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Partitioning in action, Part 2

1. Consolidating data

Introduction

In my last tip, we successfully moved /home to a new partition. Now, it's time to learn how to consolidate often-modified files onto a new partition. Why would we want to do this? Here's an example. Often-modified files contribute heavily to fragmentation. One of the best ways to contain fragmentation is to store often-modified files on their own partition. That way, the other partitions are unaffected by the fragmentation caused by the heavily modified files. In concept, this is easy to understand, but how do you actually go about doing it?

First, you must create a new partition for the specific purpose of storing frequently modified files. You might want to locate this partition on a separate disk to enhance performance. Then, I'll walk you through the steps required to move both /tmp and /var to this new partition.

Warning: The following technique describes how to move a partition(s). Although this technique is designed so that you can "back out" of a failed partition move, it doesn't protect against user error. In other words, any time that you format partitions or copy large numbers of files, there's a possibility that you will type the wrong thing, causing lots of data to be destroyed. For this reason, it's highly recommended that you take appropriate steps to back up all critical files before proceeding.

Create a filesystem on the new partition

The first step of this process is to create a new partition that's big enough to hold /var and /tmp, with a little extra space left over. You'll need either an additional drive, or a spare (unused) partition that will house the often-modified files. If you do need to use fdisk or cfdisk to create the partition, you'll need to reboot once. Then, it's time to format the new partition as follows (it's OK to be in multiuser mode while you do this; I'll let you know when to switch to single-user):

Code Listing 1.1: Creating the filesystem

# mkfs.ext2 /dev/???

2. Mount it to /mnt/rwstorage

As in my previous tip, ??? should be replaced with the device name for the new, empty partition that you are creating. Accidentally typing the wrong name will destroy data on an existing partition, so be careful! After typing this command, you'll have a brand-new ext2 filesystem on the new partition. We're almost ready to mount it, but first, let's make a new mount point.

Code Listing 2.1: Creating the mount point

# mkdir /mnt/rwstorage

I chose the name rwstorage to remind us that this particular partition is going to be specifically used to house files that are read from and written to frequently. To mount the partition, type:

Code Listing 2.2: Mounting the partition

# mount /dev/??? /mnt/rwstorage

3. Creating a new /tmp

The partition is now mounted and we're ready to create our new /tmp directory:

Code Listing 3.1: Creating the new /tmp

# cd /mnt/rwstorage
# mkdir tmp
# chmod 1777 tmp

4. Drop to single-user mode

Our new directory at /mnt/rwstorage/tmp has the right permissions for a temporary directory. Now, drop to single-user mode, since we must copy over /var. As usual, we've delayed our drop to single-user mode to the last possible moment. Right now, we don't want any programs reading or writing files in /var, so we have to stop all daemons, disconnect all users, and do some quick maintenance by typing:

Code Listing 4.1: Entering single-user mode

# init 1

If you're prompted to enter a password to perform system maintenance, do so. You should now have a root shell, and all unnecessary daemons will be stopped. Create a new location for our /var files by typing:

Code Listing 4.2: Creating the new directory

# cd /mnt/rwstorage
# mkdir var

5. Copy /var

Default permissions on our new /mnt/newstorage/var directory should be correct, so now we're ready to copy all of our original /var data over to the new partition:

Code Listing 5.1: Copying the data

# cd /var 
# cp -ax * /mnt/rwstorage/var

6. Back up and create symlinks

After this command completes, you'll have an exact copy of /var at /mnt/rwstorage/var. Now, you may be asking how exactly we get Linux to use /mnt/rwstorage/var and /mnt/rwstorage/tmp instead of the defaults in the root directory. This is easily done by the use of symbolic links -- we'll create the new symbolic links, /tmp and /var, which point to the correct directories in /mnt/rwstorage. First, let's back up the original directories:

Code Listing 6.1: Backing up the directories

# cd /
# mv var var.old
# mv tmp tmp.old

The last line probably isn't necessary, since it's very likely that you don't have anything important in /tmp, but we're playing it safe. Now, let's create the symlinks:

Code Listing 6.2: Creating the symlinks

# cd /
# ln -s /mnt/rwstorage/var /var
# ln -s /mnt/rwstorage/tmp /tmp

7. Finishing touches to /etc/fstab

Now, when any user or program uses /var, they'll automatically be transported to /mnt/rwstorage/var! Likewise for /tmp. We have one step left; however, it can be safely performed in multiuser mode. It's time to get Apache running again, and to allow all your users to log back in. Exit from runlevel 1 by pressing CTRL-D. The system should start up normally. Log in as root.

The final thing we must do is configure /etc/fstab so that /dev/??? is mounted at /mnt/rwstorage. You must add a line like the following to your /etc/fstab:

Code Listing 7.1: Configuring fstab

/dev/???   /mnt/rwstorage   ext2   defaults   1   2

Important: If you are using a kernel version in the 2.3+ range, it's very likely that you have a line in your /etc/fstab that looks like this:

Code Listing 7.2: Shared memory

none   /var/shm   shm   defaults   0   0

This line enables shared memory on your system, and by default it gets mounted in /var. In order for this line to work properly, it must appear after the line you just added. That way, when Linux starts up, /mnt/rwstorage will get mounted first (enabling /var). Then, and only then, will the shm device get mounted at /var/shm, which is really /mnt/rwstorage/var/shm. Make sure the lines are in this order:

Code Listing 7.3: Enabling shared memory

/dev/???        /mnt/rwstorage  ext2    defaults       1     2
none            /var/shm        shm     defaults       0     0

After saving the changes to /etc/fstab, your system has been successfully upgraded! After verifying that everything is working properly, you'll want to remove the /tmp.old and /var.old backup directories. Congratulations -- you've successfully reconfigured your system's partitions for optimum performance.

Resources

In Partitioning in action: Moving /home, I show you step-by-step how to move /home to a new partition.
In Maximum swappage, I show how to dramatically increase swap performance under Linux.
In Partition planning tips, I share several ways of effectively organizing your partition layout.
I always recommend checking out the howtos at The Linux Documentation Project -- they're a valuable resource.

About the author

Daniel Robbins lives in Albuquerque, New Mexico. He was the President/CEO of Gentoo Technologies Inc., the Chief Architect of the Gentoo Project and is a contributing author of several books published by MacMillan: Caldera OpenLinux Unleashed, SuSE Linux Unleashed, and Samba Unleashed. Daniel has been involved with computers in some fashion since the second grade when he was first exposed to the Logo programming language and a potentially lethal dose of Pac Man. This probably explains why he has since served as a Lead Graphic Artist at SONY Electronic Publishing/Psygnosis. Daniel enjoys spending time with his wife Mary and his new baby daughter, Hadassah. You can contact Daniel at Daniel Robbins.

Page updated March 2, 2010

Summary: In this second tip on changing partition layout on a running system, Daniel Robbins shows you how to move /tmp and /var to their own shared partition. He also covers several tricks of the trade to minimize downtime and avoid making costly mistakes.

Daniel Robbins
Author

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