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Re: UFS2 support?

From: Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxx>
Date: Thu, 3 Feb 2005 10:08:03 -0800 (PST)

:Hmm... 4.10 seems to have -C as well.
:     -C cachesize
:             Specify the cache size in megabytes.  This will greatly improve
:             performance at the cost of dump possibly not noticing changes in
:             the filesystem between passes.  Beware that dump forks, and the
:             actual memory use may be larger then the specified cache size.
:             The recommended cache size is between 8 and 32 (megabytes).
:I am not using this right now. Looks like the file system doesn't have
:to be RO to use it either. I'll try this tonight. This still doesn't
:solve the bigger problem of the time for restore though. A full backup
:taking 12 hours is bad but the restore taking 2-3 days is the real

    Guess who wrote that option ?

    revision 1.44
    date: 2003/01/13 19:42:41;  author: dillon;  state: Exp;  lines: +7 -2
    Add a caching option to dump.  Use -C.  Note that NetBSD has a caching option
    called -r but it takes 512 byte blocks instead of megabytes, and I felt a
    megabytes specification would be far more useful so I did not use the same
    option character.

:> By the way, snapshots don't require UFS2.
:> From reading Kirk McKusick's paper on soft-updates and snap-
:> shots, I get the impression that implementing snapshots in
:> FFS is pretty straight-forward and simple -- much simpler
:> than the implementation of soft-updates.  Thus I think there
:> is hope that someone might bring it to DragonFly some day,
:> too.  :-)
:That would be nice.  There is a lot about snapshots that is very
:cool.  NetApp filers have a snapshot facility and that does one
:every hour so you can get back those files you deleted by accident.

    Snapshots in UFS are a huge hack in my opinion.    They are *not*
    a pure block level snapshot.  Instead UFS plays tricks with its
    allocation bitmap amoung other things in an attempt to make the
    snapshot ultra efficient.  But like softupdates the snapshot 
    mechanism is very complex in actual implementation and I just don't
    trust it.

    A would like to see a block level snapshot device in DragonFly, but
    I don't have time to write it (right now).

					Matthew Dillon 

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