That is a very good question, which I have raised elsewhere on the postgresql lists previously.
In practice: I have *never* managed to make diskchecker fail with the BBU enabled in front of the drives and I spent days trying with plug pulls till I reached the point where as a statistical event it just can't be that likely at all. That's not to say it can't ever happen, just that I've taken all reasonable measures that I can to find out on the time and money budget I had available. In theory: It may be the fact the BBU makes the drives run at about half speed, so that the capacitors go a good bit further to empty the cache, after all: without the BBU in the way, the drive manages to save everything but the last fragment of writes. But I also suspect that the controller itself maybe replaying the last set of writes from around the time of power loss. Anyway I'm 50/50 on those two explanations. Any other thoughts welcome. This raises another interesting question. Does anyone hear have a document explaining how their BBU cache works EXACTLY (at cache / sata level) on their server? Because I haven't been able to find any for mine (Dell PERC H710/H710P). Can anyone tell me with godlike authority and precision, what exactly happens inside that BBU post-power failure? There is rather too much magic involved for me to be happy. G On 07 Jul 2015, at 18:27, Vitalii Tymchyshyn <v...@tym.im> wrote: > Hi. > > How would BBU cache help you if it lies about fsync? I suppose any RAID > controller removes data from BBU cache after it was fsynced by the drive. As > I know, there is no other "magic command" for drive to tell controller that > the data is safe now and can be removed from BBU cache. > > Вт, 7 лип. 2015 11:59 Graeme B. Bell <graeme.b...@nibio.no> пише: > > Yikes. I would not be able to sleep tonight if it were not for the BBU cache > in front of these disks... > > diskchecker.pl consistently reported several examples of corruption > post-power-loss (usually 10 - 30 ) on unprotected M500s/M550s, so I think > it's pretty much open to debate what types of madness and corruption you'll > find if you look close enough. > > G > > > On 07 Jul 2015, at 16:59, Heikki Linnakangas <hlinn...@iki.fi> wrote: > > > > > So it lies about fsync()... The next question is, does it nevertheless > > enforce the correct ordering of persisting fsync'd data? If you write to > > file A and fsync it, then write to another file B and fsync it too, is it > > guaranteed that if B is persisted, A is as well? Because if it isn't, you > > can end up with filesystem (or database) corruption anyway. > > > > - Heikki > > > > -- > Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-performance -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
