Hardware RAID Level & Performance
Aleksandar Milivojevic
amilivojevic at pbl.ca
Thu Feb 17 12:39:51 EST 2005
Norman Zhang wrote:
> May I ask has anyone consider SATA RAID yet? I seems to be a very
> inexpensive solution.
All inexpensive SATA RAID solutions are "fake RAID". This includes
almost all SATA controlers that are integrated into motherboards and
marketed as RAID capable. They are software RAID. Basically, you use
BIOS to write some metadata to the disks (configure the RAID), and than
you need to use special drivers in OS that will do the actual software
RAID stuff. Most of those specialized drivers are slow, unstable, not
available for anything but Windows, or all three of previous statements.
If you have one of those motherboards and/or controllers, you are far
better disabling RAID stuff in BIOS and using standard Linux software
RAID drivers (md) or standard *BSD RAID drivers (RAIDframe).
The only reason why would anybody use one of those "fake" RAID stuff is
if you have Windows XP Home/Professional installed on the machine. This
will give you software RAID support by using special device driver
(since native Windows software RAID driver is available only in Server
versions of Windows). Basically this is the core reason why those
"fake" RAID controlers exist: chipset manufacturers giving you something
Microsoft denied to you. This is more or less desktop/home user domain.
For servers, there is no advantage of using them (only disadvantages).
All is not dark. There are several companies making real hardware RAID
solutions that use SATA disks. 3ware is one of them and seems to be
very well supported and popular in Linux community. Device drivers are
part of official Linux kernel. Adaptec has some real SATA hardware RAID
controlers. Device drivers for some of them are part of official Linux
kernel, for others can be downloaded from Adaptec web site. So if you
want hardware RAID solution based on SATA disks, that is the only way to
go currently. If buying Adaptec, be carefull. The cheap SATA RAID card
they sell is software RAID. You need to buy one of the more expensive
ones to get hardware RAID.
Also, there's couple of "accelerated SATA RAID controlers" around.
Those are software RAIDs too, special device drivers needed, with some
accleration done in hardware. People who tested them reported that they
are slower than standard Linux software RAID (implemented by md device
driver).
--
Aleksandar Milivojevic <amilivojevic at pbl.ca> Pollard Banknote Limited
Systems Administrator 1499 Buffalo Place
Tel: (204) 474-2323 ext 276 Winnipeg, MB R3T 1L7
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