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7. Tweaking, tuning and troubleshooting

7.1 raid-level and raidtab

Some GNU/Linux distributions, like RedHat 8.0 and possibly others, have a bug in their init-scripts, so that they will fail to start up RAID arrays on boot, if your /etc/raidtab has spaces or tabs before the raid-level keywords.

The simple workaround for this problem is to make sure that the raid-level keyword appears in the very beginning of the lines, without any leading spaces of any kind.

7.2 Autodetection

Autodetection allows the RAID devices to be automatically recognized by the kernel at boot-time, right after the ordinary partition detection is done.

This requires several things:

  1. You need autodetection support in the kernel. Check this
  2. You must have created the RAID devices using persistent-superblock
  3. The partition-types of the devices used in the RAID must be set to 0xFD (use fdisk and set the type to "fd")

NOTE: Be sure that your RAID is NOT RUNNING before changing the partition types. Use raidstop /dev/md0 to stop the device.

If you set up 1, 2 and 3 from above, autodetection should be set up. Try rebooting. When the system comes up, cat'ing /proc/mdstat should tell you that your RAID is running.

During boot, you could see messages similar to these:

 Oct 22 00:51:59 malthe kernel: SCSI device sdg: hdwr sector= 512
  bytes. Sectors= 12657717 [6180 MB] [6.2 GB]
 Oct 22 00:51:59 malthe kernel: Partition check:
 Oct 22 00:51:59 malthe kernel:  sda: sda1 sda2 sda3 sda4
 Oct 22 00:51:59 malthe kernel:  sdb: sdb1 sdb2
 Oct 22 00:51:59 malthe kernel:  sdc: sdc1 sdc2
 Oct 22 00:51:59 malthe kernel:  sdd: sdd1 sdd2
 Oct 22 00:51:59 malthe kernel:  sde: sde1 sde2
 Oct 22 00:51:59 malthe kernel:  sdf: sdf1 sdf2
 Oct 22 00:51:59 malthe kernel:  sdg: sdg1 sdg2
 Oct 22 00:51:59 malthe kernel: autodetecting RAID arrays
 Oct 22 00:51:59 malthe kernel: (read) sdb1's sb offset: 6199872
 Oct 22 00:51:59 malthe kernel: bind<sdb1,1>
 Oct 22 00:51:59 malthe kernel: (read) sdc1's sb offset: 6199872
 Oct 22 00:51:59 malthe kernel: bind<sdc1,2>
 Oct 22 00:51:59 malthe kernel: (read) sdd1's sb offset: 6199872
 Oct 22 00:51:59 malthe kernel: bind<sdd1,3>
 Oct 22 00:51:59 malthe kernel: (read) sde1's sb offset: 6199872
 Oct 22 00:51:59 malthe kernel: bind<sde1,4>
 Oct 22 00:51:59 malthe kernel: (read) sdf1's sb offset: 6205376
 Oct 22 00:51:59 malthe kernel: bind<sdf1,5>
 Oct 22 00:51:59 malthe kernel: (read) sdg1's sb offset: 6205376
 Oct 22 00:51:59 malthe kernel: bind<sdg1,6>
 Oct 22 00:51:59 malthe kernel: autorunning md0
 Oct 22 00:51:59 malthe kernel: running: <sdg1><sdf1><sde1><sdd1><sdc1><sdb1>
 Oct 22 00:51:59 malthe kernel: now!
 Oct 22 00:51:59 malthe kernel: md: md0: raid array is not clean --
  starting background reconstruction 
This is output from the autodetection of a RAID-5 array that was not cleanly shut down (eg. the machine crashed). Reconstruction is automatically initiated. Mounting this device is perfectly safe, since reconstruction is transparent and all data are consistent (it's only the parity information that is inconsistent - but that isn't needed until a device fails).

Autostarted devices are also automatically stopped at shutdown. Don't worry about init scripts. Just use the /dev/md devices as any other /dev/sd or /dev/hd devices.

Yes, it really is that easy.

You may want to look in your init-scripts for any raidstart/raidstop commands. These are often found in the standard RedHat init scripts. They are used for old-style RAID, and has no use in new-style RAID with autodetection. Just remove the lines, and everything will be just fine.

7.3 Booting on RAID

There are several ways to set up a system that mounts it's root filesystem on a RAID device. Some distributions allow for RAID setup in the installation process, and this is by far the easiest way to get a nicely set up RAID system.

