Subject: Re: Abit KT7A-RAID/HPT370/2.4.14 questions... Of software RAIDs and sealing wax
From: civileme (civileme@mandrakesoft.com)
Date: Mon Nov 26 2001 - 11:27:41 AKST
OK if you want a hardware IDE RAID, there is one, and it is OS transparent.
http://www.arcoide.com/productspages/products.html
There is nothing else that is a pure hardware IDE RAID.
Now we have controllers from CMD, Promise, Highpoint, and at least one other
plus 3ware. The 3ware controller, while it existed, had GPL drivers. Those
are all to some extent Software RAID. Here's how tey do it:
Assume this line is your disk space:
M-----------------------------
where M is te master boot record.
After the controller does its thing,
MR--------------------------R
where the R are date and timestamp info. Also your drive has been destroked
in the BIOS to protect those areas. There is also a littel info there to
toggle which lie the controller will tell the system about wat is real and
what is not.
The result is that you have precisely one RAID extent, because of this
design. The R at the front of the disk is useless because LILO stomps all
over it, but the one at the other end is the one used (originally, only the
beginning was used so very early controllers are rotally incompatible with
linux and in fact their RAIDs are destroyed by an attempted install.)
OK let's look at the software RAIDs
"Hardware " Cards Linux Software RAID
Extents 1 >16
RAID Types 0, 1 0+1 0,1,4,5
Identical drives Yes Irrelevant
Volume mgr No Volumes can be pieces of
up to 16 drives
Setup at install No, requires kernel mods Yes
Cost $30-100 extra Your time
Journaling filesystems will work with both.
If you really want to use the RAID controller in RAID mode, even after all of
this, there is some _experimental_ software to do so here:
http://people.redhat.com/arjanv/pdcraid/
Of course, this is not for the faint of heart. The one advantage of having
theis software probably is with Promise FastTrak controllers which do not
fully work as IDE controllers in linux. The most useful feature is
compatibility with Windows RAID for file exchange on a stand-alone dual-boot
system. (A small network has better alternatives).
RAID0 does perform better for two drives as far as access time goes. It
performs even better if the drives are on different IDE channels, which is
achievable with linux Software RAID, but not with the controllers I have
tested.
Finally, all of the IDE-RAID controllers are closed-source at this time.
They may all admit that linux software RAID is better in implementation than
theirs, but they are sorely afraid of giving their competitiors advantage in
the Windows market.
WinRAIDs, anyone?
Civileme
This archive was generated by hypermail 2a23 : Mon Nov 26 2001 - 11:24:16 AKST