Re: HAMMER and RAID question

[Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxx>]

    I'd rather keep the functionality less integrated.

    One interesting thing with HAMMER is that it would be possible to
    integrate its synchronization mechanics with a software RAID subsystem
    and get rid of unnecessary disk syncs.  That is, the software RAID
    subsystem would allow disks in a mirror to be slightly out of sync
    during periods where HAMMER itself does not request a disk sync.
    HAMMER won't care if older data (going back to the previous disk sync)
    is returned during those periods.

    This would allow a software mirror to implement fairly optimal zone
    tracking without needing a battery backup, which in turn would allow
    portions of the mirror to go offline without requiring a complete
    recopying of an entire disk when those portions come back online again.

    One of the biggest problems software RAID systems have now, particularly
    if one is trying to implement the RAID across discrete systems or over
    internet links, is when a portion of the RAID goes offline temporarily
    then comes back online again.  With multi-terrabyte disks it is important
    to avoid any unnecessary copying that would degrade the array for
    extremely long periods of time.

						-Matt