Newer LILO distributions can handle RAID-1 devices, and thus the kernel can be loaded at boot-time from a RAID device. LILO will correctly write boot-records on all disks in the array, to allow booting even if the primary disk fails.

If you are using grub instead of LILO, then just start grub and configure it to use the second (or third, or fourth...) disk in the RAID-1 array you want to boot off as its root device and run setup. And that's all.

For example, on an array consisting of /dev/hda1 and /dev/hdc1 where both partitions should be bootable you should just do this:

grub>device (hd0) /dev/hdc
grub>root (hd0,0)       
grub>setup (hd0)

Some users have experienced problems with this, reporting that although booting with one drive connected worked, booting with both two drives failed. Nevertheless, running the described procedure with both disks fixed the problem, allowing the system to boot from either single drive or from the RAID-1

Another way of ensuring that your system can always boot is, to create a boot floppy when all the setup is done. If the disk on which the /boot filesystem resides dies, you can always boot from the floppy. On RedHat and RedHat derived systems, this can be accomplished with the mkbootdisk command.

7.4 Root filesystem on RAID

In order to have a system booting on RAID, the root filesystem (/) must be mounted on a RAID device. Two methods for achieving this is supplied bellow. The methods below assume that you install on a normal partition, and then - when the installation is complete - move the contents of your non-RAID root filesystem onto a new RAID device. Please not that this is no longer needed in general, as most newer GNU/Linux distributions support installation on RAID devices (and creation of the RAID devices during the installation process). However, you may still want to use the methods below, if you are migrating an existing system to RAID.

Method 1

This method assumes you have a spare disk you can install the system on, which is not part of the RAID you will be configuring.

If you're doing this with IDE disks, be sure to tell your BIOS that all disks are "auto-detect" types, so that the BIOS will allow your machine to boot even when a disk is missing.

Method 2

This method requires that your kernel and raidtools understand the failed-disk directive in the /etc/raidtab file - if you are working on a really old system this may not be the case, and you will need to upgrade your tools and/or kernel first.

You can only use this method on RAID levels 1 and above, as the method uses an array in "degraded mode" which in turn is only possible if the RAID level has redundancy. The idea is to install a system on a disk which is purposely marked as failed in the RAID, then copy the system to the RAID which will be running in degraded mode, and finally making the RAID use the no-longer needed "install-disk", zapping the old installation but making the RAID run in non-degraded mode.

7.5 Making the system boot on RAID

For the kernel to be able to mount the root filesystem, all support for the device on which the root filesystem resides, must be present in the kernel. Therefore, in order to mount the root filesystem on a RAID device, the kernel must have RAID support.

The normal way of ensuring that the kernel can see the RAID device is to simply compile a kernel with all necessary RAID support compiled in. Make sure that you compile the RAID support into the kernel, and not as loadable modules. The kernel cannot load a module (from the root filesystem) before the root filesystem is mounted.

However, since RedHat-6.0 ships with a kernel that has new-style RAID support as modules, I here describe how one can use the standard RedHat-6.0 kernel and still have the system boot on RAID.

Booting with RAID as module

You will have to instruct LILO to use a RAM-disk in order to achieve this. Use the mkinitrd command to create a ramdisk containing all kernel modules needed to mount the root partition. This can be done as:

 mkinitrd --with=<module> <ramdisk name> <kernel>
For example:
 mkinitrd --preload raid5 --with=raid5 raid-ramdisk 2.2.5-22

This will ensure that the specified RAID module is present at boot-time, for the kernel to use when mounting the root device.

Modular RAID on Debian GNU/Linux after move to RAID

Debian users may encounter problems using an initrd to mount their root filesystem from RAID, if they have migrated a standard non-RAID Debian install to root on RAID.

If your system fails to mount the root filesystem on boot (you will see this in a "kernel panic" message), then the problem may be that the initrd filesystem does not have the necessary support to mount the root filesystem from RAID.

Debian seems to produce its initrd.img files on the assumption that the root filesystem to be mounted is the current one. This will usually result in a kernel panic if the root filesystem is moved to the raid device and you attempt to boot from that device using the same initrd image. The solution is to use the mkinitrd command but specifying the proposed new root filesystem. For example, the following commands should create and set up the new initrd on a Debian system:

% mkinitrd -r /dev/md0 -o /boot/initrd.img-2.4.22raid
% mv /initrd.img /initrd.img-nonraid
% ln -s /boot/initrd.img-raid /initrd.img"

7.6 Converting a non-RAID RedHat System to run on Software RAID

This section was written and contributed by Mark Price, IBM. The text has undergone minor changes since his original work.

