Re: file system and raid performance

Mark Mielke wrote:
> Greg Smith wrote:
> > On Fri, 15 Aug 2008, Bruce Momjian wrote:
> >> 'data=writeback' is the recommended mount method for that file 
> >> system, though I see that is not mentioned in our official 
> >> documentation.
> > While writeback has good performance characteristics, I don't know 
> > that I'd go so far as to support making that an official 
> > recommendation.  The integrity guarantees of that journaling mode are 
> > pretty weak.  Sure the database itself should be fine; it's got the 
> > WAL as a backup if the filesytem loses some recently written bits.  
> > But I'd hate to see somebody switch to that mount option on this 
> > project's recommendation only to find some other files got corrupted 
> > on a power loss because of writeback's limited journalling.  ext3 has 
> > plenty of problem already without picking its least safe mode, and 
> > recommending writeback would need a carefully written warning to that 
> > effect.
> 
> To contrast - not recommending it means that most people unaware will be 
> running with a less effective mode, and they will base their performance 
> measurements on this less effective mode.
> 
> Perhaps the documentation should only state that "With ext3, 
> data=writeback is the recommended mode for PostgreSQL. PostgreSQL 
> performs its own journalling of data and does not require the additional 
> guarantees provided by the more conservative ext3 modes. However, if the 
> file system is used for any purpose other than PostregSQL database 
> storage, the data integrity requirements of these other purposes must be 
> considered on their own."
> 
> Personally, I use data=writeback for most purposes, but use data=journal 
> for /mail and /home. In these cases, I find even the default ext3 mode 
> to be fewer guarantees than I am comfortable with. :-)

I have documented this in the WAL section of the manual, which seemed
like the most logical location.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

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