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 From : Sergey Lentsov                       2:4615/71.10   08 Mar 2001  18:11:10
 To : All
 Subject : URL: http://lwn.net/2001/0308/kernel.php3
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    See also: [14]last week's Kernel page.
    
 Kernel development
 
    The current kernel release is still 2.4.2. Linus has resumed work
    toward 2.4.3; his current prepatch release is [15]2.4.3pre3. Alan Cox,
    meanwhile, has kept up his pace and reached [16]2.4.2ac14. A reader
    asked us to figure out how much of the "ac" patches have made it into
    Linus's kernel. Unfortunately, there is no easy answer to that
    question. Linus's changelogs just say "Alan Cox: continued merging".
    The only person who actually knows the answer, in all likelihood, is
    Alan, and he does not have the time to make a list.
    
    No new 2.2.19 prepatches have been released in the past week.
    
    SnapFS alpha release. Peter Braam and his colleagues at Mountain View
    Data have [17]announced the alpha release of SnapFS, a new filesystem
    add-on. As an alpha release, it's not something that you are likely to
    want to put on that big departmental server. It has some interesting
    features, though, that make it definitely worth a look.
    
    Essentially, SnapFS enables a filesystem to preserve its history. One
    could compare it to the old VMS file versioning scheme, but SnapFS is
    far more flexible than that. It can preserve the state of an entire
    filesystem at any given time; it can also be set up to preserve every
    revision that is ever made of every file on the system. That latter
    mode, presumably, is recommended only for users with very large disks.
    
    To many, SnapFS may just seem like a way of filling up excess disk
    space. But, in fact, there are some truly useful applications for such
    a filesystem:
      * Backups. SnapFS can be used to stabilize a version of the
        filesystem, which can then be written to tape. There is no need to
        (1) kick the users off the system to get a clean backup, or
        (2) run backups on a live system and just hope everything comes
        out OK.
      * System administration work. Imagine freezing the filesystem state
        before that next big operating system upgrade. If it doesn't work
        out, one command brings back the previous state of affairs.
      * Fat-fingered users. Most system administrators learn early on that
        the most frequent use of backups is not recovery from disk
        disasters; [SnapFS diagram] it's restoring files that some user
        (usually the same user) destroyed. With snapshots, that process is
        much easier.
        
    SnapFS works by placing itself between the Linux virtual filesystem
    (VFS) and any of a number of underlying filesystems - though only XFS
    and ext3 are implemented in the alpha version. The diagram on the
    right (stolen from a white paper on the [18]Mountain View Data SnapFS
    page) shows how the pieces fit together. The supporting filesystem
    must be of the journaling variety, since SnapFS relies on the atomic
    updates that journaling filesystems provide. The filesystem must also
    support extended attributes, essentially the hanging of additional
    metadata onto files. Thus, most users will need to apply both the ext3
    and extended attribute patches (of which there is a new release - see
    below) to be able to run SnapFS.
    
    Whenever a file is to be modified, and its contents must be preserved
    in a snapshot, SnapFS creates a new inode in the filesystem to hold
    the snapshot version. An extended attribute which points to the
    snapshot inode is then attached to the visible version of the file.
    The actual blocks of the file are shared between the current file and
    the snapshot until they are changed; at that point the SnapFS "copy on
    write" mechanism makes copies of the affected blocks. Snapshots are
    thus relatively efficient in their use of storage, especially in
    situations where only parts of files are changed. For example, a
    snapshot of that huge web server log file, which is only appended to,
    does not duplicate the log entries that are shared between the current
    and archived versions.
    
    This mechanism also makes the creation of snapshots very fast. Since
    no data is copied at that time, making a snapshot is really just a
    matter of filling in a table entry.
    
    A set of tools is provided with SnapFS to handle the management of
    SnapFS filesystems, performing rollbacks to older versions, etc.
    Mountain View Data's revenue model is starting to come into focus,
    though - a number of additional management tools will be proprietary.
    For example, there will be utilities to stabilize and quiesce Oracle
    and MySQL databases for snapshots. The basic SnapFS code, however, is
    licensed under the GPL.
    
    What should the kernel do with DOS-formatted scripts? A user recently
    [19]turned up a little problem. Imagine that you have a perl script
    that starts with the usual incantation:
         #!/usr/bin/perl
 
    You would expect the kernel to be able to run the perl interpreter
    when the script is invoked. But now imagine that the script is in DOS
    format - each line ends with a carriage-return and a line feed (\r\n)
    rather than just a line feed (\n), which has been the Unix standard
    forever. The kernel, in this case, will see the carriage return as
    part of the interpreter name; as a result, the user gets a "no such
    file or directory" complaint from the shell.
    
