[mythtv-users] Fastest RAID for HD?
rogerheflin at gmail.com
Thu Feb 14 18:47:42 UTC 2008
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> 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
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
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