[mythtvnz] Recording dropouts and disk performance...

[Rob Connolly]

On Sun, Feb 17, 2013 at 04:34:44PM +1300, Steve Holdoway wrote:
> 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
> 

D'oh, it looks like that may be (one of) my problem(s) although my
performance doesn't seem to be quite as bad as yours (possibly because
these are SATA 3 drives?).

Here is the partition table of one of my drives:

Model: ATA WDC WD20EARX-008 (scsi)
Disk /dev/sda: 2000GB
Sector size (logical/physical): 512B/4096B
Partition Table: msdos
Disk Flags: 

Number  Start   End     Size    Type     File system     Flags
 1      32.3kB  255MB   255MB   primary  ext2            boot, raid
 2      255MB   4302MB  4047MB  primary  linux-swap(v1)
 3      4302MB  2000GB  1996GB  primary                  raid

Out by 0.3k!

Cheers,

Rob