CentOS - Nov 2005 - Turning root partition into a RAID array

I have a CentOS 4.2 system that was set up VERY quickly following the demise
of its former life as a CentOS 3 server - you don't want the full story, but
it had to be done quickly to get a company up and working following a slight
disaster involving an electrician, a portable appliance safety tester and a
pulled power cable - anyway, here's where I am at...

Everything is running fine but I now want to mirror the root partition
(/dev/hda3). I have read quite a few set of notes on the theory but since
they contain conflicting advice I am hoping someone here can set me straight
- here's what IO want to do:

hda3 and hde3 become a mirrored pair (/dev/md0) and this is mounted at boot
time as /

Is it as simple as a quick mdadm command and then an edit of /etc/inittab??

If I try and use mdadm right now I get complaints that /dev/hda3 is busy
(fair enough) so I guess I need to be in less than 'full steam ahead'
mode
to make the change - over to you....

Thanks

Once you've mdadm and done you're other stuff you can use grub, to 
install a boot loader on each disk, if you are using grub...

You might find this excerpt useful:

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  From: Michael Martucci <m.martucci at gmx.net>
  To  : <linuxsa at linuxsa.org.au>
  Date: Tue, 29 Jul 2003 14:44:46 +0930

GRUB + RAID howto
 In the Software RAID howto it is mentioned that it is not known how
to set up GRUB to boot off RAID. Here is how I did it:
**Follow at your own risk. If you break something it's your fault.**
=================================================================Configuration:
 - /dev/hda (Pri. Master) 60 GB Seagate HDD (blank)
 - /dev/hdc (Sec. Master) 60 GB Seagate HDD (blank)
 - /dev/hdd (Sec. Slave) CDROM Drive

Setup Goals:
 - /boot as /dev/md0: RAID1 of /dev/hda1 & /dev/hdc1 for redundancy
 - /     as /dev/md1: RAID1 of /dev/hda2 & /dev/hdc2 for redundancy
 - swap*2 with equal priority: /dev/hda3 & /dev/hdc3 for more speed
 - GRUB installed in boot records of /dev/hda and /dev/hdc so either
    drive can fail but system still boot.

Tools:
 - mdadm (http://www.cse.unsw.edu.au/~neilb/source/mdadm/)
   (I used 1.2.0, but notice that as of 20030729 1.3.0 is available)

1. Boot up off rescue/installation CD/disk/HDD/whatever with mdadm
tools installed.

2. Partitioning of hard drives:
(I won't show you how to do this. See: # man fdisk ; man sfdisk )
But here's how stuff was arranged:
------------------------------------------------------------------
# sfdisk -l /dev/hda

Disk /dev/hda: 7297 cylinders, 255 heads, 63 sectors/track
Units = cylinders of 8225280 bytes, blocks of 1024 bytes, counting
from 0

  Device Boot Start   End  #cyls   #blocks  Id System
/dev/hda1  *      0+   16     17-   136521  fd Linux raid autodetect
/dev/hda2        17  7219   7203  57858097+ fd Linux raid autodetect
/dev/hda3      7220  7296     77    618502+ 82 Linux swap
/dev/hda4         0     -      0         0   0 Empty
------------------------------------------------------------------
To make /dev/hdc the same:
------------------------------------------------------------------
# sfdisk -d /dev/hda | sfdisk /dev/hdc
------------------------------------------------------------------
/dev/hd[ac]1 for /dev/md0 for /boot
/dev/hd[ac]2 for /dev/md1 for /
/dev/hd[ac]3 for 2*swap
It is important to make md-to-be partitions with ID 0xFD, not 0x83.

