Hammer questions

[Matthew Dillon]

:It was mentioned recently on this list that UFS softupdates is complex / fr=
:agile;
:how is complexity of Hammer compared to softupdates?

    The softupdates code is moderately complex in that it must adapt a
    filesystem (UFS) that was never designed for asynchronous operation
    to be asynchronous.  That was a major undertaking for Kirk and it
    extended UFS's life by another 10 years at least, but the code itself
    is basically a 5000-line single subsystem add-on to UFS.  It is not
    a filesystem unto itself and it would be inappropriate to classify it
    at the same level as something like HAMMER, or ZFS, or EXT4, or Reiser.

:Also: is Hammer considered to be a log structured filesystem?
:
: -thomas

    No, HAMMER definitely isn't log structured, though it uses a number of
    similar concepts.  The reblocker and the method HAMMER currently uses
    for space reclamation is probably the closest, but HAMMER's UNDO FIFO
    only stores meta-data changes and is only used for crash recovery.
    HAMMER is B-Tree-based, and even though HAMMER tends to allocate
    information linearly on the disk the low level structures support
    random and out-of-order allocations and deallocations which are
    definitely not log-like.

					-Matt
					Matthew Dillon 
					<dillon at backplane.com>