[mythtv-users] LVM2 Striping Suggestions...

Michael MacLeod mikemacleod at gmail.com
Sat Sep 23 17:08:53 UTC 2006

I have a setup almost exactly the same, and I'm very happy with it. I have
two 250gig ATA-133 drives, and I put them on opposite IDE channels as well.
I'm got them in an LVM setup with striping and XFS as the filesystem. Seems
to run flawlessly. I only have a PVR-550, so I only have to deal with three
streams concurrently at any one time, plus any bittorrent downloads that
might be going on, but it handles them fine.


On 9/23/06, Andy Speagle <sidrew at gmail.com> wrote:
> I understand completely what you're saying... I actually took your advice
> and placed an order for a PVR-500... I've been looking at doing this project
> so long that I didn't even know they came OUT with a dual-tuner card... it's
> awesome.  So... Unless I send one of the 2 PVR-350's back... I'll have the
> option of recording four streams at once... let me get your opinion of how
> well this system will function.
> Athlon XP 3200+ (2.2GHz)
> 1 GB PC3200 DDR
> 2x PVR-350
> 1x PVR-500
> 2x 250GB WD 7200RPM 8MB ATA-100 Drives
> 1x ATA DVD+/-RW Drive
> 1x Ensoniq PCI Audio
> I'm curious what your suggestions would be for setting up storage on this
> box... or if you think it would even be capable of recording four streams...
> or recording three while watching a fourth... (and potentially even remote
> front-ends)...
> I think I'm going to employ a third ATA-100/133 HD to setup the system
> partitions ... and use the two 250GB drive for media storage...
> Suggestions?
> Thanks,
> Andy
> On 9/23/06, Richard Freeman <r-mythtv at thefreemanclan.net> wrote:
> >
> > Andy Speagle wrote:
> > > I want to setup the LV using striping
> > > across the two drives to take advantage of concurrent I/O on both
> > > spindles...
> >
> > Actually, the thing I really wished that LVM had was something I'd call
> > "chunking" instead of striping.  The user would specify a chunk size
> > (maybe 50MB for common use, or 0.5-1GB for myth), and the drive would
> > store each chunk on a single drive, and each consecutive chunk on
> > sequential drives.  So, logical drive throughput for a single file is
> > only that of a single drive, and if a drive fails you end up with a LOT
> > of files completely intact.  On the other hand, if you access two random
> >
> > files on the same logical drive there is a good chance that they're on
> > different physical drives, so head seeking is greatly lowered and data
> > throughput rises to the total drive throughput if all drives are in use.
> >
> >
> > Unless you're recording uncompressed HD you don't actually need all that
> > much single-file throughput from your drives - if you're playing 3 5Mbps
> > streams then you only need 15Mbps of throughput.  The bigger issue with
> > something like myth is head-seeking - if those files are scattered on a
> > single physical drive then the head must seek.   With striping you
> > GUARANTEE that every file is on EVERY drive - so you will always have
> > competition for the drive head when reading multiple files
> > simultaneously.  If you could instead try to distribute files randomly
> > across drives you'd reduce this competition, as drives might only have
> > one file being requested on them and could read this file at full
> > throughput for a long time.
> >
> > Think of it this way - striping gives you a much higher
> > single-file-access speed.  A pattern which keeps data for individual
> > files on a single drive, but which distributes different files across
> > drives would have lower maximum burst speed for an individual file, but
> > a higher sustained throughput when multiple files are being read at
> > once.
> >
> > Granted, if these drives are ONLY doing mythtv you probably won't have
> > issues unless you have 14 front ends and 12 tuners or something like
> > that.  Things become more of an issue if you're also using these drives
> > for other tasks (which are more likely to cause head movement than
> > seeking along a few 1GB MPEG files).  Having more free RAM also helps -
> > a file server with lots of free RAM will undoubtedly do some read-ahead
> > and write caching which will reduce head thrashing (especially with a
> > filesystem like xfs which tends to hang onto dirty buffers).
> >
> > In theory there is no reason that LVM can't arrange the logical clusters
> > in any way the user might like - I believe it really is just a lookup
> > table mapping logical clusters to physical clusters.   Striping and
> > linear are only two of many possible ways to setup the map.  I'm sure a
> > better computer scientist than I could figure out the best possible
> > arrangement for any use scenario...
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> >
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