[vdr] mdadm software raid5 arrays?

Pasi Kärkkäinen pasik at iki.fi
Thu Nov 19 11:53:04 CET 2009


On Tue, Nov 17, 2009 at 03:34:59PM +0000, Steve wrote:
> 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
> 

You should use oflag=direct to make it actually write the file to disk..

> #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
> 

And now most probably the file will come from linux kernel cache. 
Use iflag=direct to read it actually from the disk.

-- Pasi

> 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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