Difference between revisions of "Ext3"
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# tune2fs -c 0 -i 0 /dev/hdXY
# tune2fs -c 0 -i 0 /dev/hdXY
Revision as of 20:23, 3 July 2007
Copyright (c) 2005 Peter Gordon (codergeek42)
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
I'm a big fan of the Third Extended ("ext3") filesystem. It's in-kernel and userspace code has been tried, tested, fixed, and improved upon more than almost every other Linux-compatible filesystem. It's simple, robust, and extensible. In this article I intend to explain some tips that can improve both the performance and the reliability of the filesystem.
In the document, /dev/hdXY will be used as a generic partition. You should replace this with the actual device node for your partition, such as /dev/hdb1 for the first partition of the primary slave disk or /dev/sda2 for the second partition of your first SCSI or Serial ATA disk.
Using The tune2fs and e2fsck Utilities
Before we begin, we need to make sure you are comfortable with using the tune2fs utility to alter the filesystem options of an ext2 or ext3 partition. Please make sure to read the tune2fs man page:
$ man tune2fs
It's generally a good idea to run a filesystem check using the e2fsck utility after you've completed the alterations you wish to make on your filesystem. This will verify that your filesystem is clean and fix it if needed. You should also read the manual page for the e2fsck utility if you have not yet done so:
$ man e2fsck
WARNING: ONLY RUN UNMOUNTED: Make sure any filesystems are cleanly unmounted before altering them with the tune2fs or e2fsck utilities! (Boot from a LiveCD such as Archie or Knoppix if you need to.) Altering or tuning a filesystem while it is mounted can cause severe corruption! You have been warned!
Using Directory Indexing
This feature improves file access in large directories or directories containing many files by using hashed binary trees to store the directory information. It's perfectly safe to use, and it provides a fairly substantial improvement in most cases; so it's a good idea to enable it:
# tune2fs -O dir_index /dev/hdXY
This will only take effect with directories created on that filesystem after tune2fs is run. In order to apply this to currently existing directories, we must run the e2fsck utility to optimize and reindex the directories on the filesystem:
# e2fsck -D -f /dev/hdXY
Note: This should work with both ext2 and ext3 filesystems. Depending on the size of your filesystem, this could take a long time. Perhaps you should go get some coffee...
Enable Full Journaling
By default, ext3 partitions mount with the 'ordered' data mode. In this mode, all data is written to the main filesystem and its metadata is committed to the journal, whose blocks are logically grouped into transactions to decrease disk I/O. This tends to be a good default for most people. However, I've found a method that increases both reliability and performance (in some situations): journaling everything, including the file data itself (known as 'journal' data mode). Normally, one would think that journaling all data would decrease performance, because the data is written to disk twice: once to the journal then later committed to the main filesystem, but this does not seem to be the case. I've enabled it on all nine of my partitions and have only seen a minor performance loss in deleting large files. In fact, doing this can actually improve performance on a filesystem where much reading and writing is to be done simultaneously. See this article written by Daniel Robbins on IBM's website for more information.
There are two different ways to activate journal data mode. The first is by adding data=journal as a mount option in /etc/fstab. If you do it this way and want your root filesystem to also use it, you should also pass rootflags=data=journal as a kernel parameter in your bootloader's configuration. In the second method, you will use tune2fs to modify the default mount options in the filesystem's superblock:
# tune2fs -O has_journal -o journal_data /dev/hdXY
Please note that the second method may not work for older kernels. Especially Linux 2.4.20 and below will likely disregard the default mount options on the superblock. If you're feeling adventurous you may also want to tweak the journal size. (I've left the journal size at the default.) A larger journal may give you better performance (at the cost of more disk space and longer recovery times). Please be sure to read the relevant section of the tune2fs manual before doing so:
# tune2fs -J size=$SIZE /dev/hdXY
Disable Lengthy Boot-Time Checks
WARNING: Only do this on a journalling filesystem such as ext3. This may or may not work on other journalling filesystems such as ReiserFS or XFS, but has not been tested. Doing so may damage or otherwise corrupt other filesystems. You do this AT YOUR OWN RISK.
Hmm..It seems that our ext3 filesystems are still being checked every 30 mounts or so. This is a good default for many because it helps prevent filesystem corruption when you have hardware issues, such as bad IDE/SATA/SCSI cabling, power supply failures, etc. One of the driving forces for creating journalling filesystems was that the filesystem could easily be returned to a consistent state by recovering and replaying the needed journalled transactions. Therefore, we can safely disable these mount-count- and time-dependent checks if we are certain the filesystem will be quickly checked to recover the journal if needed to restore filesystem and data consistency. Before you do this please make sure your filesystem entry in /etc/fstab has a positive integer in its 6th field (pass) so that it is checked at boot time automatically. You may do so using the following command:
# tune2fs -c 0 -i 0 /dev/hdXY
If you just want to limit the checks to happen less often without totally disabling them (for piece of mind). A great method is to change from a number of count's check to a time frame check. See the man. Here is once every month..
# tune2fs -c 0 -i 1m /dev/hdXY
Reclaim Reserved Filesystem Space
Ext3 partition contain a used space of 5% for special reasons by default. The main reason for such space is so root can log in even when the filesystem becomes 100% used. Without this option, the root user might not be able to log in to "clean up" because they system could become unstable, trying to write logs to a 100% full system for example. The other reason is to help with less fragmentation on the filesystem.
The issue with this is that hard drives are getting so big the 5% can add up to be quite a large amount of wasted space. (eg. 100 GB = 5 GB reserved). Now if you seperate your filesystems to like /home for example it might be a good idea to adjust these and reclaim that wasted space. It's a safe bet to leave your / filesystem at 5% reserved just in case. Leave reserved space for filesystems containing /var and /tmp also or else you'll end up with problems.
Now to change your reserved space to 1% of the drive, which is fair for non-root filesystems.
# tune2fs -m 1 /dev/sdXY
I get filesystem errors even with this tips. Do not disable lengthy Boot-Time
If you have some big partitions, do not enable full journaling on them. I experienced reproduceable freezes (about 30 seconds) when enabling this on my 250gb home partition.