On 20.5.2013 05:00, Greg Smith wrote: > On 5/16/13 8:06 PM, Tomas Vondra wrote: >> Have you considered using a UPS? That would make the SSDs about as >> reliable as SATA/SAS drives - the UPS may fail, but so may a BBU unit on >> the SAS controller. > > That's not true at all. Any decent RAID controller will have an option > to stop write-back caching when the battery is bad. Things will slow > badly when that happens, but there is zero data risk from a short-term > BBU failure. The only serious risk with a good BBU setup are that > you'll have a power failure lasting so long that the battery runs down > before the cache can be flushed to disk. That's true, no doubt about that. What I was trying to say is that a controller with BBU (or a SSD with proper write cache protection) is about as safe as an UPS when it comes to power outages. Assuming both are properly configured / watched / checked. Sure, there are scenarios where UPS is not going to help (e.g. a PSU failure) so a controller with BBU is better from this point of view. I've seen crashes with both options (BBU / UPS), both because of misconfiguration and hw issues. BTW I don't know what controller are we talking about here - it might be as crappy as the SSD drives. What I was thinking about in this case is using two SSD-based systems with UPSes. That'd allow fast failover (which may not be possible with the SAS based replica, as it does not handle the load). But yes, I do agree that the provider should be ashamed for not providing reliable SSDs in the first place. Getting reliable SSDs should be the first option - all these suggestions are really just workarounds of this rather simple issue. Tomas -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