    This user, Ivo Timmermans, included a patch that would make the kernel
    strip out the carriage return in scripts like this. The [20]initial
    response from Alan Cox was not particularly receptive: "Fix the
    script. The kernel expects a specific format." That approach makes
    sense to some - why should the kernel go out of its way to support
    scripts that are not in the specified format?
    
    It was subsequently pointed out, however, that the kernel will happily
    strip away other sorts of trailing white space, such as space
    characters and tabs. Should not carriage returns, which are generally
    recognized to be white space as well, be stripped too? Good question,
    with no answer from those who would eventually have to accept the
    patch. For now, "fix the script" is the order of the day.
    
    Other patches and updates released this week include:
    
      * Andreas Gruenbacher has [21]released version 0.7.8 of the access
        control list and extended attribute patch.
      * The "final" version of the aacraid driver, which "supports the
        on-board RAID controllers on the Dell PowerEdge 2400, 2450, and
        4400 servers, the add-in 4-channel PERC2 card, and the HP
        NetRAID-4M card," has been [22]released by Matt Domsch.
      * Brad Douglas has [23]announced that the linux-fbdev mailing list
        will be making the pilgrimage over to SourceForge.
      * Christian Worm Mortensen [24]released a new version of the "WRR"
        scheduler, which provides control over the use of an Internet
        connection shared by several systems. The message doesn't contain
        the URL for the code, however; if you're interested, the WRR page
        is [25]over here.
      * Zach Brown has released [26]a massively updated maestro sound card
        driver. It looks like it has a number of good fixes, but it's only
        for the 2.2.18 kernel, currently.
      * Steve Best has released [27]JFS beta 2 release 0.2.0, the latest
        version of IBM's journaling filesystem.
        
    Section Editor: [28]Jonathan Corbet
    March 8, 2001
    
    For other kernel news, see:
      * [29]Kernelnotes
      * [30]Kernel traffic
      * [31]Kernel Newsflash
      * [32]Kernel Trap
    
    Other resources:
      * [33]Kernel Source Reference
      * [34]L-K mailing list FAQ
      * [35]Linux-MM
      * [36]Linux Scalability Project
      * [37]Kernel Newbies
    
    
    
                                                   [38]Next: Distributions
    
    [39]Eklektix, Inc. Linux powered! Copyright Л 2001 [40]Eklektix, Inc.,
    all rights reserved
    Linux (R) is a registered trademark of Linus Torvalds
 
 References
 
    1. http://lwn.net/
    2. http://ads.tucows.com/click.ng/pageid=001-012-132-000-000-003-000-000-012
    3. http://lwn.net/2001/0308/
    4. http://lwn.net/2001/0308/security.php3
    5. http://lwn.net/2001/0308/dists.php3
    6. http://lwn.net/2001/0308/desktop.php3
    7. http://lwn.net/2001/0308/devel.php3
    8. http://lwn.net/2001/0308/commerce.php3
    9. http://lwn.net/2001/0308/press.php3
   10. http://lwn.net/2001/0308/announce.php3
   11. http://lwn.net/2001/0308/history.php3
   12. http://lwn.net/2001/0308/letters.php3
   13. http://lwn.net/2001/0308/bigpage.php3
   14. http://lwn.net/2001/0301/kernel.php3
   15. http://lwn.net/2001/0308/a/2.4.3pre3.php3
   16. http://lwn.net/2001/0308/a/2.4.2ac14.php3
   17. http://lwn.net/2001/0308/a/snapfs.php3
   18. http://www.mountainviewdata.com/us/technology/snapfs.html
   19. http://lwn.net/2001/0308/a/dos-script.php3
   20. http://lwn.net/2001/0308/a/ac-script.php3
   21. http://lwn.net/2001/0308/a/acl.php3
   22. http://lwn.net/2001/0308/a/aacraid.php3
   23. http://lwn.net/2001/0308/a/linux-fbdev.php3
   24. http://lwn.net/2001/0308/a/wrr.php3
   25. http://wipl-wrr.dkik.dk/wrr/
   26. http://lwn.net/2001/0308/a/maestro.php3
   27. http://lwn.net/2001/0308/a/jfs.php3
   28. mailto:lwn@lwn.net
   29. http://www.kernelnotes.org/
   30. http://kt.zork.net/
   31. http://www.atnf.csiro.au/~rgooch/linux/docs/kernel-newsflash.html
   32. http://www.kerneltrap.com/
   33. http://lksr.org/
   34. http://www.tux.org/lkml/
   35. http://www.linux.eu.org/Linux-MM/
   36. http://www.citi.umich.edu/projects/linux-scalability/
   37. http://www.kernelnewbies.org/
   38. http://lwn.net/2001/0308/dists.php3
   39. http://www.eklektix.com/
   40. http://www.eklektix.com/
 
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