[mythtv-users] Config suggestions for powerful MythTV Backend

[Simon Hobson]

James Robertson wrote:
>I have recently aquired a Dell Poweredge 2600 Server with Dual Xeon
>X5355 CPU's, 16GB RAM, 6x146GB 15K SAS disks and a PERC 5i RAID
>controller.

Ooh, that's a hungry beast to feed with lecky 24x7

>I have been contemplating the best ways to configure it as a MythTV
>backend, but would love some feedback and suggestions.
>
>My intitial plan is as follows:
>
>OS:  Debian Squeeze AMD64 (netinstall and no GUI)
>2x 146GB disks in RAID1 for OS and MythTV Database

Wasteful - see below

>4x 146GB disks in RAID10 to host the LiveTV directory (if I was
>feeling reckless I could do RAID0 across all of them).
>2X 1TB Sata disks in RAID1 to store pictures, movies, music and
>general file sharing on the network.

RAID is an interesting subject to debate.

>And what to do with all that RAM!! Could I utilise some of it in a RAM
>DISK? as I will probably be hard pressed to push 512MB from the OS and
>apps.

Tweak MySQL to use a chunk of it - preferably enough that it keeps 
the whole database in RAM. That will make all the arguments

>Would placing Live TV recordings in a Ram disk provide any real
>benefit?

Doubt it.


OK, my first suggestion is to ditch the PERC controller - it's a pain 
to use in Debian. You can set it up without any OS support, but 
you'll have zero visibility without installing the appropriate 
software. Dell don't support anything but RedHat, and I've had 
"mixed" results trying to get their software to work. I now have most 
of my machines with the controller in JBOD mode and use MD for raid - 
but the controller still gets in the way of accessing SMART data.

OS. I'd suggest a RAID1 partition across at least 2 disks for a /boot 
volume. The trick here is that the boot volume can be accessed from 
any of the member disks by the BIOS without needing any raid software 
loaded. Thus it's possible to boot the system from a raid volume 
before the raid software is loaded to access it !
Then I'd put the OS on at least two disks (RAID1 again). I tend to 
use a native partition/array for the OS itself (ie /), and then LVM 
to manage the rest (eg /var). That means the root filesystem is 
available even if your recovery disk doesn't have LVM on it. Not the 
only way to do it, just my preference.

That will leave you well in excess of 100G free on each of the OS 
disks to use for storage. Having the OS on the same disks as storage 
won't be an issue - with your RAM, all the regularly accessed stuff 
will stay in RAM anyway. On my home Myth system (small HP 
Microserver), I have two SATA disks, with the OS & Database across 
them (RAID1) and storage using the remaining space - it's not been an 
issue even with 5 recordings going (two DVB tuners, UK Freeview, 
Multirec) + commflagging + one playback stream.

If you use the remaining space on the OS/database disks for "read 
only" storage (ie videos and music) it will be less of a potential 
problem than it might be for TV recordings.

As for recordings, if you don't mind about redundancy, do not use 
RAID 0. Use separate disks and add a directory on each of them to a 
storage group - eg if you used all 6 disks, then you'd have 6 
directories listed in your default storage group. If you use multiple 
storage groups, then you can split the directories between them. 
Apart from performance, if one disk in a RAID 0 set fails, you lose 
the lot, if one disk in a set of 6 separate drives fails, you lose 
just what's on that disk.
Myth will default to spreading the recording load between disks, and 
with only one tuner card, you are never going to get close to 
saturating any of the disks.
With RAID 0 (striping), you will be writing all the concurrent 
recordings across the whole array and it's pot luck whether 
individual recording streams will hit the same or different disks at 
any point in time.

If you do want redundancy, then consider 3x 2 disk RAID1 arrays, so 
you have 3 directories to put in your storage groups.

-- 
Simon Hobson

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