Ext3 Filesystem Explained

sheckard writes: "The next installment of the wonderful Advanced filesystem implementor's guide, part 7, details the ext3 filesystem in all of its glory. This is another great voyage into the world of journaling filesystems, and ext3 has been rock-solid in my experience."

Comments

Re:Still same old 2GB limit?

[LunaticLeo]

ext2 doesn't have a 2GB file size limit. That was a operating system limit which went away somewhere in the middle 2.2.x stable series.

Further, ext3 is not the-next-version-of-EXT. It is an extention of ext2 which is fully compatible with ext2. Think of ext2 as two things: the format of bits on the disk, and the code to read/write those bits. Ext3 keeps the same format (actually with compatable extentions), but mostly it changes the code for reading/writing to the disk (journelling).

The ext2 filesystem is tried and true. You can go back and forth between ext2 and ext3 with no reformating or issueing of commands other than the mount command.
ReiserFS is a more "sophisticated" filesystem than ext[23], and XFS is a more "sophisticated" filesystem than ReiserFS. But I keep "sophisticated" in quotes because the utility, reliability, and speed of a FS relies more on your usage patterns, than on the genius of the filesystem designers/coders.

FFS-style: ext2,ufsFFS+journel: ext3, ufs+
B+tree directories, B+tree block layout, Journelling: ReiserFS
B+tree directories, B+tree block layout, extents, Journelling : XFS, JFS
Loggin FS: VxFS (my favorite)

I use ext3 at home. Good speed, no need to tar up all my files..reformat drives..untar all my data, journelling, mainline kernel support, tried and true.

One place I would seriously consider ReiserFS is for home directories. The place it really shines is constantly reading and writing lots of "small" files (small ~50k). For Gnome and KDE config files, Mozilla disk caches, CVS checkouts, and untaring of source, ReiserFS is going to be a leader of the benchmarking pack. You'll notice the difference.

But don't get into holy wars over FS, and don't think that Linux is whole generations behind Commercial Unixen. Linux Kernel is dramaticallly ahead in some areas and minorly behind in others. The only place it is dramatically behind is places where the computer you are running the OS on cost more than a half million dollars.

snapshots

[Anonymous Coward]

Not to "troll" for my fav OS or whatever, but I've been playing with snapshots in FreeBSD-CURRENT for the last few days, and I must say that this is quite possibly the coolest filesystem technology I have ever seen.

In short, a snapshot is approximately equal to an image of a filesystem. To create a snapshot, you run a mount command like "-u -o snapshot /var/snapshots/snap1 /var". Becase of the way snapshots work, the snapshot must reside in the same filesystem that it contains.

Now, once the snapshot is created, it can be treated like another filesystem. You can run fsck on it, dump it, or even mount it. The only difference is that within the snapshot, previous snapshots will appear as null files.

Basically, when you create a snapshot, you tell the filesystem that you want it's contents at the current time preserved, and the snapshot file is where it does this. Now, whenever said filesystem is modified, the modification is basically applied in reverse to extant snapshots. So, when a snapshot is first taken, it doesn't contain much information at first, but when you rm a file living in the directory, the file is saved into the snapshot. When you modify a file, deltas to reverse the change are saved to the snapshot.

This is extremely powerful used in the hands of a good sysadmin. Imagine your server that is backed up to tape every week. When someone comes asking for a file they clobbered or deleted by accident, you say "how old was the file?" - you know if they say "8 days", you have to go restore from tape, and if they say "2 days", you have to tell them that they are out of luck. Now imagine if a cron job was set up to take a snapshot once a day, and clear out old ones once a week. If they say "8 days", you still have to go fetch the tapes, but if they say "2 days", all you need is some mdconfig, mount, cp, and umount action to restore the file. How cool is that?

Snapshots essentially give your filesystems the "undo" capabilities that your editor has.