Hardware RAID Level & Performance

[Aleksandar Milivojevic]

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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