Notice: the following information is provided "AS IS" with no representation or warranty of any kind either express or implied. You may use it freely at your own risk, and no one else will be liable for any damages arising out of such usage.


The technote details how to convert a linux system with non RAID devices to run with a Software RAID configuration.


This scenario was tested with Redhat 7.1, but should be applicable to any release which supports Software RAID (md) devices.

Pre-conversion example system

The test system contains two SCSI disks, sda and sdb both of of which are the same physical size. As part of the test setup, I configured both disks to have the same partition layout, using fdisk to ensure the number of blocks for each partition was identical.

/dev/sda1   /           2048MB      /dev/sdb1               2048MB
/dev/sda2   /boot       80MB        /dev/sdb2               80MB
/dev/sda3   /var/       100MB       /dev/sdb3               100MB
/dev/sda4   SWAP        1024MB      /dev/sdb4   SWAP        1024MB
In our basic example, we are going to set up a simple RAID-1 Mirror, which requires only two physical disks.

Step-1 - boot rescue cd/floppy

The redhat installation CD provides a rescue mode which boots into linux from the CD and mounts any filesystems it can find on your disks.

At the lilo prompt type

    lilo: linux rescue

With the setup described above, the installer may ask you which disk your root filesystem in on, either sda or sdb. Select sda.

The installer will mount your filesytems in the following way.

/dev/sda1   /           /mnt/sysimage
/dev/sda2   /boot       /mnt/sysimage/boot
/dev/sda3   /var        /mnt/sysimage/var
/dev/sda6   /home       /mnt/sysimage/home

Note: - Please bear in mind other distributions may mount your filesystems on different mount points, or may require you to mount them by hand.

Step-2 - create a /etc/raidtab file

Create the file /mnt/sysimage/etc/raidtab (or wherever your real /etc file system has been mounted.

For our test system, the raidtab file would like like this.

raiddev /dev/md0
    raid-level              1
    nr-raid-disks           2
    nr-spare-disks          0
    chunk-size              4
    persistent-superblock   1
    device                  /dev/sda1
    raid-disk               0
    device                  /dev/sdb1
    raid-disk               1

raiddev /dev/md1
    raid-level              1
    nr-raid-disks           2
    nr-spare-disks          0
    chunk-size              4
    persistent-superblock   1
    device                  /dev/sda2
    raid-disk               0
    device                  /dev/sdb2
    raid-disk               1

raiddev /dev/md2
    raid-level              1
    nr-raid-disks           2
    nr-spare-disks          0
    chunk-size              4
    persistent-superblock   1
    device                  /dev/sda3
    raid-disk               0
    device                  /dev/sdb3
    raid-disk               1

Note: - It is important that the devices are in the correct order. ie. that /dev/sda1 is raid-disk 0 and not raid-disk 1. This instructs the md driver to sync from /dev/sda1, if it were the other way around it would sync from /dev/sdb1 which would destroy your filesystem.

Now copy the raidtab file from your real root filesystem to the current root filesystem.

(rescue)# cp /mnt/sysimage/etc/raidtab /etc/raidtab

Step-3 - create the md devices

There are two ways to do this, copy the device files from /mnt/sysimage/dev or use mknod to create them. The md device, is a (b)lock device with major number 9.

(rescue)# mknod /dev/md0 b 9 0
(rescue)# mknod /dev/md1 b 9 1
(rescue)# mknod /dev/md2 b 9 2

Step-4 - unmount filesystems

In order to start the raid devices, and sync the drives, it is necessary to unmount all the temporary filesystems.

(rescue)# umount /mnt/sysimage/var
(rescue)# umount /mnt/sysimage/boot
(rescue)# umount /mnt/sysimage/proc
(rescue)# umount /mnt/sysimage
Please note, you may not be able to umount /mnt/sysimage. This problem can be caused by the rescue system - if you choose to manually mount your filesystems instead of letting the rescue system do this automatically, this problem should go away.

Step-5 - start raid devices

Because there are filesystems on /dev/sda1, /dev/sda2 and /dev/sda3 it is necessary to force the start of the raid device.

(rescue)# mkraid --really-force /dev/md2

You can check the completion progress by cat'ing the /proc/mdstat file. It shows you status of the raid device and percentage left to sync.

Continue with /boot and /

(rescue)# mkraid --really-force /dev/md1
(rescue)# mkraid --really-force /dev/md0

The md driver syncs one device at a time.

Step-6 - remount filesystems

Mount the newly synced filesystems back into the /mnt/sysimage mount points.

