DragonFly kernel List (threaded) for 2008-07
Re: Hammer questions
:It was mentioned recently on this list that UFS softupdates is complex / fr=
: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?
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.