September 2, 2010

2 Cent Tip - Extend (resize) a whole device partition.

Occasionally I have to resize partitions on iSCSI or Fiber-Channel attached SAN storage.  Both technologies allow you to easily extend the available storage for a host by extending LUNs, or volumes.  A common problem after extending the size of the LUN, or volume, is resizing partitions to fill out the new size.


For the most part, I usually fire up PartedMagic  and its a snap, even with Fiber-Channel attached enterprise storage.  Once the HBA's have been zoned to Fiber-Channel switches, then the HBAs do all the heavy lifting.  In other words on Fiber-Channel, it doesn't matter if you're using PartedMagic, or Knoppix, the server just knows where the storage is, and that its certainly attached.  The only dependency for this working on a Live boot disk are drivers for the HBA cards.


iSCSI is a bit different.  Because, iSCSI relies on commodity Network Interface Cards, this technology is largely implemented in software.  One perceived advantage is iSCSI may seem less complicated to use than Fiber-Channel storage.  Unfortunately, in this case, PartedMagic did not have open-iscsi software, and I could install open-iscsi in the Knoppix Live ramdisk. However, because Knoppix came with an outdated iSCSI kernel module, it was not new enough to inter-operate with the open-iscsi software.


Furthermore, the version of parted that shipped with RHEL 5 threw an incompatible filesystem error, refusing to modify the filesystem. So, in the end, I twiddled some bits on the partition table with fdisk, and used resize2fs to extend the partition.


Assuming you have a backup of the filesystem you are working on, you can proceed with the following steps to extend a single partition to the end of the extended volume.  If you have multiple partitions on a volume, you may want to stick to more reliable methods of resizing and extending.  If you screw up the cylinder boundaries on a device with multiple partitions, you'll definitely lose data.  A single partition, in this example, is a much simpler scenario.


The device name in this example is /dev/mapper/u02, the first partition is /dev/mapper/u02p1:

  • Runfdisk -l /dev/mapper/u02 to get the starting cylinder.
Disk /dev/mapper/u02: 100.9 GB, 108340550042 bytes

255 heads, 63 sectors/track, 13171 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot Start End Blocks Id System
/dev/mapper/u02p1 1 13171 26450329 83 Linux

  • Reboot the server, after extending the volume or LUN on your SAN, and before proceeding to extend the partition in your Operating System.  The Operating System needs to re-read sector 0 on the extended SAN volume, before continuing. Note, that fdisk will report 26109 cylinders instead of 13171, after I rebooted the server.
  • Next, we will run: fdisk /dev/mapper/u02, and then hit the keys: d, n, p, 1, [enter], [enter], w
WARNING: DOS-compatible mode is deprecated. It's strongly recommended to
switch off the mode (command 'c') and change display units to
sectors (command 'u').

Command (m for help): d
Selected partition 1

Command (m for help): n
Command action
e extended
p primary partition (1-4)
Partition number (1-4): 1
First cylinder (1-26109, default 1):
Using default value 1
Last cylinder, +cylinders or +size{K,M,G} (1-26109, default 26109):
Using default value 26109

Command (m for help): w
The partition table has been altered!

Calling ioctl() to re-read partition table.
Syncing disks.
  • Finally, run resize2fs on /dev/mapper/u02p1. If you are using ext3, you can do an on-line resize while the volume is mounted.  It is probably safest to umount the partition to be re-sized, however.
resize2fs 1.39 (29-May-2006)
Filesystem at /dev/mapper/u02p1 is mounted on /u02; on-line resizing required
Performing an on-line resize of /dev/mapper/u02p1 to 52430127 (4k) blocks.
The filesystem on /dev/mapper/u02p1 is now 52430127 blocks long.

Refer to the resize2fs for more information on the command, and its proper usage.

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