Raid 5 (was: Mounting of Linux volumes)

Nicholas Leippe nick at byu.edu
Thu Dec 1 17:36:11 MST 2005


On Thursday 01 December 2005 05:05 pm, Chris Carey wrote:
> On 12/1/05, Ross Werner <ross at agilestudios.com> wrote:
> > On Thu, 1 Dec 2005, James Clawson wrote:
> > > I will access the volume as root as you suggested.  As for the second
> > > disk, it was RAID 5, and your response was pretty much what I
> > > suspected.  Oh, well.
> >
> > Since we're on the subject of RAID, what's the speed difference like
> > between RAID 1 and RAID 5? Is that pretty much the only benefit of RAID
> > 5?
> >
> > RAID 5 seems nearly impossible to recover from anything apart from a
> > single drive failure--any sort of data corruption, or a power outage, or
> > pretty much anything unexpected can render your data unsalvageable,
> > especially if you're using software RAID ... whereas with RAID 1, you
> > always have the option of just treating the disk like a normal drive.
>
> Im no expert on the performance on these but as far as space:
>
> RAID 1 - 2x40GB drives = 40GB
> RAID 5 - 3x40GB drives = 80GB
>
> You get more bang for your buck with RAID 5.
>
> As I understand, RAID 1 will read from both drives simultaneusly,
> making reads quicker. Writes go to both drives so its similar speed to
> a single drive setup. Reads *and* writes on RAID 5 are quicker.

Except for some hardware implementations, writes will be slower with RAID 1, 
since the data has to be sent to two locations, and for blocking writes, it 
must wait until both have completed.  (Some hardware setups synchronize the 
spindles which helps to mitigate this, and hardware lets the system send the 
data once--offloading sending it to the individual units from the system 
bus.)

Reads from RAID 1 can be faster, but most implementations (including some 
hardware) only get you the same speed as a single drive.  Linux is software 
RAID 1 is among them.

There is a 'RAID 1.5' that some people sell, which is really just RAID 1
with RAID 0 performance on the reads (the optimum that you'd hope for).
Linux 'RAID 1.5' may still be a very long ways off.  The way the block layer 
and md layer are structured make it very difficult to do--the elevator code 
is in the wrong layer to make it work.

RAID 5 is a time/money tradeoff.  Reads can be much faster, faster than RAID 0 
even, disk usage is more economical, but writes are much slower.

As for salvageability of data, I've personally never had any problems with a 
RAID 5 setup.  But, as with any other RAID level that does not provide full 
redundancy I'd imagine this would always be a concern.

Of course, a RAID array does not remove the need for backups.  (rm -rf can 
blow right through an array ;)


-- 
Respectfully,

Nicholas Leippe
Sales Team Automation, LLC
1335 West 1650 North, Suite C
Springville, UT  84663 +1 801.853.4090
http://www.salesteamautomation.com



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