Date: Wed, 17 Apr 1996 23:43:24 -0600 (MDT) From: Dave Andersen <angio@aros.net> To: freebsd-questions@freebsd.org Cc: gpalmer@freebsd.org, thekind@NETural.com Subject: Re: SCSI RAID controller support? Message-ID: <199604180543.XAA15760@shell.aros.net> In-Reply-To: <199604172259.PAA02918@phaeton.artisoft.com> from Terry Lambert at "Apr 17, 96 03:59:37 pm"
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Lo and behold, Terry Lambert once said:
> > I'm surprised that you need RAID for web serving at all. You'd need a
> > VERY high hit rate, or be pumping out large documents to need such
> > access speed.
>
> What? What makes you think RAID is faster? It's slower, without
> hardware acceleration (like an NVRAM write cache). You have to do
> two writes for each write, otherwise...
>
> If he wants addes speed, he should use striping with spindel-sync,
> not RAID.
>
> RAID is for fault tolerance and error recovery.
That depends entirely on the level of RAID you're using. You can use
RAID for fault tolerance, -or- for disk striping, or both.
RAID 0: Striping, no parity.
Not true "raid" but often sold as it.
RAID 1: Mirroring on two disks - redundancy - same speed
as a normal drive (in theory. :) The ECC may slow things down.
RAID 2: Hamming ECC - basically for data redundancy
About as fast as raid 1, perhaps a bit faster.
RAID 3: Striping with parity checking
A bit more reliable than RAID 0, not quite as fast,
but still considerably faster than a straight disk
RAID 4: Parity Checking on a special parity disk.
RAID 5: Parity checking with parity distributed across data disks
Quite obviously, RAID 0 and RAID 3 have the potential to be
considerably faster than an ordinary disk. I don't know how they compare
to software striping as in the ccd, I don't think anyone's done any
comparisons.:) I've used some of these in graphic design applications,
and they *really* fly. A RAID level 0 array like the FWB Jackhammer is a
very pretty piece of equipment, though a tad expensive for most people. :)
Granted that RAID 0 isn't really "R"aid in the sense that it isn't
redundant, but even RAID 3 is faster than a standalone disk (and has the
advantage of redundancy), and many manufacturers sell striped
non-redundant disk arrays as RAID drives.
-Dave Andersen
--
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