[mythtv-users] Fastest RAID for HD?

[Roger Heflin]

Markus Schulz wrote:
> Am Freitag, 15. Februar 2008 schrieb Marc:
> [...]
>> Don't assume people on this list are experts. Some are, some are
>> informed users who have done research, some just make assumptions
>> based off of one post or one article they read.
>>
>> >From a statistical standpoint..
>>
>> Raid0 would be the fastest, it is common sense. Your speed for any
>> action would be dependent on the number of drives you had or N.
>> However if one drive fails you lose all data.
>>
>> Raid3 or 5 would be the second fastest and would be safer due to the
>> ability to continue working without loss of data if you had a single
>> drive fail. The difference between these 2 raid types is the way they
>> setup parity. Raid3 has a dedicated parity drive. Raid5 spreads
>> parity across all the drives.
>> Speed of any action on either of these raid configs would be the
>> number of drives you had minus 1 or N-1.
> 
> sorry but thats not true. Show me a raid5 with 5-6 drives which will got 
>> 300MB/s _write_ performance...
> 
> Fastest Raid _WITH_ redundancy was raid10.
> 
> 

The high dollar hardware raid controllers will get close to the N-1 rate, I
have benchmarked them under sustained load, but the hardware raid are in the
$500-$1000 range, and raid10 with 6 disks will in a perfect world be 
180-210mb/second (3 effective drives-60-70mb/second/drive).   RAID10 is really
on the fasted RAID if you are benchmarking seek times (database), if you are 
benchmarking raw sequential read and write rates with a good controller
both are about the same speed with raid5/6 or with raid10, and even raid0
only gives a 10-20% gain over raid5/6.    Raid10 only really wins when one
is doing a lot of seeks as small write seeks are expensive on raid5.

Most software raid has a problem that most of the older MB controllers and
cheaper controller cards really don't have independent channels and interfere
with each other.   On mine I get 70MB/second with one disk, and then it goes
to 108mb/second with a second dd added and then to 115 with a 3rd, so the
controller is utter crap, but still I can get 40MB/second writes out of it
with raid5 and it is good enough.    With the good controllers that goes 70,140,
210,280 and I have tested good controllers and they do exist, one just has to
be more careful in finding them, and a lot of the recent MB controllers are
good controllers.