Re: Raid and SBS 2003
From: root (root_at_buchanangc.com)
Date: 02/17/04
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Date: Mon, 16 Feb 2004 19:21:04 -0800
Some of the stuff here about RAID is on track:
http://www.microsoft.com/mspress/books/sampchap/6515a.asp
"root" <root@buchanangc.com> wrote in message
news:uvJwlJQ9DHA.2672@TK2MSFTNGP10.phx.gbl...
>
> "John LeMay" <jlemay@njmc.com> wrote in message
> news:MPG.1a9b35e6ed8004ef98970c@news.individual.net...
> > In article <OLPu57O9DHA.2416@TK2MSFTNGP10.phx.gbl>, root@buchanangc.com
> > says...
> > > Proper >hardware< RAID 5 will be nearly as fast as RAID 0 but require
> one
> > > more drive. www.3ware.com
> > >
> >
> > You had me until this. What testing can you show that displays RAID5 can
> > be nearly as fast as RAID0?
>
> Don't need testing really as it's definitional. RAID 5 is striping +
> parity. Writes can be slower but reads are about the same speed as RAID 0
> because such a read IS a RAID 0 read but with one less drive.
>
> > Note that cheating by caching writes doesn't
> > count.
> >
> > I never understand why it's a surprise that RAID systems are slower than
> > writing to a single disk.
>
> Such is NOT slower than writing to single disk if a hardware RAID
controller
> is used and often not slower for software/firmware RAID 0 & 1.
>
> > With RAID 1 systems - the fastest RAID that
> > still provides redundancy
>
> No, RAID 5 is as and usually faster than RAID 1 in most situations.
>
> > - you are still writing the data twice.
>
> Not true for hardware RAID. The host only sees one write. True however
for
> software/firmware but that wont be a significant issue in many cases.
>
> > With
> > RAID 5 you have to divide the data, stripe it across a lot of disks,
> > then compute the parity and stripe it across the lot of disks as well
> > (ok, the computation is probably done prior to either write, but the
> > point remains). Writing data more than once, or writing extra data, is
> > always going to be slower than writing data once.
>
> No, because in RAID 0 and similarly RAID 5 those writes are done
> concurrently. One has multiple heads on multiple drives putting data on
the
> disk surfaces simultaneously; that is the limiting process (where the
heads
> meets the oxide). Clearly RAID 5 parity writes do add overhead compared
to
> RAID 0.
>
> A hardware RAID 5 4 drive array will write faster than a single drive.
>
> > Even in RAID 0 systems
> > there is still some overhead in deviding the data to be written,
>
> A small amount of overhead for software/firmware RAID 0 and for hardware
> RAID 0 there's no host CPU processing overhead.
>
> > however
> > these systems usually outperform a direct write since smaller amounts of
> > data (and no extra data) is written to each disk in the array.
>
> Huh?
>
> >For this
> > reason, several enterprises have selected RAID-10 (or 0+1) for heavily
> > loaded datasets. The speed of RAID-0 plus RAID-1 (striping the data,
> > then mirroring it) outperforms RAID-5 by a long shot and still allows
> > for larger arrays than RAID-1.
>
> That's false in most cases. Take eight drives and put it into a good
> hardware RAID 5 array and in most cases that'll outperform a 4+4 RAID 0+1
> array. AND the RAID 5 array is 1.75 times bigger. For cases where
writing
> vs reading is a high then the RAID 0+1 gains on the RAID 5 to the point
> where RAID 0+1 is the preferred type for write only disk stores<g>.
>
> A 16 drive RAID 0+1 array is faster than an 8 drive RAID 5 array. That
may
> be where the myth about the speed advantage of RAID 0+1 comes from. Take
> those 16 drives and put them in a RAID 5 array and the balance swings back
> to RAID 5.
>
>
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