[mythtv-users] Fastest RAID for HD?

[Roger Heflin]

>> _______________________________________________
>> mythtv-users mailing list
>> mythtv-users at mythtv.org
>> http://mythtv.org/cgi-bin/mailman/listinfo/mythtv-users
>  From what I have read, people seem to find RAID 5 too slow for writes.
> 

Not for mythtv, the people that complain about slow writes are writing
orders of magnitude more data, and care about writing it a lot faster.

Consider a 20Mbit/second HD stream, that breaks down to 2.5MBytes/second or
so, a bad raid will do around 45MB/second for a single stream of writes.

Doing multiple dd if=/dev/zero of=test1 bs=64k results in me getting
a total of 30MB/second for 10 streams at the same time, or 3 MB/second
per stream, enough to record a 20Mbit/second HD stream, and once I passed
4 streams the total rate was no longer falling, so 10 recordings at the
same time should be doable with a cheap raid setup.   Some things that
could be done to improve things would be to use larger buffers in the
backend (if file fragmentation becomes an issue).

This is with a cheap Sata Sil card and 4 x 500 GB under raid5 using
md software raid, the 4port Sil card was $20-$30 and is a basic low
end sata card.  You would be better off using 2 cheap cards and 2
separate raid5 setups than messing with any of the expensive hardware
raid solutions.

Going to more disks in the raid setup may hurt, but I would expect
that the total write rate is going to stay about the 30MB/second,
and unless you are talking about a huge number of recorders/players
at the same time I don't think it will be a problem.

Now as mentioned, don't put the database on raid5, raid5 does suck
for databases.

                              Roger