On Thursday, Aug 7, 2003, at 13:25 Europe/Amsterdam, Dave Anderson wrote:
Hello,
I've had a good look round at the howtos and the archives of this list, and I'd like to try and summarise the situation with PDC20276 as I understand it. I am fairly new to RAID, but not Linux.
I just got ataraid working with RH7.3 (plus all updates) and kernel 2.4.20-19.7 (custom, with drivers compiled in)
As I understand it, the hardware controller does no hardware raid. This was a big surprise to me, as I thought it was a RAID controller, but I guess the intention was to always do it in the driver? (even though the PDC bios is happy to tell me it has a raid1 mirrored array)
I have seen a post implying that hardware raid is coming for this controller. Is that correct?
I also saw that the pdc and ataraid drivers crash the system if a disk goes. I know you get what you pay for with OS, but this is a real shame. I need security not performance, and I guess the only use of RAID1 here is that I still get a working disk after the reboot. At least I can autonotify if the system crashes, to find out about the disk ;-) small consolation.
In the light of this, what use is the PDC hardware. Is it not just mainly an IDE controller, or have I missed something.
Is the equivalent of what I have not the same as two ide drives and md software raid? I read that md does not crash the system if it loses a disk. Why would anyone then use ataraid? Is that easier to use with 'raid' controllers.
If I wanted to switch from ataraid to md, is that easy with my PDC20276. Should I keep it in RAID mode (or IDE) Should I just have the kernel see two ide drives, as it currently does, and use md (yes, I won't mix ide and ataraid ;-)
Many thanks for any answers - I'm trying to get my head round all this.
The card supports booting from either one of the two disks, but uses software to actually do the write to both.
Since there is no abstraction layer and/or this is not done in hardware, technically speaking, it is not doing
hardware raid. However, commercially speaking, it is, since it uses hardware to do the boot from either disk
and there is raid. If you look at it from Promises viewpoint, it does do hardware raid, if you look at it from a coders
viewpoint, it doesn't.
As far as crashing disks crashing your system, I don't know, I suggest you try on a non-production box if you
want to be sure. I know that promise more or less claims you can do an online rebuild of your raid1 on
windows2000 if you have the promise tools installed and a hotswap compliant drivecasing for your drives.
However, I do not know about the read/write or readonly status of the array under these conditions.
What use it is, well, it helps you in protecting your data and keeping your downtime due to a diskfailure
to a miminum.This because you still have a bootable disk with all your data on it if the other one goes. Other
then that, it's not really such a fantastic device in my opinion.
As far as I know, software raid is not able to do a reboot from a single disk of a mirror set, or do an online
(r/w)rebuild of the array, but I would love to be corrected on this. Preferably with configuration examples or
pointers to decent documentation on this. :)
Switching to MD from ataraid, I'd advise to just let the kernel see two drives and forget about ataraid if you
want to go to MD. Ataraid devices are scary and currently do not properly flush hd write caches before
shutdown/reboot. They should be avoided unless you have a multiboot system with windows and want to
be able to use the drives on the ataraid while running linux.
Homme
