Re: [gentoo-amd64] Re: Is my RAID performance bad possibly due to starting sector value?

[Bob Sanders <rsanders@sgi.com>]

Rich Freeman, mused, then expounded:
> On Fri, Jun 21, 2013 at 3:31 AM, Duncan <1i5t5.duncan@cox.net> wrote:
> 
> > The single down side to raid1 as opposed to raid5/6 is the loss of the
> > extra space made available by the data striping, 3*single-device-space in
> > the case of 5-way raid6 (or 4-way raid5) vs. 1*single-device-space in the
> > case of raid1.  Otherwise, no contest, hands down, raid1 over raid6.
> 
> This is a HUGE downside.  The only downside to raid1 over not having
> raid at all is that your disk space cost doubles.  raid5/6 is
> considerably cheaper in that regard.  In a 5-disk raid5 the cost of
> redundancy is only 25% more, vs a 100% additional cost for raid1.  To
> accomplish the same space as a 5-disk raid5 you'd need 8 disks.  Sure,
> read performance would be vastly superior, but if you're going to
> spend $300 more on hard drives and whatever it takes to get so many
> SATA ports on your system you could instead add an extra 32GB of RAM
> or put your OS on a mirrored SSD.  I suspect that both of those
> options on a typical workload are going to make a far bigger
> improvement in performance.
>

However, the incidence of failure is less with RAID1 than RAID5/6.  As
the number of devices increases, the failure rate increases.  Indeed,
the performance and total space can outweigh the increase in device
failure.  However, more devices - especially more devices that have
motrs and bearings, takes more power, generates more heat, and increases
the need for more backups to avert an increase in failures.

Bob
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