lazy mirror / live backup

Mike Wolman mike at
Sat Apr 21 01:25:49 UTC 2007

On Sat, 21 Apr 2007, Bernd Walter wrote:

> On Sat, Apr 21, 2007 at 01:38:31AM +0100, Mike Wolman wrote:
>> 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.
> Yes - that's unfortunately true.
>> Im sure there are other situations where running zfs on the available
>> hardware is not an option compared to gmirror - im not sure what the
>> recommended amount for freebsd but as far as i can rember the suggested
>> about for solaris is 1Gb - comparing to gmirror i think i have run it on a
>> machine doing simple home fileserving with 128Mb.
> Ever thought about UFS snapshots backed with rsync?
> You get a consitent pseudo image of your running filesystem with
> unallocated blocks represented as zeros.
> rsync now allows comparing file chunks and copies only differences.
> Still every block need to be read, though.

Yea i use rsync and snapshots quite a bit, but unfortunately rsync works 
at the filesystem level so you cant really get a bootable image of the whole 
device.  It would be nice if this could be done without user interaction, 
ie if the ggatec component of a mirror disappears and reappears gmirror 
justs gets to work syncing things up.

> 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.

I'll take a peek as it sounds interesting,


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