Wednesday, March 9, 2011

Optimising Linux for minimal writes - useful for an SSD

Tip 1: Use ext2 instead of ext3/ext4.
ext3 and ext4 are journaled filesystems and so maintain a log of all filesystem changes (which can be used for recovery if need be).  If you're using a laptop chances are sudden power failure isn't really a threat so you'll lose less by not having a journaled filesystem - if you're using a desktop your choice to use a non-journaled filesystem should be an informed one.

Tip 2: Don't update file access times.
Modify your /etc/fstab adding noatime to the options for mounted drives.  This will stop writing to the file system every time a file is accessed.  (This is also used to improve disk I/O performance for critical applications - git does this for example, though they may have switched to relatime).

Tip 3: Mount "non-essential" write-heavy directories as tmpfs.
Update /etc/fstab to mount /tmp, /var/tmp and /var/log as tmps:
#              
tmpfs           /tmp            tmpfs   defaults        0       0
tmpfs           /var/tmp        tmpfs   defaults        0       0
tmpfs           /var/log        tmpfs   defaults        0       0
Unfortunately when /var/log is now mounted it won't have any of the necessary directories so we'll need to create them each time on boot.  Update /etc/rc.local with the following snippet just before the "exit 0" line.
for dir in apparmor apt ConsoleKit cups dist-upgrade fsck gdm installer libvirt news samba speech-dispatcher unattended-upgrades; do
if [ ! -e /var/log/$dir ] ; then
mkdir /var/log/$dir
fi
done
You can get the list of directories you need by running cd /var/log; ls -d */

These tips can be found repeated on the web but I gathered most of my info from here.  That page also lists some further optimisations for the kernel to take full advantage of an SSD's speed.

Tuesday, March 8, 2011

Mounting an encrypted logical volume

I found this useful when trying to do some recovery work on my computer which uses dm-crypt for full disk encryption.  The following was done using the Ubuntu Maverick (10.10) livecd.

Mount the encrypted partition
root@ubuntu:/home/ubuntu# sudo cryptsetup luksOpen /dev/sda4 data
Enter passphrase for /dev/sda4:
root@ubuntu:/home/ubuntu# ls /dev/mapper
control  data

Mount the logical volume
root@ubuntu:/home/ubuntu# aptitude install lvm2
root@ubuntu:/home/ubuntu# vgscan
  Reading all physical volumes.  This may take a while...
  Found volume group "vg0" using metadata type lvm2
root@ubuntu:/home/ubuntu# vgchange -ay vg0
  2 logical volume(s) in volume group "vg0" now active
root@ubuntu:/home/ubuntu# mkdir /mnt/tmp
root@ubuntu:/home/ubuntu# sudo mount /dev/vg0/root /mnt/tmp