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

[David Rees]

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