3. Set up md devices: (both are RAID1 [mirrors])
------------------------------------------------------------------
# mdadm --create /dev/md0 --level=1 \
    --raid-devices=2 /dev/hda1 /dev/hdc1
# mdadm --create /dev/md1 --level=1 \
    --raid-devices=2 /dev/hda2 /dev/hdc2
------------------------------------------------------------------

4. Make filesystems:
------------------------------------------------------------------
# mke2fs /dev/md0
# mkreiserfs /dev/md1
# mkswap /dev/hda3
# mkswap /dev/hdc3
------------------------------------------------------------------

5. Install Your distribution:
 Simply treat /dev/md0 and /dev/md1 as the partitions to install on,
and install the way your normally do. Eg, for Gentoo:
------------------------------------------------------------------
# mkdir newinst
# mount -t reiserfs /dev/md1 ./newinst
# cd newinst
# mkdir boot
# mount -t ext2 /dev/md0 ./boot
# tar -xvjpf ../stage1-x86-1.4_rc2.tbz2
# mount -o bind /proc ./proc
# chroot ./
...
------------------------------------------------------------------
Here're the relevant entries /etc/fstab for the newly created
partitions:
------------------------------------------------------------------
/dev/md0      /boot        ext2       noauto,noatime          1 1
/dev/md1      /        reiserfs       noatime                 1 1
/dev/hda3     none         swap       sw,pri=1                0 0
/dev/hdc3     none         swap       sw,pri=1                0 0
------------------------------------------------------------------
The "pri=1" for each of the swap partitions makes them the same
priority so the kernel does striping and that speeds up vm. Of
course, this means that if a disk dies then the system may crash,
needing a reboot. Perhaps it would be wiser to make hd[ac]3 a RAID1
array too, and just use /dev/md2 as swap.

6. Setting up GRUB: (assuming you've already installed it)
------------------------------------------------------------------
# grub
grub> root (hd0,0)
 Filesystem type is ext2fs, partition type 0xfd

grub> setup (hd0)
 Checking if "/boot/grub/stage1" exists... yes
 Checking if "/boot/grub/stage2" exists... yes
 Checking if "/boot/grub/e2fs_stage1_5" exists... yes
 Running "embed /boot/grub/e2fs_stage1_5 (hd0)"...  16 sectors are
embedded.
succeeded
 Running "install /boot/grub/stage1 (hd0) (hd0)1+16 p
(hd0,0)/boot/grub/stage2 /boot/grub/grub.conf"... succeeded
Done.

grub> root (hd1,0)
 Filesystem type is ext2fs, partition type 0xfd

grub> setup (hd1)
 Checking if "/boot/grub/stage1" exists... yes
 Checking if "/boot/grub/stage2" exists... yes
 Checking if "/boot/grub/e2fs_stage1_5" exists... yes
 Running "embed /boot/grub/e2fs_stage1_5 (hd1)"...  16 sectors are
embedded.
succeeded
 Running "install /boot/grub/stage1 (hd1) (hd1)1+16 p
(hd1,0)/boot/grub/stage2 /boot/grub/grub.conf"... succeeded
Done.

grub> quit
------------------------------------------------------------------
Here is how /boot/grub/grub.conf is: (/dev/md0 mounted as /boot)
(Assuming kernel is installed as /boot/bzImage, and RAID1 support
compiled into the kernel).
------------------------------------------------------------------
# Boot automatically after 30 secs.
timeout 30

# By default, boot the first entry.
default 0

# Fallback to the second entry.
fallback 1

# For booting with disc 0 kernel
title  GNU/Linux (hd0,0)
kernel (hd0,0)/bzImage root=/dev/md1

# For booting with disc 1 kernel, if (hd0,0)/bzImage is unreadable
title  GNU/Linux (hd1,0)
kernel (hd1,0)/bzImage root=/dev/md1
------------------------------------------------------------------

Now you should be able to reboot your system and play!
=================================================================
Please let me know of any errors, feedback, etc.

 Michael Martucci.

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Nigel kendrick wrote:
>I have a CentOS 4.2 system that was set up VERY quickly following the demise
>of its former life as a CentOS 3 server - you don't want the full story,
but
>it had to be done quickly to get a company up and working following a slight
>disaster involving an electrician, a portable appliance safety tester and a
>pulled power cable - anyway, here's where I am at...
>
>Everything is running fine but I now want to mirror the root partition
>(/dev/hda3). I have read quite a few set of notes on the theory but since
>they contain conflicting advice I am hoping someone here can set me straight
>- here's what IO want to do:
>
>hda3 and hde3 become a mirrored pair (/dev/md0) and this is mounted at boot
>time as /
>
>Is it as simple as a quick mdadm command and then an edit of /etc/inittab??
>
>If I try and use mdadm right now I get complaints that /dev/hda3 is busy
>(fair enough) so I guess I need to be in less than 'full steam
ahead' mode
>to make the change - over to you....
>
>Thanks
>
>_______________________________________________
>CentOS mailing list
>CentOS at centos.org
>http://lists.centos.org/mailman/listinfo/centos
>  
>

you probably have to boot from the cd using rescue mode as well:

at the cd boot prompt:

linux rescue

this will release the "busy" devices as you can unmount them as you
go...

