Understand. I will be looking for a way to upstream the functionality we need, but will spend time trying to work out how to integrate it into dm-snap or leverage dm-snap in some other way to avoid duplication.
Thanks,
Paul
On Thu, Nov 15, 2018 at 3:15 PM Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
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
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