Thushan Fernando Uncut

I’ve been working heavily with Red Hat Enterprise Linux (and subsequently CentOS) the past few months (shh! dont tell my MSFT homey!) and one of the great things about CentOS and RHEL is that they both install using LVM – which is a helluvah lot easier when time passes and you realise your running out of space on a drive.

But today I had to recover some data from an LVM partition and copy over some bits to another partition without actually booting the CentOS install (it was bj0rked by yours truely!). What to do? Throw in a Ubuntu LiveCD (or another) and just mount the partitions 🙂

First thing we need to do is install LVM – remember we need to be sudo for these to work.

$ aptitude install lvm2

Then scan for any available physical volumes on any of the drives.

$ pvscan

Scan for any Volume Groups that may be present.

$ vgscan

Now activate any of the Volume Groups that it finds, running this makes the logical volumes known to the kernel.

$ vgchange –available y

Then let it scan for any Logical Volumes on any drives

$ lvscan

After running the logical volume scan it will show the path to the LVM mount path, for my boxen it gives something like this

ACTIVE            ‘/dev/LVM/Data‘ [5.26 TB] inherit

You simply mount the path specified and browse like normally 🙂

$ mount /dev/LVM/Data /mnt

Enjoy.

File Systems are a hairy topic, on Windows you should be using NTFS (the days of FAT are long gone!) but on Linux, BSD and *Solaris we still have a wide variety to pick and choose depending on our needs. I’ve always been a JFS and XFS fan (previously ReiserFS) until Btrfs goes mainstream (which is one thing to hangout for in Linux Kernel 2.6.29!) and often I’d have a mixture of all three. Our main server at home – affectionately dubbed Zeus, after our lovable Australian Customs puppy Zeus, uses XFS, JFS and Ext3.

JFS to manage the home directories and core file system, ReiserFS for the temp folder and XFS for the heavy file shares – which span multiple terrabytes of files over a LVM (with each file being 1-2Gb in size). The reasoning behind opting for XFS over another file system for the file server was that XFS performs incredibly well under heavy load and scales well when you know the files are big (over 500Mb). Overall I’ve always felt that XFS does provide consistent performance and scalabilty in comparison to the others – but you may think otherwise.

Unfortunately, XFS – whilst quite an excellent file system for managing large files, it seems, suffers from fragmentation over time (especially true if you use your file system for DVR – eg, a Myth backend host) or if the disk gets close to filling up. Luckily there are two utilities that XFS has to manage this fragmentation.

• xfs_db – XFS Debug Information
  Used to examine an XFS filesystem for problems or gather information about the XFS file system.
• xfs_fsr – File System Organiser
  Improves the organisation of mounted file systems. The reorganisation algorithm operates on one file at a time, compacting or otherwise improving the layout of the file extents (contiguous blocks of file data).
  

In Debian/Ubuntu (and derivatives) these two utilities are found in the package xfsdump. Using these two utilities we can workout the health of the file system (xfs_db) and hopefully tune/optimise it (xfs_fsr). I took the plunge last night and optimised Zeus’s main file storage partition:

Filesystem            Size Used Avail Use% Mounted on
/dev/sdf7              40G  3.5G   37G   9% /
varrun               1014M  4.5M 1010M   1% /var/run
varlock              1014M  8.0K 1014M   1% /var/lock
udev                 1014M  112K 1014M   1% /dev
devshm               1014M     0 1014M   0% /dev/shm
lrm                  1014M   34M  980M   4% /lib/modules/2.6.22-15-generic/volatile
/dev/sdf6            1023M   38M  986M   4% /boot
/dev/sdf10            235G  173G   63G  74% /home
/dev/sdf9              10G  544K   10G   1% /opt
/dev/sdf8              10G  2.7G  7.4G  27% /var
/dev/mapper/Storage
                      2.3T  1.9T  408G  83% /media/LVM/Storage
/dev/sde1             466G  396G   71G  85% /media/Backups

As you can see, the LVM “Storage” mount has just under 20% free space and the non-LVM partition for “Backups” has 15% free space. Both these are XFS volumes, to find the health of the two use the xfs_db command to gather some information.

$ sudo  xfs_db -c frag -r /dev/mapper/Storage
$ sudo  xfs_db -c frag -r /dev/sde1

Here we’re asking xfs_db to open the file system in a readonly mode (-r) passing in a command (-c)  to get the file fragementation data (frag) for the device (/dev/*). When we use the frag command, it returns information only pertaining to the file data in the filesystem as opposed to the fragmentation of freespace (which we can guage with passing the freesp command). The output of the commands appear below for Zeus.

thushan@ZEUS:~$ sudo  xfs_db -c frag -r /dev/sde1
actual 189356, ideal 148090, fragmentation factor 21.79%

thushan@ZEUS:~$ sudo  xfs_db -c frag -r /dev/mapper/Storage
actual 406056, ideal 21584, fragmentation factor 94.68%

Wow! The LVM partition (which spans 4 drives) has around 95% fragementation! Yikes!!! The parition has quite a few Virtual Machine images, various large files (DV Captures etc). The “Backup” (sde1) on the other hand isnt as badly fragmented.

So right now we’ve found our problem and its time to fix it. First thing to do – and realise that we can fix this on a live running system – is to try and find a time where the partition will be used very little (like overnight) so you let its do its thing without unnecessary burden. Then lets make use of the File System Organiser utility (xfs_fsr) and ask it to reorganise our parition to the best of its ability.

$ sudo xfs_fsr -t 25200 /dev/mapper/Storage -v
$ sudo xfs_fsr -t 25200 /dev/sde1 -v

Now this is much simpler, the xfs_fsr utility is being told to reorganise /dev/* with a timeout (-t) of 7hrs  (60 * 60 * 7 = 25200) which is specified in seconds. Because I like to see how much is done I also specified the verbose output option (-v). Let it do its thing and hopefully when you return you will have the last bit of output showing the extents before, how many after and the inode, something like this:

extents before:5 after:1 DONE ino=4209066103
ino=4209066107
extents before:5 after:1 DONE ino=4209066107
ino=4209066101
extents before:4 after:1 DONE ino=4209066101
ino=4209066091
extents before:3 after:1 DONE ino=4209066091
ino=4209066093
extents before:3 after:1 DONE ino=4209066093
ino=4209066105
extents before:2 after:1 DONE ino=4209066105
ino=4209066143
extents before:27 after:1 DONE ino=4209066143

Now its time to go back and check how well the file system reorganising was:

$ sudo  xfs_db -c frag -r /dev/mapper/Storage

And the results?

thushan@ZEUS:~$ sudo  xfs_db -c frag -r /dev/mapper/Storage
actual 21652, ideal 21584, fragmentation factor 0.31%

Lovely! What a difference and you’ll notice the improvement immediately if you start moving or transfering files around.

Ideally, you may want to setup a cron task to let this process run (maybe with a lower timeout) overnight or when theres low-load. Whats great about the xfs_fsr utility is that its smart enough to remember where it finished up last time and continue from there. Its a shame Ubuntu doesnt do this already.