Re: LMDB persistent snapshots

[Howard Chu <hyc@symas.com>]

Michael Conrad wrote:
> On 07/10/2017 12:16 PM, Howard Chu wrote:
> > Michael Conrad wrote:
> > > Hi, I'm scoping out my options for databases that support snapshotting.  
> > > From what I've read so far, this is a natural feature of LMDB,
> >
> > Not really. The persistent state only records the 2 most recent transactions. 
>
> Hm, well so much for that idea ;-)   But could you clarify what would happen 
> in a scenario like:
>
> Proc1: Begin Read Transaction t1
> Proc2: Write Transaction t2
> Proc2: Write transaction t3
> Proc3: Begin Read Transaction t4
> Proc2: Write Transaction t5
> Proc2: Write Transaction t6
>
> After those steps, are process 1 and process 3 still able to continue reading 
> their data, and is all the data they see consistent with the state of the 
> database at the time they began the transaction?

Yes. They each retain an in-memory copy of the DB metadata at the start of 
their transaction, and none of the pages they require can be reused by any 
write transactions.

--
  -- Howard Chu
  CTO, Symas Corp.           http://www.symas.com
  Director, Highland Sun     http://highlandsun.com/hyc/
  Chief Architect, OpenLDAP  http://www.openldap.org/project/