Re: New server: SSD/RAID recommendations?

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 <vit(at)tym(dot)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(dot)bell(at)nibio(dot)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 <hlinnaka(at)iki(dot)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
>
>
>
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