[mythtv-users] LVM2 Striping Suggestions...
sidrew at gmail.com
Sat Sep 23 16:48:19 UTC 2006
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 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
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...
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...
> mythtv-users mailing list
> mythtv-users at mythtv.org
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