Re: SSD + RAID

Bruce Momjian wrote:
> I thought our only problem was testing the I/O subsystem --- I never
> suspected the file system might lie too. That email indicates that a
> large percentage of our install base is running on unreliable file
> systems --- why have I not heard about this before? Do the write
> barriers allow data loss but prevent data inconsistency? It sound like
> they are effectively running with synchronous_commit = off.
>
You might occasionally catch me ranting here that Linux write barriers
are not a useful solution at all for PostgreSQL, and that you must turn
the disk write cache off rather than expect the barrier implementation
to do the right thing. This sort of buginess is why. The reason why it
doesn't bite more people is that most Linux systems don't turn on write
barrier support by default, and there's a number of situations that can
disable barriers even if you did try to enable them. It's still pretty
unusual to have a working system with barriers turned on nowadays; I
really doubt it's "a large percentage of our install base".

I've started keeping most of my notes about where ext3 is vulnerable to
issues in Wikipedia, specifically
http://en.wikipedia.org/wiki/Ext3#No_checksumming_in_journal ; I just
updated that section to point out the specific issue Ron pointed out.
Maybe we should point people toward that in the docs, I try to keep that
article correct.

--
Greg Smith 2ndQuadrant Baltimore, MD
PostgreSQL Training, Services and Support
greg(at)2ndQuadrant(dot)com www.2ndQuadrant.com