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Re: SSD + RAID

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Greg Smith <greg(at)2ndquadrant(dot)com>
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 12:08:22
Message-ID: 200911301208.nAUC8MP29058@momjian.us (view raw or flat)
Thread:
Lists: pgsql-performance
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. +

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