Re: fsync, rdiff-backup, wapbl, and WD Elements 1T drive

[Thor Lancelot Simon <tls%panix.com@localhost>]

On Tue, Nov 01, 2011 at 09:54:45AM -0400, Greg Troxel wrote:
> 
> My man page on 5.1 matches Mathew's.
> 
> But, does sync do cache flushes on all disks as well?
> 
> Does SUS require this?

I believe fsync_range with FDATASYNC is required to.  Note that
since it's guaranteed to sync "sufficient metadata to.." it should
force directory updates, file size updates, etc. out to disk as
well -- without flushing the entire kernel page or metadata cache.

Unfortunately, since *disk* cache flushes are rather a blunt
instrument this will still harm performance more than it ought.
It'd be nice to have a concept of ordered tags as barriers like
Linux does (see my old B_BARRIER proposal) but that'd still need
tagged queueing support for ATA disks to be really useful today.

-- 
Thor Lancelot Simon                                    tls%panix.com@localhost
  "All of my opinions are consistent, but I cannot present them all
   at once."    -Jean-Jacques Rousseau, On The Social Contract