lazy mirror / live backup

[Bakul Shah]

> >> My only concern is zfs is quite heavy weight (memory wise) compared to
> >> gmirror and ufs for a simple laptop/desktop setup which you may just want
> >> to replicate the entire drive and grab the ggatec device and be ready to
> >> run should anything happen to the machine.

If you are using a mirrored device, you just need to keep
track of changed block numbers -- no need to get down to
sector numbers or even fragments.  Assuming 16KB block size
you need to find disksize/16384 bytes somewhere for a bitmap
of changed blocks.  This is not too onerous.  Probably a
variation of ggatec.  And you can let a user mode program
send them to a remote site.

> > vbackup from devel/plan9port stores checksums and allows offering
> > only different blocks to the other side.
> > venti - the backing store behind vbackup - allows compression and
> > single storage of different blocks with same data, which reduces
> > the required backup capacity very impressive.
> > My expirience with vbackup is that this mechanism is fast enough
> > as long as there are no hughe differences.

vbackup is a good replacement for dump.  But it will read all
the blocks in use (even if after the initial dump most of
them won't be written to venti).  This means for very large
disks it will take a long time.  For instance, on my laptop
vbackup does a little over 1GB/minute.  If we can combine
vbackup with a bitmap of changed blocks it can go much
faster!