Little UFS2 FAQ

Jeroen C. van Gelderen jeroen at
Wed Apr 23 06:20:46 PDT 2003

Little UFS2 FAQ (20023/04/23), culled from contributions by:

Peter Schultz <peter at>
"Poul-Henning Kamp" <phk at>
Marcin Dalecki <mdcki at>
CARTER Anthony <a.carter at>
Robert Watson <rwatson at>
Terry Lambert <tlambert2 at>
Frank van der Linden <fvdl at>
Manfred Antar <null at>
Narvi <narvi at>
Takahashi Yoshihiro <nyan at>
Alexander Pohoyda <alexander.pohoyda at>
Bruce Evans <bde at>
"Wilkinson,Alex" <Alex.Wilkinson at>

What is UFS1?

The long-time BSD(?) native filesystem.

What is UFS2?

UFS2 is an extension to the well-known UFS. It adds 64 bit block 
pointers (breaking the 1T barrier), support for extended file storage, 
and a few other things.

Short summary of changes:
  o 64-bit pointers up the wazoo
    (implies that inodes have doubled in size, and now are 256 bytes)
  o Layout and functional changes to help support variable-size blocks
    (extent-like allocation)
  o Extension of various flag fields
  o Addition of per-inode extended attribute extent
  o Lazy inode initialization (watch newfs(8) fly)

Apart from these modifications all UFS1 code is being used in UFS2 

What is the difference between UFS and FFS?

UFS (and UFS2) define on-disk data layout. FFS sits on top of UFS (1 or 
2) and provides directory structure information, etc, etc. This FAQ is 
about a revision of UFS named UFS2.

What is the rationale for UFS2?

The motivating factor in the layout change was the need for better 
Extended Attribute (EA) support, and while the developers were at it 
they figured they'd do a bunch of other useful things too.  UFS2 uses 
the same basic technologies as modern UFS1 (inodes, linear directory 
layout, soft updates, snapshotting, background file system checking, 
etc) so it was a relatively low-risk change.

Why did you not add <feature> while you were at it?

It would most likely require significant changes whereas the developers 
wanted to restrict themselves to low-risk modifications only. See 
previous question.

Which OSes support UFS2?

FreeBSD and NetBSD. (Others?)

What is the UFS2 status on FreeBSD?

As of 2003/04/20, newfs(8) and sysinstall(8) will create UFS2 file 
systems by default, unless explicitly specified.  Users wanting to 
create UFS1 file systems for whatever reason (interoperability with 
earlier versions, etc) should be sure to employ the -O1 flag to 
newfs(8), or hit '1' in the label editor in sysinstall(8) to select 

What is the UFS2 status on NetBSD?

As of 2003/04/02 UFS2 is not (yet) the default type for FFS 
filesystems. newfs(8) will create a normal FFS filesystem by default. 
If you want an UFS2 fileystem, specify "-O 2" as an option.

No additional kernel options are needed for UFS2 support, it's 
contained within the FFS code.

Please note that older fsck binaries will complain a bit about 
filesystems if you boot a new kernel, because of some superblock 
changes. This is harmless. However, if you have 1.6 fsck binaries, they 
will signal a fatal superblock mismatch with the first alternate, 
because they compare too many fields (even ones that aren't useful). 
This is annoying, and I'd advise peole to upgrade their fsck_ffs binary 
before using a new kernel. 1.6.1 will have an fsck that is forward 
compatible. Again, none of this signals actual filesystem damage, but 
it's still annoying.

Does the /boot/loader now understand UFS2 on the root filesystem (i386)?

Yes, modulo the restriction that your root filesystem cannot be larger 
than 1.5TB. David Schultz et al. proposed a patch to remove this 

Does the /boot/loader now understand UFS2 on the root filesystem (PC98)?

Nope. It is unknown if work is underway to address this.

Is there a UFS to UFS2 conversion tool?

No. You can however dump/restore from UFS to UFS2.

Will "dump" on UFS and "restore" on UFS2 filesystem work?

Yes, that will work.

Does UFS2 dynamically allocate inodes?

No it does not.  Inodes are preallocated, but UFS2 lazily initializes 
them.  This mainly means that newfs(8) runs much faster.

Does Grub work with UFS2?

No. Not yet(?).

Jeroen C. van Gelderen - jeroen at

"They accused us of suppressing freedom of expression.
This was a lie and we could not let them publish it."
   -- Nelba Blandon,
      Nicaraguan Interior Ministry Director of Censorship

More information about the freebsd-current mailing list