(rescue)# mount /dev/md0 /mnt/sysimage
(rescue)# mount /dev/md1 /mnt/sysimage/boot
(rescue)# mount /dev/md2 /mnt/sysimage/var

Step-7 - change root

You now need to change your current root directory to your real root file system.

(rescue)# chroot /mnt/sysimage

Step-8 - edit config files

You need to configure lilo and /etc/fstab appropriately to boot from and mount the md devices.

Note: - The boot device MUST be a non-raided device. The root device is your new md0 device. eg.



Alter /etc/fstab

/dev/md0               /                       ext3    defaults        1 1
/dev/md1               /boot                   ext3    defaults        1 2
/dev/md2               /var                    ext3    defaults        1 2
/dev/sda4              swap                    swap    defaults        0 0

Step-9 - run LILO

With the /etc/lilo.conf edited to reflect the new root=/dev/md0 and with /dev/md1 mounted as /boot, we can now run /sbin/lilo -v on the chrooted filesystem.

Step-10 - change partition types

The partition types of the all the partitions on ALL Drives which are used by the md driver must be changed to type 0xFD.

Use fdisk to change the partition type, using option 't'.

(rescue)# fdisk /dev/sda
(rescue)# fdisk /dev/sdb

Use the 'w' option after changing all the required partitions to save the partion table to disk.

Step-11 - resize filesystem

When we created the raid device, the physical partion became slightly smaller because a second superblock is stored at the end of the partition. If you reboot the system now, the reboot will fail with an error indicating the superblock is corrupt.

Resize them prior to the reboot, ensure that the all md based filesystems are unmounted except root, and remount root read-only.

(rescue)# mount / -o remount,ro

You will be required to fsck each of the md devices. This is the reason for remounting root read-only. The -f flag is required to force fsck to check a clean filesystem.

(rescue)# e2fsck -f /dev/md0

This will generate the same error about inconsistent sizes and possibly corrupted superblock.Say N to 'Abort?'.

(rescue)# resize2fs /dev/md0

Repeat for all /dev/md devices.

Step-12 - checklist

The next step is to reboot the system, prior to doing this run through the checklist below and ensure all tasks have been completed.

Step-13 - reboot

You can now safely reboot the system, when the system comes up it will auto discover the md devices (based on the partition types).

Your root filesystem will now be mirrored.

7.7 Sharing spare disks between different arrays

When running mdadm in the follow/monitor mode you can make different arrays share spare disks. That will surely make you save storage space without losing the comfort of fallback disks.

In the world of software RAID, this is a brand new never-seen-before feature: for securing things to the point of spare disk areas, you just have to provide one single idle disk for a bunch of arrays.

With mdadm is running as a daemon, you have an agent polling arrays at regular intervals. Then, as a disk fails on an array without a spare disk, mdadm removes an available spare disk from another array and inserts it into the array with the failed disk. The reconstruction process begins now in the degraded array as usual.

To declare shared spare disks, just use the spare-group parameter when invoking mdadm as a daemon.

7.8 Pitfalls

Never NEVER never re-partition disks that are part of a running RAID. If you must alter the partition table on a disk which is a part of a RAID, stop the array first, then repartition.

It is easy to put too many disks on a bus. A normal Fast-Wide SCSI bus can sustain 10 MB/s which is less than many disks can do alone today. Putting six such disks on the bus will of course not give you the expected performance boost. It is becoming equally easy to saturate the PCI bus - remember, a normal 32-bit 33 MHz PCI bus has a theoretical maximum bandwidth of around 133 MB/sec, considering command overhead etc. you will see a somewhat lower real-world transfer rate. Some disks today has a throughput in excess of 30 MB/sec, so just four of those disks will actually max out your PCI bus! When designing high-performance RAID systems, be sure to take the whole I/O path into consideration - there are boards with more PCI busses, with 64-bit and 66 MHz busses, and with PCI-X.

More SCSI controllers will only give you extra performance, if the SCSI busses are nearly maxed out by the disks on them. You will not see a performance improvement from using two 2940s with two old SCSI disks, instead of just running the two disks on one controller.

If you forget the persistent-superblock option, your array may not start up willingly after it has been stopped. Just re-create the array with the option set correctly in the raidtab. Please note that this will destroy the information on the array!

If a RAID-5 fails to reconstruct after a disk was removed and re-inserted, this may be because of the ordering of the devices in the raidtab. Try moving the first "device ..." and "raid-disk ..." pair to the bottom of the array description in the raidtab file.

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