Re: MDB v2: Replace meta pages with "meta position" word

[Howard Chu <hyc@symas.com>]

Hallvard Breien Furuseth wrote:
> I wrote:
> > One thing about ACID - IIRC there are some potential data races which my
> > single-word meta info in the header will fix.  E.g. if the user does ^Z
> > at an unfortunate time, and with WRITEMAP updating the meta page while a
> > txn is reading it.
>
> Reproducible by sending frequent SIGSTOPs and SIGCONTs, with mdb.c
> patched to verify (memcmp or checksum) the meta info after copying.
> Happens even with 1 MDB process.
>
> (Re comments elsewhere: Selecting a meta page is atomic, but
> copying/writing it is not.)

True, but writing the meta page doesn't need to be atomic - it won't be 
considered current until its txnid is updated. Until then, its txnid will be 
less than the other page's anyway.

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