cvs commit: src/sys/vfs/hammer hammer.h hammer_disk.h hammer_io.c hammer_ondisk.c

Matthew Dillon dillon at
Fri Aug 29 14:52:21 PDT 2008

:What's the status of UFS with soft updates with respect to disk cache 
:flushing? Doesn't do it currently and therefore can't provide its 
:guarantees for consistency after a power failure?

    It's almost impossible to do with UFS unless you want to slow
    performance to a crawl.  While softupdates can gang unrelated writes
    together, it can't gang dependant writes together at all so each
    element in a dependancy chain would require its own flush command.

    HAMMER dependancies work differently.   HAMMER dependancies can be
    ganged together into two groups of writes, typically requiring only
    two flush commands to clean out the entire buffer cache.

					Matthew Dillon 
					<dillon at>

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