Re: SSD + RAID

Greg Smith wrote:
> 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".

Ah, so it is only when write barriers are enabled, and they are not
enabled by default --- OK, that makes sense.

> 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.

Yes, good idea.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ If your life is a hard drive, Christ can be your backup. +