On Sep 20, 2012, at 10:05 PM, Stefan Ring <stefan...@gmail.com> wrote:

> On Fri, Sep 21, 2012 at 6:31 AM, andy thomas <a...@time-domain.co.uk> wrote:
>> I have a ZFS filseystem and create weekly snapshots over a period of 5 weeks
>> called week01, week02, week03, week04 and week05 respectively. Ny question
>> is: how do the snapshots relate to each other - does week03 contain the
>> changes made since week02 or does it contain all the changes made since the
>> first snapshot, week01, and therefore includes those in week02?
> Every snapshot is based on the previous one and store only what is
> needed to capture the differences.

This is not correct. Every snapshot is a complete point-in-time view of the 

You can send differences between snapshots that can be received, thus satisfying
a requirement for incremental replication. Internally, this is easy to do 
because the
birth order (in time) of a block is recorded in the metadata.

>> To rollback to week03, it's necesaary to delete snapshots week04 and week05
>> first but what if week01 and week02 have also been deleted - will the
>> rollback still work or is it ncessary to keep earlier snapshots?
> No, it's not necessary. You can rollback to any snapshot.
> I almost never use rollback though, in normal use. If I've
> accidentally deleted or overwritten something, I just rsync it over
> from the corresponding /.zfs/snapshots directory. Only if what I want
> to restore is huge, rollback might be a better option.

Yes, rollback is not used very frequently. It is more common to copy out or 
clone the older snapshot. For example, you can clone week03, creating 
what is essentially a fork.
 -- richard

illumos Day & ZFS Day, Oct 1-2, 2012 San Fransisco 

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