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Gmane
From: Theodore Tso <tytso <at> MIT.EDU>
Subject: Re: Introducing Next3 - built-in snapshots support for Ext3
Newsgroups: gmane.comp.file-systems.ext4
Date: Saturday 8th May 2010 11:48:49 UTC (over 7 years ago)
On May 8, 2010, at 1:43 AM, Amir G. wrote:

> 1. No features are added to Ext3, so there is no concern for the
> stability of Ext3.
> The feature is added as a new f/s, with the slight overhead of
> duplicate code in the
> kernel tree and an extra loadable module in the system.

This is where it's important to understand exactly what is meant by a
***file system***.   Are you referring to the format, or the
implementation?   The way I've always treated it, and it's the way I
believe most of the ext234 developers have treated it is, that what users
call ext2, ext3, and ext4 are different _implementations_ of the same
_file_ _system_ [format].    That is to say, ext4 simply happens to be a
fuller, more complete implementation  of the same file system as ext2 and
ext3.   Ext2 doesn't support certain features such as journaling and
directory indexing; ext3 doesn't support some advanced features such as
delayed allocation and extents, and requires that the journal always be
present.   Ext4 is a superset of ext2 plus ext3 plus delayed allocation,
extents, a multi-block 
 allocator, and a few other new features.   But they are all the same file
system.

Nor are they the only implementations of that file system.  The BSD file
systems have a compatible (although feature-restricted) implementation,
which was independently implemented.  So does the GNU HURD.   And there are
others.  And note that all of these folks all use the same userspace
utilities, e2fsprogs, for all of these various implementations: BSD, GNU
HURD, and the Linux implementations of ext2, ext3, and ext4 all use the
same set of tools: mke2fs, e2fsck, tune2fs, debugfs, and so on.

The same this is true for NTFS.   There are features in NTFS that you will
find in Windows Vista that don't exist in Windows NT or Windows Vista.  But
everybody treats them as the same file system, even though they have more
advanced features in newer versions of the operating system.

The "ext" in ext2 stands for "extended", as in the "the second extended
file system" for Linux.   It perhaps would be better if we had used the
term "extensible", since that's the main thing about ext2/3/4 that has
given it so much staying power.  We've been able to add, in very carefully
backwards and forwards compatible way, new features to the file system
format.  This is why I object to why Next3 uses some fields that overlaps
with ext4.   It means that e2fsprogs, which supports _one_ and _only_ _one_
file system format, will now need to support two file system formats.  And
that's not something I want to do.

Put another away, it should be possible to add your "Next3" snapshots to
ext4.   Even if today, no one has the time and energy to do the work, it is
something that should be _theoretically_ possible.   In another e-mail
message, you've made the claim: "Unfortunately, merging Next3 snapshots
feature into Ext4 is not an easy task,
because extent mapped files break the design concepts of Next3 snapshots." 
   But aside from stealing fields already assigned to various features
supported by ext4, this isn't true!  I don't see anything that
fundamentally incompatible with Next3 and extent-mapped files.    (Unless
you mean that the snapshot file might not be as efficiently stored using
extent-mapped files, but [a] it's not clear the lack of efficiency will
matter, since most files are contiguously stored, and there can be over 380
extents in a extent tree leaf block, and [b] we could always use an
indirect block mapped file for the snapshot file --- ext4 is fully
backwards compatible with ext2, so you can use an old-style direct/indirect
block mapped file for the snapshot if you really wanted.)

-- Ted

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