[mythtvnz] Recording dropouts and disk performance...

[criggie]

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.

In practice this theoretical stuff goes out the window, because it takes 
time for either software or hardware to calulate the parity bits Then 
the OS has to write each part to each disk.  SATA controllers can still 
only address one drive at a time, and noone has one controller per disk.

Remember RAID0 values speed over redundancy, RAID1 values redundancy, 
and RAID5 tries to be redundant without wasting half your disk while 
staying faster than RAID1.

Have you considered memory buffers instead of disk buffers to be the 
buffers in question?

> Personally I'd suspect LVM, and I must ask, why are you using LVM at all?

LVM is awesome for the OS, or your files, or almost anything.

Except recording storage in mythtv.

Storage pools are more useful, faster, and fewer layers of abstraction 
means they're simpler.

No need to worry about "balancing" your storage either - myth sorts that 
quite happily by itself.

-- 
Criggie

http://criggie.org.nz/