Re: [dm-devel] [RFC] dm-bow working prototype

[Mikulas Patocka <mpatocka@redhat.com>]

On Mon, 29 Oct 2018, Paul Lawrence wrote:

> 
> > The snapshot target could be hacked so that it remembers space trimmed
> > with REQ_OP_DISCARD and won't reallocate these blocks.
> > 
> > But I suspect that running discard over the whole device would degrade
> > performance more than copying some unneeded data.
> > 
> > How much data do you intend to backup with this solution?
> > 
> > 
> We are space-constrained - we will have to free up space for the backup before
> we apply the update, so we have to predict the size and keeping usage as low
> as possible is thus very important.
> 
> Also, we've discussed the resizing requirement of the dm-snap solution and
> that part is not attractive at all - it seems it would be impossible to
> guarantee that the resizing happens in a timely fashion during the (very busy)
> update cycle.
> 
> Thanks everyone for the insights, especially into how dm-snap works, which I
> hadn't fully appreciated. At the moment, and for the above reasons, we intend
> to continue with the dm-bow solution, but do want to keep this discussion
> open. If anyone is going to be at Linux Plumbers, I'll be presenting this work
> and would love to chat about it more.

dm-snapshot took 9 years to fix the last data corruption bug (2004-2013 - 
the commit e9c6a182649f4259db704ae15a91ac820e63b0ca).

And with the new target duplicating the snapshot functionality, it may be 
the same.

Mikulas