RAID? Was: PATA hard disks, anyone?

[Grant Taylor]

On 03/28/2018 12:32 PM, Fred Cisin via cctalk wrote:
> With very unreliable drives, that isn't acceptable.  If each "drive" 
> within the RAID were itself a RAID, . . .  Getting to be a complicated 
> controller, or cascading controllers, . . .

Many of the SCSI / SAS RAID controllers that I've worked with over the 
last 10+ years have this cascading controller functionality.  Most of 
the RAID controlelrs that I've worked with would let you build a mirror 
or stripe across some sort of underlying RAID.  Typical examples are 
striping (RAID 0) across mirrors (RAID 1) or multiple RAID 5 arrays.

> 'course not.  Besides MTBF for calculating the probability of a second 
> drive failing within N hours, must also consider other factors, such as 
> external influences causing more than one drive to go, and the 
> essentially non-linear aspect of a failure rate curve.
You also need to take into account the additional I/O load imposed on 
the remaining drives during a rebuild.

I usedto routinely run into software (Solstice Disk Suite?) RAID 1 
mirrors on Solaris boxen for the OS (/) where different parts of each 
drive would fail.  So we'd end up with a situation where we had a decent 
RAID, but we couldn't replace either disk.  This usually involved taking 
an entire backup of the machine, replacing both disks, and restoring the 
data.



-- 
Grant. . . .
unix || die