P.



Nigel kendrick wrote:
>I have a CentOS 4.2 system that was set up VERY quickly following the demise
>of its former life as a CentOS 3 server - you don't want the full story,
but
>it had to be done quickly to get a company up and working following a slight
>disaster involving an electrician, a portable appliance safety tester and a
>pulled power cable - anyway, here's where I am at...
>
>Everything is running fine but I now want to mirror the root partition
>(/dev/hda3). I have read quite a few set of notes on the theory but since
>they contain conflicting advice I am hoping someone here can set me straight
>- here's what IO want to do:
>
>hda3 and hde3 become a mirrored pair (/dev/md0) and this is mounted at boot
>time as /
>
>Is it as simple as a quick mdadm command and then an edit of /etc/inittab??
>
>If I try and use mdadm right now I get complaints that /dev/hda3 is busy
>(fair enough) so I guess I need to be in less than 'full steam
ahead' mode
>to make the change - over to you....
>
>Thanks
>
>_______________________________________________
>CentOS mailing list
>CentOS at centos.org
>http://lists.centos.org/mailman/listinfo/centos
>  
>

Nigel kendrick <support-lists at petdoctors.co.uk>
wrote:
> I have a CentOS 4.2 system that was set up VERY quickly
> following the demise of its former life as a CentOS 3
server -
> you don't want the full story, but it had to be done
quickly
> to get a company up and working following a slight disaster
> involving an electrician, a portable appliance safety
tester
> and a pulled power cable - anyway, here's where I am at...
Er, um, would RAID solve that problem?  I.e., did you fry the
drive or get physical errors?  Or did the filesystem get
toasted?  I mean, drive failures are one thing, but
electrical/surges are a completely other thing (and RAID
might not help you there ;-).
> If I try and use mdadm right now I get complaints that
> /dev/hda3 is busy (fair enough) so I guess I need to be in
> less than 'full steam ahead' mode to make the change - over
> to you....
As many other people have pointed out, you'll need to have
the system off-line to do this.  The Rescue CD has pretty
much everything you need -- from mdadm to grub.  There are a
few HOWTOS, although I find some of their example hardware
configurations to be a bit disturbing (like putting an ATAPI
device as a slave to an ATA drive).

It used to be more of an issue when /boot (with the initrd)
couldn't be a md device, but that no longer seems to be a
constraint.

-- Bryan

P.S.  I'll reiterate that a $100-125 3Ware Escalade 7006-2
(ATA) or 8006-2 (SATA) card gives you quite "piece of mind"
for the price when it comes to RAID-1.  And since it doesn't
use block striping (unlike RAID-0, 10 or 5), you can remove
the disks from the RAID-1 volume and use them without the
card.

With 3Ware, you don't have the boot-time issues, you don't
have the ATA chipset/drive timeout issues, and the admin
tools do their job quite well (with more and more standard
support software like smartd being able to monitoring 3Ware
kernel message, etc... as well).


-- 
Bryan J. Smith                | Sent from Yahoo Mail
mailto:b.j.smith at ieee.org     |  (please excuse any
http://thebs413.blogspot.com/ |   missing headers)

RIP (Recovery is Possible) also has a script to allow you to install and
boot
it from USB flash/pen drive.

Chris
 
 
>>>spamcatcher at lantrix.no 11/15/05 1:03 pm >>> 
>That's the ticket.   I wonder if anyone has tried building a
"rescue
USB 
 
>thumb drive" since the 1GB devices are so cheap these days.  That would
>be great for this kind of situation. 
 
FWIW, I'm booting my firewall from an usb pen drive with devil-linux 
installed, so a rescue environment should be doable. But then again, 
devil-linux works like a rescue environment for CentOS anyway. :) 
 
Regards, 
Harald 
 
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