Date: Sat, 17 Feb 2007 22:13:14 +0000 From: Matthew Seaman <m.seaman@infracaninophile.co.uk> To: Garrett Cooper <youshi10@u.washington.edu> Cc: freebsd-questions@freebsd.org Subject: Re: gvinum with RAID capable SATA card Message-ID: <45D77DFA.9090504@infracaninophile.co.uk> In-Reply-To: <45D77216.40308@u.washington.edu> References: <45D77216.40308@u.washington.edu>
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Garrett Cooper wrote:
> Hello,
> Motherboards usually don't support more than 2 SATA ports, and sinc=
e
> SATA is 1:1, I have to invest in a SATA RAID card to get RAID-5 support=
=2E
> After reading the handbook a bit I came across chapter 19, which
> goes into some detail about why and how to the configure one's disks in=
> a software RAID with gvinum. However, I'm not sure what mode (if one
> exists) that I could put a RAID card in to make the OS see single disks=
=2E
> So I guess my point is, can I turn off RAID functionality on the
> card and make the disks into single, separated disks available via the
> disk controller, or is that impossible with a RAID card?
> I think the problem lies with my (limited) knowledge of RAID.
Most RAID controllers certainly will support acting in JBOD mode.=20
Check the docco before buying. =20
However, if you've got a hardware RAID card, then you'll almost
certainly be better off using it for doing RAID5 than doing it in
software with gvinum. The RAID card will have hardware to do the
parity calculations needed for RAID5 and offloading that from the
CPU is a big win.
The balance of advantage between hardware and software RAID is not so
clear cut for RAID1 or RAID10 (mirror or mirror+stripe). Software RAID
is a lot cheaper, can be monitored by native system tools and is pretty
much as performant as hardware RAID unless you have a battery backup
unit on the RAID card [in which case you can set the card to tell the OS
the data is secure as soon as it is in battery backed RAM on the card
(which takes nanoseconds) rather than actually written to disk (which
takes milliseconds), hence decreasing IO latency enormously].
When buying a RAID card, an important consideration is that there
are FreeBSD compatible management tools available -- otherwise, for
example, you'll not get alerted to disk problems other than by the
onboard alarm buzzer on the card.
Cheers,
Matthew
--=20
Dr Matthew J Seaman MA, D.Phil. 7 Priory Courtyard
Flat 3
PGP: http://www.infracaninophile.co.uk/pgpkey Ramsgate
Kent, CT11 9PW
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