[mythtv-users] Slow video on XFS raid 0, ok video on ext3 on single drive.

[devsk]

Here is what you need to try: Slowly up (negative higher, 'man renice') the nice value of mythbackend when watching livetv. Stop doing that when the artifacts go away. Record that value and put it in a boot script.

the reasoning: we did a lot of experiments to see how the I/O latency changes with priority of the process. As we increased the priority, the IO latency went down but thruput (number of IOs per sec) also went down. This meant that the kernel was making sure that the higher priority tasks got their IO completed quickly but in smaller chunks. This effect is more pronounced with RAID0 because your latency is the worst case latency of all the drives in the raid. And a 'good citizen' nice value doesn't help backend.

One more thing to try is to remake the raid on hde-hdf or hdf-hdg and see if that
helps (particularly, you have a lot of disks, so backing up and remaking should be no problem). It is possible that one of hde or hdg is dying and suffering huge
latencies. Your PCI card could be a culprit too.

Regardless of whether the renice works or not, get yourself a better PCI card and move to SATA if you've got the dough.

-devsk


----- Original Message ----
From: Listbox <listbox at hymerfania.com>
To: Discussion about mythtv <mythtv-users at mythtv.org>
Sent: Saturday, December 30, 2006 11:04:07 PM
Subject: Re: [mythtv-users] Slow video on XFS raid 0, ok video on ext3 on single drive.

The RAID devices are hde and hdg, the/ partition is hdc.... to me this looks
like all my physical drives give comparable throughput.

/dev/hda:
 Timing cached reads:   1668 MB in  2.00 seconds = 834.10 MB/sec
 Timing buffered disk reads:   88 MB in  3.05 seconds =  28.87 MB/sec

/dev/hdb:
 Timing buffered disk reads:  
/dev/hdc:
 Timing cached reads:   1696 MB in  2.00 seconds = 847.36 MB/sec
 Timing buffered disk reads:   88 MB in  3.05 seconds =  28.82 MB/sec

/dev/hde:
 Timing cached reads:   1680 MB in  2.00 seconds = 838.49 MB/sec
 Timing buffered disk reads:  168 MB in  3.03 seconds =  55.46 MB/sec

/dev/hdf:
 Timing cached reads:   1688 MB in  2.00 seconds = 843.57 MB/sec
 Timing buffered disk reads:  164 MB in  3.00 seconds =  54.59 MB/sec

/dev/hdg:
 Timing cached reads:   1696 MB in  2.00 seconds = 846.68 MB/sec
 Timing buffered disk reads:  162 MB in  3.03 seconds =  53.45 MB/sec

So, given the same hardware throughput, I would expect better performance
with the XFS+RAID setup. This is not the case.
My adapter is a:
RAID bus controller: Silicon Image, Inc. PCI0680 Ultra ATA-133 Host
Controller (rev 02)
I also tried upping the PCI latency of the RAID card with 
        setpci -v -s 01:07.0 latency_timer=B0 # PCI0680 Ultra ATA-133 Host
But this had no effect. The DVB card has a latency of 32(decimal).

I just reformatted with ext3, and tried again, and got the same degradation.
It's looking like the RAID is the problem, but I do not,NOT,NOT! want to
un-stripe the disks. 

What else to try?

Listbox

-----Original Message-----
From: Andrew Lyon [mailto:andrew.lyon at gmail.com] 
Sent: Saturday, December 30, 2006 11:31 AM
To: Discussion about mythtv
Subject: Re: [mythtv-users] Slow video on XFS raid 0,ok video on ext3 on
single drive.

On 12/30/06, Listbox <listbox at hymerfania.com> wrote:
> Hi folks!
>
> I have two Seagate 160gb drives in a RAID 0 array on an SiI0680 PCI 
> ATA raid controller. I formatted an XFS filesystem on a single 
> partition, and set mythbackend to use it. My DVB card is a DviCO 
> Fusion Gold 3,( Conexant CX23880).
>
> When I watch live TV, I get terrible artifacts on digital TV, and slow 
> frame rates on analog. When I reset mythbackend to use a ext3 
> partition on a single drive, performance is acceptable.
>
> I went to considerable trouble to get the RAID and XFS working on my 
> Fedora
> 5 system, and it's pretty discouraging to see that it actually 
> degrades instead of enhances performance. What is likely the problem? 
> Is it the controller, the raid or the filesystem?
>
> Thanks for any advice!
>
> Listbox
>
> _______________________________________________
> mythtv-users mailing list
> mythtv-users at mythtv.org
> http://mythtv.org/cgi-bin/mailman/listinfo/mythtv-users
>

Please use hdparm -tT to benchmark the speed of your drives, and your raid
array, e.g.

hdparm -tT /dev/sda /dev/sdb /dev/sdc /dev/md0

You should do this when the system is idle, perhaps even in single user
mode.

That should shed some light on the situation.

Andy
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