7/2/2003 This page gets a lot of hits, so we tried to clean it up a bit.
Our main purpose of the mirror is to backup the main disk, but not as a hot standby. The mirror will be removed after synchronized and put in a secure place. The only major changes will be to the data, which can be backed up by CD. So, a monthly mirror re-sync is envisioned. The other goal is have the mirrored drive simply installed in another PC, then let it find new hardware and a new server is up and running.
Disk mirroring was accomplished - with much pain! So, the process we used follows: The drives for mirrors chosen are a Master IDE of 20 GB and a Slave IDE of 13GB. The mirroring for Linux is a mirroring of partitions, not drives. So, the Master Boot Record (MBR) is not part of the mirror, therefore the mirrored drive is not inherently bootable. The process of making a mirror destroys all present data on both drives. So boot from Linux CD Disk1 and use FDISK and Disk Druid (no program exists on disk by that name, a Redhat phenom) , which can be easier than using " mkraid " command.
When you get to the point of choosing between FDISK or Disk Druid, choose FDISK (don't worry, you will use both!) FDISK is easier to sort out the partitions. You may want to get familiar with FDISK at the shell prompt before this whole process. It was hard to see the FDISK screen in the Linux Setup.
There are two commands for disk partitions /sbin/fdisk and /sbin/parted. Neither one seems complete for setting a disk mirror. Parted almost does but uses disk sizes instead of cylinders - unwieldy. To set up partition sizes, you need to work backward starting from the swap size (swap = 2X RAM [256MB RAM in this case] ). The root (/ ) sectors = boot (/boot ) + 1, to max cylinders - swap +1. The number of swap and boot cylinders was taken from the original default Linux setup, read with FDISK.
So for example, the 13GB (limit for both drives) had 1665 Cylinders. From a previous Linux install, the /boot partition had 13 cylinders (approx. 100MB) and the swap had 66 cylinders (approx. 500MB), so the / directory (root) was set from 14 to 1599 cylinders.
The partitions on both drives were setup, using fdisk, before doing Disk Druid, as follows (hda = 20GB, hdb = 13GB):
Disk /dev/hda: 255 heads, 63 sectors, 2431 cylinders (20 GB Western Digital)Units = cylinders of 16065 * 512 bytes
Partition table (* = boot partition):
Device Boot Start End Blocks Id System
/dev/hda1 * 1 13 104391 fd Linux raid autodetect
/dev/hda2 14 1599 12739545 fd Linux raid autodetect
/dev/hda3 1600 1665 530145 fd Linux raid autodetect
Disk /dev/hdb: 255 heads, 63 sectors, 1665 cylinders (13 GB Matrox)
Units = cylinders of 16065 * 512 bytes
Device Boot Start End Blocks Id System
/dev/hdb1 * 1 13 104391 fd Linux raid autodetect
/dev/hdb2 14 1599 12739545 fd Linux raid autodetect
/dev/hdb3 1600 1665 530145 fd Linux raid autodetect
All partitions are set with ID = fd - Linux raid autodetect.
Drive size in bytes = cylinders X 16065 X 512
hda = 19,995,655,680 bytes
hda1 (/boot - 13 cylinders) = 106,928,640 bytes
hda2 (/ - 1586 cylinders) = 13,045,294,080 bytes
hda3 (swap - 66 cylinders) = 542,868,480 bytes
So, 6,300,564,480 bytes on the 20GB are unused. The whole Maxtor 13GB drive is used. Both /boot and swap sizes are apparently generous. You can have 4 partitions, but many volumes.
Using FDISK -ul /dev/hda shows the file system on each partition - as PARTED does.
# /sbin/parted /dev/hdb > parted.txt (then P enter, Q enter, to write to parted.txt)
GNU Parted 1.4.24
Using /dev/hdb
Information: The operating system thinks the geometry on /dev/hdb is 1665/255/63.
Therefore, cylinder 1024 ends at 8032.499M.
(parted) p
Disk geometry for /dev/hdb: 0.000-13063.640 megabytes
Disk label type: msdos
Minor Start End Type Filesystem Flags
1 0.031 101.975 primary ext3 boot, raid
2 101.975 12542.937 primary ext3 raid
3 12542.937 13060.656 primary linux-swap raid
After the partitions are set (don't forget to set the boot partition, and write the data) hit the back button and choose Disk Druid.
The pain comes from not paying close attention to the settings in Disk Druid. We wound up re-loading Linux with only the packages wanted, several times. There is no Disk Druid settings memory for RAID configurations, as there is for FDISK partition data. There looks like there is Disk Druid settings memory, but watch out for the mount point being dropped (e.g. /boot, /) and the RAID number (RAID1 for mirror). So, check all of the settings.
In Disk Druid, each partition is set as Linux raid and after the RAID button is pressed and partitions selected to mirror ( ex. /dev/hda1 & /dev/hdb1), the file system is selected (ex. format as ext3 or swap). Remember to pick RAID1 and the mount point ([/boot] and [/] [no mount point for swap partition])!
After Linux was installed, #cat /proc/mdstat showed that md1 & md0 were not mirrored ([2/1] [U_] - (they are in the below example).
Personalities : [raid1]
read_ahead 1024 sectors
md2 : active raid1 hda3[0] hdb3[1]
530048 blocks [2/2] [UU]
md1 : active raid1 hdb2[1] hda2[0]
12739456 blocks [2/2] [UU]
md0 : active raid1 hdb1[1] hda1[0]
104320 blocks [2/2] [UU]
unused devices: <none>
A look at the System Log showed that there was a " superblock update time inconsistency " with hda1 & hdb1; hda2 & hdb2. So, hdb1 & hdb2 were kicked out of the mirror.
The fix is to manually put the drive kicked in the mirror: # /sbin/raidhotadd /dev/md1 /dev/hdb2 .... etc.
All the RAID info is hard to piece together - trial & error on the internet.
There were a couple of ways found to get a mirrored drive bootable, from the internet. Could not get LILO bootable to work except by doing the following (GRUB is below):
With the IDE drives on different buses (hda is the bootable), at the shell prompt type: " dd if=/dev/hda of=/dev/hdc " hit Enter key and wait for approx. 1 minute, then do Ctrl + C to stop the process. The two drives have different disk parameters (less cylinders on the 13 GB). One minute is enough time to copy the MBR, and then some.
Possible data corruption could have occurred on the mirrored drive, so then re-synched the mirror partitions: " /sbin/raidhotadd /dev/md0 /dev/hdc1 " and so on for all the partitions. The boot on the 13GB worked. If both drives are the same, you can just let the dd command finish (a few hours?) and you will have a bootable mirror.
If you have a problem getting the system to boot-off and load a good mirror instead of the damaged disk (superblock being newer - the kernel kept picking the damaged disk). Run fdisk and change the type ID on the bad disk to 83 - hdc1(/boot), 83 - hdc2 (/ ), 82 - hdc3 (swap), for the 3 partitions on the bad disk. Re-boot with the good disk as primary master (hda), then with the good disk loaded change the bad disk partitions back to ID type fd - Linux raid auto-detect. Then raidhotadd the partitions of the bad disk to the good.