[CentOS-virt] Slightly OT: FakeRaid or Software Raid
grantmasterflash at gmail.com
Thu Dec 3 14:41:18 UTC 2009
On Thu, Dec 3, 2009 at 6:08 AM, Christopher G. Stach II <cgs at ldsys.net>wrote:
> ----- "Grant McWilliams" <grantmasterflash at gmail.com> wrote:
> > On Wed, Dec 2, 2009 at 9:48 PM, Christopher G. Stach II <
> > cgs at ldsys.net > wrote:
> > ----- "Grant McWilliams" < grantmasterflash at gmail.com > wrote:
> > > a RAID 10 (or 0+1) will never reach the write... performance of
> > > a RAID-5.
> > (*cough* If you keep the number of disks constant or the amount of
> > usable space? "Things working" tends to trump CapEx, despite the
> > associated pain, so I will go with "amount of usable space.")
> > No.
> > --
> > Christopher G. Stach II
> > Nice quality reading. I like theories as much as the next person but
> > I'm wondering if the Toms Hardware guys are on crack or you disapprove
> > of their testing methods.
> > http://www.tomshardware.com/reviews/external-raid-storage,1922-9.html
> They used a constant number of disks to compare two different hardware
> implementations, not to compare RAID 5 vs. RAID 10. They got the expected
> ~50% improvement from the extra stripe segment in RAID 5 with a serial
> access pattern. Unfortunately, that's neither real world use nor the typical
> way you would fulfill requirements. If you read ahead to the following
> pages, you have a nice comparison of random access patterns and RAID 10
> coming out ahead (with one less stripe segment and a lot less risk):
> Christopher G. Stach II
So if I have 6 drives on my RAID controller which do I choose? If I have to
add two more drives to the RAID 10 to equal the performance of a RAID 5 I
could just make it a RAID 5 and be faster still. RAID 5 is faster than RAID
10 for reads and writes.
However, you are right on the IOs. The RAID 10 pretty much trounced RAID 5
on IOs in all tests.
What wasn't in the test (but is in others that they've done) is RAID 6. I'm
not sure I'm sold on it because it gives us about the same level of
redundancy as RAID 10 but with less performance than RAID 5. Theoretically
it would get soundly trounced by RAID 10 on IOs and maybe be slower on r/w
transfer as well.
Some people, when confronted with a problem, think "I know, I'll use
Now they have two problems.
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