[mythtv-users] recommendations for building new FE/BE HDTV system?

Gordon McCrae gordon.mccrae at googlemail.com
Sun Apr 5 18:55:08 UTC 2009


Brian Wood wrote:
>
> One word: Performance.
>
> I don't mean to answer for the person to whom the question was directed, but I 
> can tell you why I use RAID0 instead of Storage Groups.
>
> RAID0 will increase the performance of the disk system, while Storage Groups 
> (or LVM) will not. Note this example (sda is a single drive, md1 is a 
> two-drive RAID0 array:
>
> /dev/sda:
>  Timing cached reads:   1822 MB in  2.00 seconds = 911.75 MB/sec
>  Timing buffered disk reads:  174 MB in  3.00 seconds =  57.98 MB/sec
>
> /dev/md1:
>  Timing cached reads:   1798 MB in  2.00 seconds = 899.41 MB/sec
>  Timing buffered disk reads:  522 MB in  3.00 seconds = 173.98 MB/sec
>
>
> I'm not sure why the RAID array is more than twice as fast as the single 
> drive, but it appears to be, at least according to this somewhat limited 
> test.
>
> I understand that using the 2 drives in RAID0 doubles my chances of losing 
> data to a drive failure, and that a single drive failing will cost me ALL the 
> data on the array, but I'm willing to put up with that. The array is backed 
> up regularly, and I like the performance bump.
>
> In fact, with most Myth systems, a single drive is fast enough, but the 
> improvement is real and significant.
>
>   
The simple reason why it's faster is that this is what RAID-0 is
designed to do; it stripes the data across multiple disks so that you
get them running in parallel, all transferring data (up to your bus
limits of course). So whenever access a file from a RAID-0 array it
actually read parts from each disk in parallel, thus reducing the time
you need to wait for it. In theory it could be as much as twice the
speed, but reality kick in with overheads, bus bottlenecks etc. and
reduces it below that.

As to whether it's wise to do it, that's entirely up to the individual.

See http://en.wikipedia.org/wiki/Redundant_array_of_independent_disks
for a fuller explanation of the RAID levels.

Cheers
Gordon


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