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

[David Rees]

On 5/2/06, Steven Adeff <adeffs.mythtv at gmail.com> wrote:
> Would a 4 disk RAID10 be better than RAID5 for a recordings drive
> considering I may have times where I've got my 3x HD recorders and my
> PVR150, (possibly swaping for a PVR500) plus having at most 2x HD
> capable frontends running?

In any raid 5 setup, small writes will be basically limited to the
speed of 1 disk as each write has to go to every single drive. In some
cases small random writes will be even slower as parity data has to be
read before a stripe is rewritten. Large streaming writes can approach
N-1 disk speeds, but rarely do in practice. I know that mythbackend
attempts to frequenty write out small chunks of data which does not
help. It would be nice if you could configure the data write interval
or how much data to queue before forcing a write to disk with myth.

Read speed of a raid 5 will depend on the size of the read and the
stripe size. If the read is smaller than the chunk size, read
performance will be the same as a single disk. If the read covers more
than one chunk read speeds can approach N-1 performance where N is the
number of drives in the array.

Now for a RAID10 array (a stripe of mirrors). Each mirror has
basically the same write performance as a single drive, but up to
double read performance. Each time you stripe an additional mirror,
you double read and write performance. So performance will be N for
reads and N/2 for writes with no write performance penalty for small
writes.

For mythtv performance where you have multiple tuners writing frequent
small chunks of data all over the disk would suggest that RAID10 would
outperform RAID 5. Careful tuning of the stripe size in a RAID 5 setup
is necessary to obtain optimal performance.

-Dave