Re: a question about Direct I/O and double buffering

From: Jim C. Nasby
Subject: Re: a question about Direct I/O and double buffering
Date: ,
Msg-id: 20070418180758.GW72669@nasby.net
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In response to: Re: a question about Direct I/O and double buffering  (Erik Jones)
List: pgsql-performance

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a question about Direct I/O and double buffering  (Xiaoning Ding <-state.edu>, )
 Re: a question about Direct I/O and double buffering  (Erik Jones, )
  Re: a question about Direct I/O and double buffering  (Xiaoning Ding <-state.edu>, )
   Re: a question about Direct I/O and double buffering  (Erik Jones, )
    Re: a question about Direct I/O and double buffering  ("Alex Deucher", )
     Re: a question about Direct I/O and double buffering  (Xiaoning Ding <-state.edu>, )
      Re: a question about Direct I/O and double buffering  ("Alex Deucher", )
      Re: a question about Direct I/O and double buffering  (, )
   Re: a question about Direct I/O and double buffering  (, )
    Re: a question about Direct I/O and double buffering  (Erik Jones, )
     Re: a question about Direct I/O and double buffering  (, )
  Re: a question about Direct I/O and double buffering  (Mark Lewis, )
   Re: a question about Direct I/O and double buffering  (Erik Jones, )
    Re: a question about Direct I/O and double buffering  (Mark Lewis, )
     Re: a question about Direct I/O and double buffering  (Erik Jones, )
      Re: a question about Direct I/O and double buffering  ("Jim C. Nasby", )

On Thu, Apr 05, 2007 at 03:10:43PM -0500, Erik Jones wrote:
> Nope.  What we never tracked down was the factor of 10 drop in
> database transactions, not disk transactions.  The write volume was
> most definitely due to the direct io setting -- writes are now being
> done in terms of the system's block size where as before they were
> being done in terms of the the filesystem's cache page size (as it's
> in virtual memory).  Basically, we do so many write transactions that
> the fs cache was constantly paging.

Did you try decreasing the size of the cache pages? I didn't realize
that Solaris used a different size for cache pages and filesystem
blocks. Perhaps the OS was also being too aggressive with read-aheads?

My concern is that you're essentially leaving a lot of your memory
unused this way, since shared_buffers is only set to 1.6G.

BTW, did you ever increase the parameter that controls how much memory
Solaris will use for filesystem caching?
--
Jim Nasby                                            
EnterpriseDB      http://enterprisedb.com      512.569.9461 (cell)


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