Monday, March 16, 2009

xfs filesystem fragmentation

Many filesystems use many techniques to overcome filesystem fragmentation and XFS filesystem is one of them. But in long usage, you may face with fragmentation problems.Here, I will show you how to find fragmentation ratio in your xfs filesystem and how to re-organize your xfs filesystem with xfs_fsr utility to reduce fragmentation on a live xfs filesystem. First, let's check our xfs filesystem's fragmenation ratio using xfs_db (xfs debugger) utility. As shown below, we used two parameters. First one is "-r", this means that we use debugger in read-only mode. We do not want to accidently do anything bad to our filesystem. Also, this option is useful if we are debugging a mounted filesystem. Second option is "-c" command, which simply takes it's parameter as xfs_db command and run it. Without "-c" parameter, xfs_db will run it's interactive shell where you can run xfs_db commands.

We see that our xfs filesystem on /dev/sdb device has 0.14% fragmentation. This is very low fragmentation. We don't need to defrag it. But here how to do it with xfs_fsr. This utility re-organizes a xfs filesystem while it's mounted file by file. As you can see in sample below, it runs with "-t" parameter, this tells xfs_fsr utility to run maximum that given seconds and quit.

There is one more option you can use while using xfs filesystem to avoid future fragmentations. It's called "allocsize", where it preallocates disk space before writing to a file. You have to set this preallocation size while mounting your xfs filesystem. This will prevent fragmentation on your xfs filesystem if you give a reasonable size depending on your average file sizes.

mount -t xfs -o noatime,allocsize=8M /dev/sdb /mydata

you can also set this option in your /etc/fstab.



Anonymous said...

Very informative, thanks for posting.

vishnurao said...

thanks for very informative post.

had a question though.

What should be an alarming 'fragmentation factor'?

is there a rule of thumb for this? say > 2%

can i safely assume that anything in double digits is bad ?

thanking you,
vishnu rao