Making the Case for Partitioning

What is it about partitioning? The old school rule was that there were separate partitions for /, /usr, /home, /var, and /tmp. In fact, default server installations (including Solaris) still use this partitioning set up.

Has partitioning outlived its usefulness?

This question has come up before. There are negative and positive aspects to partitioning, and the case for partitioning might not be as strong as it once was.

Partitioning means that you may have a situation with no space in one partition and lots in another. This, in fact, is the most common argument against partitioning. However, using LVM or ZFS where disks can be grown dynamically makes this a moot point. With technologies such as ZFS and LVM, you can expand a disk and filesystem any time you need to.

However, this still means that the system will require maintenance – but that is what administrators are for, right? If a filesystem fills – or is going to fill – it is up to a system administrator to find the disk space and allocate it to the disk.

Another argument against partitioning says “Disk is cheap.” Well, if this is true, then why do companies still balk at getting terabytes of disk into their SANs? The phrase “Disk is cheap” is trite but untested: in truth, buying 144Gb disks is not done in bulk. Companies still have to watch the budget, and getting more disk space is not necessarily going to be a high priority until disk runs out.

So, what are the benefits to partitioning disks? There are many.

Each partition can be treated seperately – so the /usr filesystem can be mounted read-only, the /home directory can be mounted with the noexec and nosuid options, which makes for a more secure and more robust system.

Also, if there are disk errors, then a single partition can be affected rather than the entire system. Thus, on a reboot, the system still comes up instead of being blocked because the root filesystem is trashed. In this same vein, if a filesystem requires a check, going through a 144Gb filesystem check could take a very long time – whereas, if the partition was 10Gb it would be not nearly as long and the system would come back up that much faster.

Backups – and restores – are another thing that is simplified by having multiple partitions. For example, when backing up an HP-UX system using make_tape_recovery, you specify which partitions to back up to tape. These partitions are then restored when the tape is booted. If you used a single partition for everything (data, home, etc.) then you would probably not be able to make this sort of backup at all.

One of the nicest reasons to partition is the ability to separate user data from system data. This allows the reinstallation of the system while keeping user data (and application data) untouched. This saves time and effort. I recently installed Ubuntu Server in place of Red Hat Enterprise Linux, and since the system was a single partition, there was no way to install Ubuntu Server without wiping out 200Gb of application data and restoring it – which took around 9 hours each way on a gigabit network link (if nothing else was sharing the network). Alternately, I converted my OpenSUSE laptop to using Xubuntu – and was able to keep all of my user settings because /home was on a separate partition. Keeping a system on a single partition cost the company somewhere in the order of a full day’s worth of time and effort – how much money would the company have saved by having a separate partition for /var/lib/mysql?

Performance is another reason for partitioning – but this is only relevant for separate disks. If you have multiple disks, you can segregate them into separate partitions, which means then that if a disk is heavily used for one purpose it can be dedicated to that purpose – you won’t have your database accesses slowing down because of system log writes, for example. However, this problem can be reduced or eliminated – or made moot – by moving data to a striped volume, and possibly with disk cache as well. Yet, as long as there are disk accesses, do you want them competing with your database?

Having said all of this, how does using a virtual machine change these rules? Do you need partitioning in a virtual machine?

A virtual machine makes all of the arguments relating to the physical disk moot – performance on a virtual disk doesn’t have a high correlation with true physical hardware unless the virtual machine host is set up with segregated physical disk. Even so, the disks may actually be separate LUNs created in a RAID.

However, the ability to secure a filesystem (such as /usr), save filesystem check time, prevent excessive /home usage, and other reasons suggest that the case for partitions is still valid.

An Easier Way to Mount Disks in Linux (by UUID)

When mounting a disk, the traditional way has always been to use the name given to it by the operating system – such as hda1 (first partition on first hard drive) or sdc2 (second partition on third SCSI drive). However, with the possibility that disks may move from one location to another (such as from sdc to sda) what can be done to locate the appropriate disk to mount?

Most or all Linux systems now install with the /etc/fstab set up to use the Universally Unique Identifier (UUID). While the disk’s location could change, the UUID remains the same. How can we use the UUID?

First, you have to find out the UUID of the disk that you want to mount. There are a number of ways to do this; one traditional method has been to use the tool vol_id. This tool, however, was removed from udev back in May of 2009. There are other ways to find the UUID of a new disk.

One way is to use blkid. This command is part of util-linux-ng. The vol_id command was removed in favor of this one, and is the preferred tool to use instead of vol_id. Find the UUID of /dev/sda1 like this:

$ blkid /dev/sda1
/dev/sda1: UUID="e7b85511-58a1-45a0-9c72-72b554f01f9f" TYPE="ext3"

You could also use the tool udevadm (which takes the place of udevinfo) this way:

$ udevadm info -n /dev/sda1 -q property | grep UUID

Alternately, the tune2fs command (although specific to ext) can be used to get the UUID:

# tune2fs -l /dev/sda1 | grep UUID
Filesystem UUID:          e7b85511-58a1-45a0-9c72-72b554f01f9f

The tune2fs utility is also only available to the superuser, not to a normal user.

