[vdr] mdadm software raid5 arrays?

Steve steve-vdr at deltabbs.org
Tue Nov 17 16:34:59 CET 2009


Alex Betis wrote:
> I don't record much, so I don't worry about speed.

While there's no denying that RAID5 *at best* has a write speed
equivalent to about 1.3x a single disk and if you're not careful with
stride/block settings can be a lot slower, that's no worse for our
purposes that, erm, having a single disk in the first place. And reading
is *always* faster...

Example. I'm not bothered about write speed (only having 3 tuners) so I
didn't get too carried away setting up my 3-active disk 3TB RAID5 array,
accepting all the default values.

Rough speed test:
#dd if=/dev/zero of=/srv/test/delete.me bs=1M count=1024
1073741824 bytes (1.1 GB) copied, 13.6778 s, 78.5 MB/s

#dd if=/srv/test/delete.me of=/dev/null bs=1M count=1024
1073741824 bytes (1.1 GB) copied, 1.65427 s, 649 MB/s

Don't know about anyone else's setup, but if I were to record all
streams from all tuners, there would still be I/O bandwidth left.
Highest DVB-T channel bandwidth possible appears to be 31.668Mb/s, so
for my 3 tuners equates to about 95Mb/s - that's less than 12 MB/s. The
78MB/s of my RAID5 doesn't seem to be much of an issue then.

Steve





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