Re: Interactive responsiveness under heavy I/O load

On Mon, Jan 26, 2004 at 02:12:24PM -0600, Johan A. van Zanten wrote:
> 
> John Goerzen <[email protected]> wrote:
> 
> > Have I stumbled across the reason softdep is not enabled by default, or
> > is there some other logic behind this?
> 
> My understanding of how soft dependencies works is that file system
> metadata is cached in memory for a short period of time (like 20
> seconds?), which means it's at risk (of being lost) if the machine
> were to loose power.

Two points:

 - The question of "on by default" is shady; softdeps aren't enabled
   unless you specify the mount option, but I understand sysinst now
   creates fstab's with that option by default.  So most users would
   have them enabled on new systems at least. I may be wrong about
   sysinst, its not something I really ever use.

 - You are right about the metadata being delayed, but it's still
   written in-order and before the relevant file data. The essential
   point of softdep is to allow the ordering up updates to the on-disk
   data structures to be preserved, so the ffs and fsck semantics that
   depend on these still work, but without requiring synchronous
   writes that make everything stop and wait. If you sync or fsync at
   the right time, the previous semantics about data on disk or not
   still hold.

   Speaking very broadly, if your system crashes, the resulting
   filesystem state with softdep is similar to if the machine had
   crashed a little earlier without (unless you sync).  

   Softdep is such a huge performance win, especially for tasks like
   extracting a pkgsrc tree, because lots of metadata writes update
   the same disk block repetitively (think about adding files one at a
   time to the same directory). With softdep, each of these updates is
   done in memory (without the sync disk wait) and the resulting final
   directory blocks written to disk (again, speaking very broadly).
   
--
Dan.

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