to the frontend?

Dear Gabriel,

I am not sure I understand the numbers you quote above.

You show a dd to disc achieving 48MBps but the read test shows only
97MBps. Where does 800 mega bits per second come into it?

Incidentally, the write test is unrealistic. mythbackend does not queue
up 1GB data chunks in memory before flushing to disc. I don't know what
the block size is, but I would hazard a guess of a few megabytes (a
third perhaps of the size of the ring buffer). Moreover whilst
mythbackend flushes video data to disc it synchronously inserts rows
onto the end of the recordedseek table. If this is on the same disc
(you don't say) it could have a significant impact upon write
performance. Even if recordedseek is on another disc it will still have
an impact, unless it is on SSD.

A good way to assess the performance of a disc system is to use a
standard test. Many people use bonnie++. That said, the numbers you
quote don't seem unreasonable.

Here's the same unrealistic test of performance on my old myth system
(5 year old 2GHz Celeron with Seagate 2TB drive of the same vintage):

# dd if=/dev/zero of=/export/home52/foo bs=1G count=1 oflag=dsync
1+0 records in
1+0 records out
1073741824 bytes (1.1 GB) copied, 15.9102 s, 67.5 MB/s
#

It's not great, but it does have two layers of lvm and cryptography to
deal with. It can record 4 channels and play on 3 screens
simultaneously without judders. The o/s and database are on the same
disc.

Mike.