Re: Question regarding MDB_NOLOCK

[Howard Chu <hyc@symas.com>]

David Barbour wrote:
> Okay, I think I see what's happening here:
>
> mdb_find_oldest(): will return the most recent snapshot if no readers exist
>
> mdb_page_alloc(): will search FREE_DBI for a transaction `last` that is
> less than oldest, and will try to find a contiguous range of pages that
> were free'd by said transaction, potentially merging free pages from
> many transactions. If nothing is found, will instead grow the database.
>
> Since `last` < `oldest` when we reuse any old pages, and we're only
> using the 'freed' pages from last (not the data pages), we know that at
> the data pages for the eldest two transactions are protected.
>
> Is this right?

Yep.

> My earlier assumption (before reading mdb_page_alloc) was that LMDB
> would be aggressive about grabbing pages freed by transactions that are
> not actively being read. If we're relying on `last < oldest` to create a
> two page discrepancy, this means when we actually have readers on older
> transactions that we're being little more conservative than necessary.

More than necessary? I don't think so.

> But it does protect the last two snapshots.

Yes, always.

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