[mythtv-users] recording drive bottle neck suggestions..

[Mark Knecht]

On Mon, Aug 31, 2009 at 7:13 AM,
sonofzev at iinet.net.au<sonofzev at iinet.net.au> wrote:
> Hi All
>
> On Sunday night I ended up having a bottle neck as it was the first time I ever
> had 5 recordings going at one (1 HD 4 SD)... the system choked and I couldn't
> watch as the recordings were going (although it appears the recordings ended up
> fine).. The system couldn't seem to playback live TV or even a recording while it
> was being recorded without major stuttering (F1 race so it was important!!)
>
> The cpu seemed fine so I am assuming this is a bottle neck but it's only dual
> core.. do I need more cores for this type of operation?
>
> Ultimately I would like to be able to record 3 HD streams and 3 SD streams
> simultaneously and playback 1 of those streams.. .. but that's just a wish.. not
> a necessity..
>
> My MBE recording drive is on an LVM partition on a 4-disk RAID 5 configuration...
> With 300 GB set aside for recording... Itis on XFS with tuning for the RAID
> configuration (large extensts and sunit and swidth)
>
> I am about to migrate to a bigger case and will have enough room for at least 2
> more drives.
>
> Here are the options I see.
>
> 1. Add 2 more drives to the RAID 5 partition. This is my first preference as I am
> hoping this will add enough bandwidth to record 5 shows.
>
> 2. Create a new RAID 0 for recording only removing the original 300GB partition
> to utilise for extra space for transcoded videos. (will this be enough?)
>
> 3. Add 2 disks not in RAID and include them in the list of recording drives
> deleting the original partition
>
> 4. Add 2 disks not in RAID and include them in the list of recording drives
> keeping the original partition
>
>
> I'd love your opinions on these or other potential options (that don't involve
> dismantling the existing RAID)
>
>
> Also, is XFS still worthwhile, or should I consider something like NILFS, btrfs
> or ext4?
>
>
> I know that the simple answer is to restrict my tuners to record one channel at a
> time but I have found recently that the multiple recordings per instance is good
> especially with overlapping programs, now that it seems in Australia almost no
> channel runs to schedule anymore..
>
> cheers
>
> ALlan

In addition to the other inputs you've received don't forget that
there can be internal bottlenecks within the PC at places where data
converges such as the PCI bus. (Assuming your machine is not some
state of the art machine using other interfaces.) If you are recording
these streams and if the processor is doing anything at all with them
then you have raw data coming in across the PCI bus to the processor
(Myth), getting processed and then being written back out across that
bus to a disk, and likewise streams being played back have to come
into the processor across PCI and then back out to be displayed. While
a bus like PCI is theoretically 132MB/S in reality it will often start
showing signs of stress above 50MB/S. Consider what the system itself
is doing - network traffic, any graphics work that might access hard
drives or other PCI devices. Sometimes this traffic overwhelm machines
in strange ways.

HTH,
Mark