> No, I had read some megacli related docs about SSD, and the advice was
> to put writethrough on disks. (see
> http://wiki.mikejung.biz/LSI#Configure_LSI_Card_for_SSD_RAID), last section.
> Disks are already in "No Write Cache if Bad BBU" mode. (wrote on
> splitted line on my extract)
====
The advice in that link is for maximum performance e.g. for fileservers where people are dumping documents, temporary
workingspace, and so on.
It is not advice for maximum performance of DB systems which have unique demands in terms of persistence of data
writes.
For postgres, if you use WT with SSDs that are not tested as having data-in-flight protection via capacitors, YOU WILL
GETA CORRUPTED DB WITH WRITETHROUGH (the first time the power is cut). It is quite likely you will not be able to
recoverthat DB, except from backups. Potentially, the consequences of corrupted data could affect your slave depending
onwhich version of postgres you're using.
Don't believe me? test it yourself with that diskchecker.pl program I linked on the other post. You will see the
corruptionhappening to your data even when the disk assures you that it is safely stored.
Do not use WT if you value your data or your uptime.
It may still be acceptable to use writethrough if you can accept a DB becoming SILENTLY CORRUPT after a power cut
reboot.In some use cases that's ok (we have a experimental machine with software raid0 that we just reclone
occasionallyfrom another machine, set for maximum performance, who cares if that db gets corrupt?).
Also, relating to your problem. The issue of unexplained load spikes that don't go away, is something I've heard can
happenwith corrupt dbs. Can anyone else contribute anecdotes?
So, I'm sorry to bring bad news, but there is a possibility your DB is already corrupt because of your previous use of
WT.
Graeme.
