[mythtv-users] Storage Questions. LVM, RAID, NFS

[Steven Adeff]

On 5/2/06, David Rees <drees76 at gmail.com> wrote:
> On 5/2/06, Robin Hill <myth at robinhill.me.uk> wrote:
> > With a 4 disk RAID 5 you'll be writing 3 blocks plus parity at once,
> > whereas with a 4 disk RAID 10 you'll be writing 2 blocks (plus mirrors)
> > at once.  In terms of raw I/O the RAID 5 should be faster.  The slight
> > complication is that RAID 5 will need to calculate the parity before
> > writing - this shouldn't have any impact in a modern system though.  For
> > reading the speeds will probably be about the same - both should be able
> > to read from all 4 disks at once.
>
> Read speeds from a RAID 5 is N-1 because parity blocks aren't used for
> reads unless a drive has failed so a RAID 10 will be faster for reads.
> Each write in a RAID 5 array requires a write to all disks in the
> array and possibly a read if the write is smaller than the stripe
> size. With RAID 10, writes smaller than the stripe size only go to one
> mirror, writes larger than the stripe size get written in parallel.
>
> > So, IMO, RAID 10 won't give you any advantage.  It's probably a win for
> > continuous small reads & writes (e.g. databases) since you could read
> > from one miror while writing to the other - this'll only work for small
> > writes though as you wouldn't want the mirrors to be out of sync for
> > more than a fraction of a second.  For writing a number of large files
> > simultaeously (with redundancy) I'd say RAID 5 would be your best bet.
>
> As I mentioned earlier, mythtv forces frequent writes of data while
> recording. This kills write performance in a RAID 5 setup, the RAID 10
> will vastly outperform RAID 5 when recording with multiple tuners.
> Careful tuning and the ability to modify the amount of data to buffer
> before flushing writes would help RAID 5 performance.

Dave, thanks for the explanations. I'll definitely be doing a RAID10
then as write performance with the number of tuners I have is key.

Another question...
The motherboard I intend to use for this new standalone master backend
has 2 SATA and 3 PATA ports. I plan on using the 2 SATA and 2 of the
PATA ports (with one drive on each) for the RAID10, from the talk on
this list, I assume I will want to have the two RAID1 arrays contain 1
SATA and 1 PATA each to match the performance of the two arrays and
then RAID0 those two arrays? All four of the drives I have are the
same model except for the PATA/SATA difference.

Thanks!
(if only they made an 8port SATA PCI card with that didn't cost more
than a new computer...)

--
Steve
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