stressfree - raid

Tuning Ubuntu's software RAID

David — Fri, 23 Apr 2010 03:04:31 +0000

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:

#!/bin/sh -e
#
# rc.local
#
# 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

exit 0

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.

Enable LVM on software-RAID during a Suse installation

David — Wed, 13 Sep 2006 22:42:35 +0000

If you have tried setting up LVM on top of a LVM-enabled software-RAID partition during a standard Suse installation you may have hit a brick wall where the installer tells you that there are no available LVM partitions. This is a bug in the installer, it looks as if the LVM setup tool only checks physical partitions for LVM labels and ignores software-RAID partitions that have LVM labels.

Steve Wray has pointed out that you can get this functionality to work by creating a small physical partition and giving it an LVM label. The LVM setup tool will see this partition and allow the configuration to continue quite happily with the software-RAID partitions available for use. I have not tested this but I think it is worth giving a spin as the data integrity of RAID-1 coupled with the flexibility of LVM is a compelling combination (though performance maybe an issue).

Note: From a bit of Googling it appears that this limitation is not apparent in Suse Linux Enterprise Server by the looks of this IBM documentation.

Linux software RAID housekeeping

David — Sun, 29 Jan 2006 22:19:38 +0000

I performed some operating system maintanence this morning with the resizing and recreation of a few software RAID disks. It did result in some downtime whilst the mirrors were rebuilt but it was only for 30 minutes which is not bad. Overall the operation went pretty smoothly and I found the following two references very useful: