Re: New server: SSD/RAID recommendations? - Mailing list pgsql-performance
From | Graeme B. Bell |
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Subject | Re: New server: SSD/RAID recommendations? |
Date | |
Msg-id | BEA346EE-F8C1-42C6-81A4-C6B05B732E91@skogoglandskap.no Whole thread Raw |
In response to | Re: New server: SSD/RAID recommendations? (Vitalii Tymchyshyn <vit@tym.im>) |
Responses |
Re: New server: SSD/RAID recommendations?
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List | pgsql-performance |
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 daystrying with plug pulls till I reached the point where as a statistical event it just can't be that likely at all. That'snot to say it can't ever happen, just that I've taken all reasonable measures that I can to find out on the time andmoney 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 furtherto empty the cache, after all: without the BBU in the way, the drive manages to save everything but the last fragmentof writes. But I also suspect that the controller itself maybe replaying the last set of writes from around the timeof 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 (atcache / sata level) on their server? Because I haven't been able to find any for mine (Dell PERC H710/H710P). Can anyonetell 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@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 itwas fsynced by the drive. As I know, there is no other "magic command" for drive to tell controller that the data is safenow and can be removed from BBU cache. > > Вт, 7 лип. 2015 11:59 Graeme B. Bell <graeme.bell@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 closeenough. > > G > > > On 07 Jul 2015, at 16:59, Heikki Linnakangas <hlinnaka@iki.fi> wrote: > > > > > So it lies about fsync()... The next question is, does it nevertheless enforce the correct ordering of persisting fsync'ddata? If you write to file A and fsync it, then write to another file B and fsync it too, is it guaranteed that ifB 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
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