Mailinglist Archive: opensuse (2348 mails)

< Previous Next >
Re: [opensuse] Software IDE RAID versus hardware IDE RAID
  • From: Richard Creighton <ricreig@xxxxxxxxx>
  • Date: Thu, 03 Apr 2008 05:54:18 -0400
  • Message-id: <47F4A94A.9090404@xxxxxxxxx>
David C. Rankin wrote:
Sandy Drobic wrote:

On modern systems the cpu load is no longer an argument, even if you were to use software RAID5. The real argument is ease of use:
software raid cannot include /boot, so you have to provide redundancy for /boot in some other way.


Hmmm. Sandy,

I don't know if that is true anymore. The last 3 software raid systems I set up, using Yast, everything except swap was included in the raid array. That included / /boot and /home. I agree that you are only using one disk to "boot" from with grub using:

title openSUSE 10.3 - 2.6.23.17-ccj64
root (hd0,4)

However, hd0 is mapped to the raid array where:

[22:40 nirvana/boot] # cat grub/device.map
(hd0) /dev/mapper/nvidia_hacfgfda

So I am booting from the 5th partition (or the pc slice number in the BSD terminology) where "4" the partition is counted from "0". hd0 is the software raid array built from /dev/sda and /dev/sdb in raid1:

22:28 nirvana~> sudo dmraid -r
/dev/sda: nvidia, "nvidia_hacfgfda", mirror, ok, 976773166 sectors, data@ 0
/dev/sdb: nvidia, "nvidia_hacfgfda", mirror, ok, 976773166 sectors, data@ 0

with partitions:

22:18 nirvana~> cat /etc/fstab
/dev/mapper/nvidia_hacfgfda_part7 / ext3 acl,user_xattr 1 1
/dev/mapper/nvidia_hacfgfda_part5 /boot ext3 acl,user_xattr 1 2
/dev/mapper/nvidia_hacfgfda_part8 /home ext3 acl,user_xattr 1 2
/dev/mapper/nvidia_hacfgfda_part6 swap swap defaults 0 0

So in my case, the system is booting off from (hd0,4) or the 5th partition on the raid array with is /boot which is mirrored between /dev/sda and /dev/sdb.

I don't know whether grub has internal logic to select an individual disk to boot from, but I believe that the physical disk boot selection is governed by the BIOS selection of hard disk boot priority.

So it appears that /boot is mirrored and also used to boot as well.


David,
I think you are right. I have a RAID only system under 10.3. I think the only constraint is that the partition that contains /boot needs to be mirrored as a RAID 1. In my case / and /home and all the rest except for /boot are in a RAID 5 1.5TB array. I even have SWAP in a RAID 0 array (that was just to see if it would work for 'purity' sake). I know some say Swap should not be in a RAID n because it can do it itself, but for sake of both testing and disk management of the 4 disks I use as arrays, it was expedient to do and it works flawlessly. I DO NOT have ANY separate partititon used for the purpose of booting. The only concession I had to make was to ensure that the MBR was written to BOTH sda and sdb or it was a crap-shoot as to which one BIOS would pick for initial boot before the OS got involved. Usually, it would work, but sometimes I had to reset several times before I tried the workaround. Now it is rock solid. Don't ask me why, as the theory as I understand it says the BIOS shouldn't know or care about SuSE's software raid schemes. In addition, I have a HARDWARE RAID 5 controller for an additional 4 drives with another 2 TB of space which becomes available as soon as the OSs loads the driver module at boot time, but I can't boot from those drives because BIOS can't see them. I think the support for mirrored /boot started with 10.2 but I didn't start playing with it until beta of 10.3. Support for 'fake-raid' on the motherboard is 'iffy' in SuSE. I've seen reports where it works, but on my ASUS, as of 10.3, it doesn't work correctly. Maybe 11.0 will work. I bought another set of drives to test that and I have a 2nd MB with a 'fake-raid' controller that I will use to test it. I'll probably switch to software only raid regardless so I don't lose everything if the MB goes south and the replacement uses a different support scheme, but it would be nice to know if it works :)
--
To unsubscribe, e-mail: opensuse+unsubscribe@xxxxxxxxxxxx
For additional commands, e-mail: opensuse+help@xxxxxxxxxxxx

< Previous Next >