There are utilities similar to tune2fs for other filesystems – and most probably report the UUID. For instance, the XFS tool xfs_info reports the UUID (in the first line) like this:

$ xfs_info /dev/sda6
meta-data=/dev/disk/by-uuid/c68acf43-2c75-4a9d-a281-b70b5a0095e8 isize=256    agcount=4, agsize=15106816 blks
         =                       sectsz=512   attr=2
data     =                       bsize=4096   blocks=60427264, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0
log      =internal               bsize=4096   blocks=29505, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0

The easiest way is to use the blkid command. There is yet one more way to get the UUID – and it also allows you to write programs and scripts that utilize the UUID. The filesystem contains a list of disks by their UUID in /dev/disk/by-uuid:

$ ls -l /dev/disk/by-uuid
total 0
lrwxrwxrwx 1 root root 10 2011-04-01 13:08 6db9c678-7e17-4e9e-b10a-e75595c0cacb -> ../../sda5
lrwxrwxrwx 1 root root 10 2011-04-01 13:08 c68acf43-2c75-4a9d-a281-b70b5a0095e8 -> ../../sda6
lrwxrwxrwx 1 root root 10 2011-04-01 13:08 e7b85511-58a1-45a0-9c72-72b554f01f9f -> ../../sda1

Since a disk drive is just a file, these links can be used to “find” a disk device by UUID – and to identify the UUID as well. Just use /dev/disk/by-uuid/e7b85511-58a1-45a0-9c72-72b554f01f9f in your scripts instead of /dev/sda1 and you’ll be able to locate the disk no matter where it is (as long as /dev/disk/by-uuid exists and works).

For example, let’s say you want to do a disk image copy of what is now /dev/sda1 – but you want the script to be portable and to find the disk no matter where it winds up. Using dd and gzip, you can do something like this:

dd if=/dev/disk/by-uuid/${UUID} of=- | gzip -c 9 > backup-$UUID.gz

You can script the retrieval of the UUID – perhaps from a user – this way:

eval `blkid -o udev ${DEV}`
echo "UUID of ${DEV} is ${ID_FS_UUID}"

In the /etc/fstab file, replace the mount device (such as /dev/sda1) with the phrase UUID=e7b85511-58a1-45a0-9c72-72b554f01f9f (or whatever the UUID actually is).

Be aware that the UUID is associated with the filesystem, not the device itself – so if you reformat the device with a new filesystem (of whatever type) the UUID will change.

Mirroring HP-UX Root Disks (LVM)

Mirroring the root disk in HP-UX is not necessarily as hard as it sounds. There are excellent directions out there. Firstly, go get the PDF from HP titled “When Good Disks Go Bad” (make sure you have the most recent!). This is frequently recommended and you should have it if you don’t already.

Appendix D: Procedures contains detailed instructions on both mirroring PARISC boot disks and Itanium disks. Here is a broad overview to get you prepped:

  • 1. Prepare the disk for LVM. This means using the insf command to add the disk files as appropriate, and using pvcreate to lay down LVM structures.
  • 2. Add the new volume to the root volume group. Use vgextend to add the new disk to the volume group.
  • 3. Make the disk bootable. This is a hairy part. You have to copy the proper data from the original disk, and so forth. Follow the directions closely.
  • 4. Mirror the logical volumes on the first disk to the new one. This has to be done in a specific order or it will fail. It’s not hard, just time consuming.
  • 5. Update system data with new boot information. This involves configuring the machine to boot with the new disk.

Setting up Itanium boot mirrors requires a few additional steps: the disk must be partitioned into three parts: 1) EFI boot partition; 2) HP-UX root disk data; and 3) HP Service Partition. The second partition is equivalent to the entire disk in a PARISC system. The first partition (EFI boot) is like a mini-disk that contains all of the data needed to boot HP-UX.

The last partition – the HP service partition – is not configured unless you copy it from the original source. If you do not do this, you do not actually have a service partition – which could be a nasty surprise when you need it!

Identifying the Right Disk…

When it comes time to replace a disk – such as when a disk goes bad in an external hot-swappable disk cage – it can be catastrophic to remove the wrong disk (to say the least). Assuming that you have “read” capabilities on this drive, how do you identify the disk associated with the one you know is bad (which you’ve identified from the operating system)?

Well, in a UNIX-based environment, you can use dd. How?

Make the disk light on the disk glow by entering this command:

dd if=/dev/baddsk of=/dev/null

This will read the entire disk, starting at the beginning, and send the output to…. nowhere! It does, however, make the disk light light up.

In system administration, it is never wrong to double-check. So: execute the command, and check which disk has lit up. Then stop the command and watch to see that the light goes out (or is no longer solidly lit). Restarting the command should then light up the disk. So now you know.