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IBM JFS for Linux FAQ

(Frequently Asked Questions)

Version 1.0

Last edited on: July 1, 2001

By Moshe Bar, moshe@moelabs.com

This document is under the GPL License

Q: What is the current latest version of JFS for Linux. Is in released already?

A: We have released JFS 1.0!!

Q: What is the maximum size of a file system under JFS?

A: The maximum aggregate size is between 4 and 32 petabytes. A 512 block size allows 4 PB and a 4k block size allows 32 PB. Actually, JFS is limited by the addressability of the block number on the block device. In struct buffer_head, b_blocknr is declared as an unsigned long, although, several places in the vfs code use an int when dealing with the block number. If we can assume we can use 32 bits for the block number (we may be stuck with 31), a 4K blocksize would allow us a filesystem of 4K * 2^32 = 2^44 = 16 TB. The block device drivers may limit this further.

Kernel 2.3.x supports LFS. The only 32bitism left  is file locks There is also a 2.2 patch for LFS now. The 2.2 LFS patch is available from http://www.scyld.com/

Q: Which Linux kernel versions does JFS support?

A: As of the last edition of this FAQ, the following versions are supported: 2.2.12, 22.13, 2.2.14, 2.2.15, 2.2.16, 2.2.17 and 2.2.18, 2.2.19 and the new kernel series with 2.4.

Q: Where can I obtain the jfs software?

A: Current snapshots as well as patches for the supported Linux kernel versions can be downloaded from: http://oss.software.ibm.com/developerworks/opensource/jfs/

Q: Is JFS a journaling or a logging file system?

A: It is both. JFS runs a current log of updates to the file system and journals the entries in a transactional manner. For a more thorough treatise of the internals of JFS, await the book Linux File Systems by Moshe Bar to be published in November 2000 by McGraw-Hill.

Q: I'm looking for documentation of the Linux Virtual File-system Layer?

A: Neil Brown neilb@cse.unsw.edu.au and others. have created documentation about the Linux Virtual File-system Layer. This document is at the following url.  http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html#toc1

Q:.Is Logical Volume Manager (LVM) part of the JFS port?

A:LVM isn't included with the JFS port. JFS will have the file system support that LVM needs from the file system to grow the file system size, etc. There is a LVM project http://linux.msede.com/lvm/ in Linux that has been added to the 2.4  kernel  release.

Q: Will the GPL'd JFS handle hot growing of the filesystem as it does in AIX? (say if an LVM is used)

A:Yes JFS has the support in the file system to grow and under our TODO list in the readme it is called "get EXTENDFS capabilities operational." We still need to port this functionality to Linux as of drop 7.

Q: What are the features of  a journaled file system?

A: Take a look at the JFS Overview white paper at http://www-4.ibm.com/software/developer/library/jfs.html

In general a journaling file system ensures that if a power outage or  other system failure, the file system won't be corrupted and no disk modifications will be left incomplete. The structure of the disk volume is restored to a consistent state when the system is started again.

Q: What will the maximum file size be on JFS for linux on an x86? 

A:JFS will support a maximum file size of 512 terabytes (with block size 512 bytes) to 4 petabytes (with block size 4 Kbytes). The virtual file system layer could  limit the maximum file size that a file system can support. This is what is happening on Linux in 2.2.14 or below tree. I  (Steve Best) haven't checked yet if the LFS patch has made it into  the 2.2.x series of the kernel yet.

Q: Is the IBM Linux JFS the only JFS being developed for Linux?  I didn't even know IBM was doing this and I was sure that in the past I had read of other  folks doing it.

A:There are at least 3 other efforts going on to add journaling to Linux. Ext3, ReiserFS, XFS. Links to those projects are included at the bottom of this JFS article. http://oss.software.ibm.com/developer/opensource/features/jfs_feature.html

Q: Why should I use JFS instead of ReiserFS or another file system?

A:We (the developers) can't compare JFS vs ResierFS, since we are not experts on ReiserFS or other file systems. Both are journaling file systems and by being journaling file systems the user will have fast restart time after crash or power interruption. Our JFS technology has been available in IBM enterprise servers for several years and several thousands of customers are running their businesses using this file system.

By having more than one journaling file system under development for Linux is goodness. The file system community is working together to have a better file system frame work for Linux. Also, by both file systems being licensed under GPL we can in the future share algorithms and make both file systems better. Having more than one journaling file system for Linux in the long run with make Linux a better OS.

Q: Would like to see JFS licensed as non-GPL, so it could be ported to BSD.

A:We have decided not to do this at this time for a variety of internal IBM reasons. Although this didn't work out, we're certainly open to hearing similar suggestions in the future.

Q: When will JFS be merged with the standard Linux kernel?

A: Linus Torvalds and Alan Cox are constantly updated on the progress of JFS. Because we are making good progress, it is quite probable that we will JFS included in the 2.4 kernels soon.

Q: Does JFS work with software RAID on Linux?

A: No. There are major problems with that. But, then why would you run software RAID?

Q: Does JFS work with hardware RAID?

A: Yes, works perfectly.

Q: Does JFS work with NFS?

A: Yes, it does

Q: How fast is JFS compared to ext2?

A: Folks, there is no way around it: journaling is slower inherently compared to non-journaling file systems. Some code has to make for the added security and that code slows down I/O operations. The good news is that not even ext2fs is as fast as it could be theoretically, so JFS tries to improve things by being fast at basic I/O operations, be they for journaling or for data.

Q: Do I need to run logreply before fsck.jfs?

A: Conceptually, yes. But if you run fsck.jfs with the -f switch it will invoke the logreply for you.

Q: Can I mount AIX JFS disks on my Linux box? What about JFS/OS2?

A: AIX disks are not mountable under JFS/Linux because of the different volume labels under the AIX LVM. JFS for OS/2 disks however can be read regularly under Linux.