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From: Greg Smith <greg(at)2ndquadrant(dot)com>
To: Bruce Momjian <bruce(at)momjian(dot)us>
Cc: Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com>, Merlin Moncure <mmoncure(at)gmail(dot)com>, Brad Nicholson <bnichols(at)ca(dot)afilias(dot)info>, Karl Denninger <karl(at)denninger(dot)net>, Laszlo Nagy <gandalf(at)shopzeus(dot)com>, pgsql-performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: SSD + RAID
Date: 2009-11-30 05:12:41
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
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 ; 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

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Subject: Re: Any have tested ZFS like PostgreSQL installation filesystem?
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Subject: Re: SSD + RAID

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