Re: [RFC] Heads up on sys_fallocate()

[Jan Kara]

On Tue 06-03-07 12:23:22, Eric Sandeen wrote:
> Jan Kara wrote:
> > On Tue 06-03-07 06:36:09, Ulrich Drepper wrote:
> >> Christoph Hellwig wrote:
> >>> fallocate with the whence argument and flags is already quite complicated,
> >>> I'd rather have another call for placement decisions, that would
> >>> be called on an fd to do placement decissions for any further allocations
> >>> (prealloc, write, etc)
> >> Yes, posix_fallocate shouldn't be made more complicated.  But I don't
> >> understand why requesting linear layout of the blocks should be an
> >> option.  It's always an advantage if the blocks requested this way are
> >> linear on disk.  So, the kernel should always do its best to make this
> >> happen, without needing an additional option.
> >   Actually, it's not that simple. You want linear layout of blocks you are
> > going to read. That is not necessary a linear layout of blocks in a single
> > file - trace sometime a start of some complicated app like KDE. You find
> > it's seeking like a hell because it needs a few blocks from a ton of
> > distinct files (shared libs, config files, etc). As these files are mostly
> > read only, it's advantageous to interleave them on disk or at least keep
> > them close.
> 
> At some point shouldn't the apps be fixed, rather than do crazy things
> with the filesystem?  :)
  Yes :) That's basically what we told KDE developpers when they were
complaining ;) But it's hard to fix it for them too (because of some
desktop specs requiring lots of different text config files which can
change anytime - don't ask me who designed it). Moreover for example for
loading shared libraries from which you need just a few blocks scattered
all over the place the problem is in ELF itself.
  I'll probably first write some userspace fs-reorganizer to find out how
much these changes in layout are able to give you in performance (i.e.
whether it's worth the effort of more complicated kernel online
defragmenter).

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SuSE CR Labs
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