Gvinum RAID5 performance

[Richard Coleman]

Brad Knowles wrote:
> Keep in mind that if you've got a five disk RAID-5 array, then for 
> any given block, four of those disks are data and would have to be 
> accessed on every read operation anyway, and only one disk would be 
> parity.  The more disks you have in your RAID array, the lower the 
> parity to data ratio, and the less benefit you would get from
> checking parity in background.

My understanding is that this is true only when the system is running in 
a degraded mode.  When you blow a disk, instead of hitting that disk, 
you access all the other disks and calculate the desired block using the 
parity.  If this system is healthy, a single disk access should be 
sufficient.

If a system with a 9 disk raid5 volume (pretty common) had to hit each 
disk on a block access, it would majorly suck.

I think detecting and handling bad physical blocks is a whole other matter.

Richard Coleman
rcoleman at criticalmagic.com