[mythtvnz] Recording dropouts and disk performance...

[Steve Holdoway]

On Sat, 2013-02-16 at 22:16 +1300, criggie wrote:
> On 16/02/13 21:48, Steve Hodge wrote:
> > On Sat, Feb 16, 2013 at 8:36 PM, Robin Gilks <g8ecj at gilks.org
> >     I do find it odd that a Raid 1 system is so slow - my Raid 5 system,
> >     which
> >     in theory is 1/4 the speed (at best!) handles 8 simultaneous
> >     recording OK
> >     with a mixture of DVB-T, DVB-S and analog off a set=top box.
> >
> >
> > RAID1 and RAID5 have theoretically identical write performance - the
> > same as the speed of the slowest disk. In practice it'll be slightly
> > slower just because there are more disks involved.
> 
> Agreed - RAID5 should be N times faster than one disk where N is number 
> of disks in the raid.   A file is written in N blocks, with 1/N per disk 
> (approximately) so it should be done in 1/N the time of a single disk 
> writing the lot.

Not quite. RAID5 writes 1/(N-1) data + 1 X parity. Trivial difference
sure, but still there. Performance can vary depending on how good a fit
the stripe size is.

Could the performance be caused by that awful time when suppliers were
transferring from 512B to 4k blocksize, but not correctly reporting as
such? I had terrible problems with WD EARS disks in RAID1 a couple of
years ago...
http://www.greengecko.co.nz/content/western-digital-can-i-have-my-2-days-back

Cheers,


Steve
-- 
Steve Holdoway BSc(Hons) MIITP 
http://www.greengecko.co.nz
Skype: sholdowa
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