delayed write tuning (Re: Best file system)

[Claus Herwig <lists@checon.de>]

Hi,

>>Delayed writes in XFS makes all of the above useless.  It simply doesn't
>>send the data to the disk, and that makes the potential data loss window a
>>order of magnitude (or more) bigger than the time it takes to flush data to
>>the disk with such a setup.   Although it is very possible that one can tell
>>XFS to shorten or close the delayed-write window (I didn't check).
> 
> 
> /proc/sys/vm.

ok, I read /usr/src/linux/Documentation and found some tuning parameters
(my comments, as I understood the docs):

/proc/sys/vm/dirty_writeback_centisecs (500)
Wakeup interval of pdflush daemon, which writes buffers to disk

/proc/sys/vm/dirty_expire_centisecs (3000)
Age threshold, after which dirty buffers get written to disk

/proc/sys/fs/xfs/xfssyncd_centisecs (3000)
xfssyncd metadata write interval

/proc/sys/fs/xfs/xfsbufd_centicecs (100)
Wakeup interval for xfsbufd metadata buffer daemon (uh, another metadata
daemon??)

/proc/sys/fs/xfs/age_buffer_centisecs (1500)
Max age for dirty metadata buffers

But how does this all fit together? Or, to ask the one simple question:
Whats the maximum age of new/changed data which gets lost on an xfs in
case of power failure?

Thanks,
  Claus

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