[mythtv-users] combined myth/server

[Travis Tabbal]

On Wed, Apr 21, 2010 at 1:03 PM, Indulis Bernsteins
<indulis.b at au1.ibm.com>wrote:

>
> >Oh, and do yourself a favor and use a separate disk for the OS and
> database.
> >I didn't do that the first time out and ended up having to change things
> >around. An old 40G IDE drive worked great for that, then I dedicated the
> >500G SATA drive to recordings and solved some irritating drop-outs in
> >recordings. Don't skimp on RAM either, particularly if you are going to
> >attempt an FE install on that box as you will want to dedicate 512M to
> video
> >for VDPAU.
>
> I have been running for a couple of months on one drive- I use LVM to
> "split up" the drive into separate OS and data partitions or Logical VOlumes
> (like disk partitions but in a nice flexible way cf PITA disk partitions).
>  Nice because you can migrate them between drives, shrink, grow (as long as
> filesystem you choose can do likewise).  This is a 1TB SATA drive 3 Gbps and
> it can record 3-4 digital broadcasts at a time (and do some web surfing)
> with no problems at all- have never had a stutter/glitch.  I am using XFS as
> the filesystem tuned up as documented in the mythtv wiki.  This probably
> makes a big difference compared to ext3/4 etc- especially cluster size.
>
> Actually the LVM is on top of RAID ("md" s/w RAID done by the Linux kernel)
> but I haven't set up the 2nd drive as a mirror yet- just noticed it the
> other night! I am tempting fate now...
>
> If you use LVM then once you get a 2nd drive you can set up a "pending
> mirror" RAID10 on it, then use LVM to just migrate your data to the 2nd
> drive.  Once done, set up mirror on 1st drive, and let "md" synchronise the
> two!
>
> I dont agree with using an old 40GB drive as drives have a shelf life, you
> are asking for trouble using an old drive.  IMHO...
>


I was using a very similar setup and had the problems described. If it works
for you, great, but you'll see a lot of others on this list recommend the
exact same thing. I was using ext4 on the OS/Database partition and XFS on
the recordings partition. The problem is that you end up losing a lot of
throughput to seeking on the disk. Keeping the seeks from the DB away from
the seeks from the recordings helps a lot.

Please stop quoting the interface speed of the drive like it's meaningful.
We do very little that allows the tiny cache on those things to work, so
very little of the data is going to/from cache. You're waiting on the
platter, and I'm not aware of any platter drive that can push 3Gbps. Some
SSDs certainly can, if you can afford them. And, as I said, it's the seek
latency that's killing you here, the raw transfer rate is near meaningless
with Myth as you will never get close to it.

The old drive was just one I had sitting around. The point was that it
doesn't need to be expensive, large, or fast to work well for the OS/DB. A
USB stick would work (yes, I've done that too. 8GB USB flash stick booting
Mythbuntu 9.10). As for "shelf life", meh, I have old 1GB drives that worked
fine last year (wiping old drives, took me forever to find the stupid 50pin
SCSI cable). It's certainly hit and miss, so I would recommend buying
something new, but if you want to be cheap or re-use parts you already have,
go for it. Just make sure you're doing backups so you can restore your
Mythbox when the drive dies, as they all do. On the main OS/DB drive, it's
really just the database you have to backup. The rest can be installed from
the CD if you like.

That said, my current backend is running in a VM under OpenSolaris/Xen and
the same array hosts the recordings and the OS image without trouble.
However, I have 2 6-disk arrays in that pool so things are a little
different on that box. If you think LVM/md is cool, try ZFS. :)
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