Recently I encountered an issue where the read/write performance of Ubuntu's software RAID configuration was relatively poor. Fortunately, others have encountered this problem and have documented a potential cause and solution here:
The short story is that Ubuntu uses some very conservative defaults for RAID caching. Whilst this may ensure reliable behavior across a range of hardware, it does mean that for many read/write performance will be lacklustre. The solution to this problem is to define a more aggressive caching options on any software RAID partitions that are in use.
Setting the stripe_cache_size and read ahead caches
The following example assumes that the Ubuntu server has two software-based RAID-5 partitions, /dev/md0 (the root partition) and /dev/md1 (the /var partition).
Set the stripe_cache_size and read ahead caches in the /etc/rc.local script. In the example below the stripe_cache_size is set to 8192, and the read ahead cache 4096:
# This script is executed at the end of each multiuser runlevel.
# Make sure that the script will "exit 0" on success or any other
# value on error.
# In order to enable or disable this script just change the execution bits.
# By default this script does nothing.
# Tune the RAID5 configuration
echo 8192 > /sys/block/md0/md/stripe_cache_size
echo 8192 > /sys/block/md1/md/stripe_cache_size
blockdev --setra 4096 /dev/md0
blockdev --setra 4096 /dev/md1
Restart Ubuntu to apply these settings.
Note: It is possible to apply these changes without a restart by executing each directive at the command line.
The pages linked to above explain how to test the influence of these cache changes. In general I have found that the parameters given in the example above have improved performance without influencing the reliability of the system, or the data stored on it.