Re: Asynchronous commit | Transaction loss at server crash

Jesper Krogh wrote:
> A Battery Backed raid controller is not that expensive. (in the range
> of 1 or 2 SSD disks).
> And it is (more or less) a silverbullet to the task you describe.

Maybe even less; in order to get a SSD that's reliable at all in terms
of good crash recovery, you have buy a fairly expensive one.  Also, and
this is really important, you really don't want to deploy onto a single
SSD and put critical system files there.  Their failure rates are not
that low.  You need to put them into a RAID-1 setup and budget for two
of them, which brings you right back to

Also, it's questionable whether a SSD is even going to be faster than
standard disks for the sequential WAL writes anyway, once a non-volatile
write cache is available.  Sequential writes to SSD are the area where
the gap in performance between them and spinning disks is the smallest.

> Plugging your system (SSD's) with an UPS and trusting it fully
> could solve most of the problems (running in writeback mode).

UPS batteries fail, and people accidentally knock out over server power
cords.  It's a pretty bad server that can't survive someone tripping
over the cord while it's busy, and that's the situation the "use a UPS"
idea doesn't improve.

--
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com   www.2ndQuadrant.us