[SATLUG] OT: RAID setup recommendations

John Pappas j at jvpappas.net
Thu Jan 27 12:25:20 CST 2011

On Wed, Jan 26, 2011 at 3:18 PM, Alan Lesmerises <alesmerises at satx.rr.com>

> Since there are so many practicing sysadmin types on the list, I'd like to
> get some recommendations for a RAID setup on a server I have based on your
> real-world experiences.  The server has 7 hot-swappable SCSI drives (that I
> plan to fully populate) and I wanted to use it as file repository/backup
> location for other PC's we have on our home network.  Speed is not a
> priority, but data integrity is.

Software RAID:  If you are using md-raid in the linux kernel, then RAID
throughput depends on the CPU, especially on a rebuild/degraded array.  I
would do a RAID-6 +1 Hot spare (unless you think/have easy access to spare
drives is available), then just RAID-6.  You could do a RAID-5 since the
drives are reasonably small (thus rebuilds are faster and possibility of a
read error is also smaller).

If you want to have no write hole and capacity is not the main concern,
Todd's 6xR10 suggestion is good (Striped Mirrors) or you could also go
6xR0+1 (Mirrored Stripes) with a hot spare.  Mirroring/striping is also CPU
inexpensive, so not nearly the CPU XOR requirement of R5/6 So from a
capacity perspective (Where N is number of drives):

R5=N-1 or N-2 with Hot Spare
R6=N-2 or N-3 with Hot Spare
R10=N-4 (works with even drive counts) with or without Hot Spare
R0+1=same as R10

Hardware RAID: This is a bit pricey (8-drive cards are not cheap), but the
advantage is that there is a possibility for battery backup modules to
guard against power failures and the exposure to the RAID5/6 write hole data
integrity vulnerability, as well as the dedicated XOR hardware to remove
that responsibility from the CPU.

> From what I have read, there are several ways I could configure the RAID,
> but there are pros & cons with each.  So what I'd like to know is what would
> be your recommendations for the RAID and why?

I use RAID5 for my stuff, as the drives I use have a long tail
(readily available for several years) and the drive sizes/drive counts are
reasonably small - <1TB & <6 drives.  As the drive count & drive size
increases the probability for drive failure + Read error during rebuild.  In
cases where the array size is > 6TB there would be a strong consideration
for RAID6.  The use of hot spare is driven by two things: Availably of
replacement and time to repair.  If the parts are readily available and you
don't mind being exposed during the hunt/acquisition of replacement, then no
hot spare is needed.  If parts/time is scarce then hot sparing is advised.

Really it boils down to Capacity/Resources.  If Capacity needs are modest
and resources are available, then a mirroring RAID is good.  The best
balance is one of the error correcting RAID levels (3, 5, 6).

Hope that helps!

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