Re: ext3 reservation question.

[Andrew Morton <akpm@osdl.org>]

Linus Torvalds <torvalds@osdl.org> wrote:
>
> 
> 
> On Wed, 21 Apr 2004, Badari Pulavarty wrote:
> > 
> > I am worried about a case, where multiple threads writing to 
> > different parts of same file - there by each thread thrashing 
> > reservation window (since each one has its own goal).
> 
> Didn't we have a patch two years ago or something floating around with
> doing lazy (delayed) block allocation on ext2 - doing the actual
> allocation only when writing the thing out? Then you shouldn't have this
> problem under any normal load, hopefully.

That would certainly help.  I had delayed allocation for ext2 all up and
running in 2.5.7 or thereabouts - most of the complexity is in managing
filesystem space reservations.  If you don't care about ENOSPC the VFS at
present "just works".

I do recall deciding that there were fundamental journal-related reasons
why delalloc couldn't be made to work properly on ext3.

ummm.

The code I had at the time would reserve space in the filesystem
correspnding to the worst-case occupancy based on file offset.  When we
actually hit ENOSPC in prepare_write(), we force writeout, which results in
those worst-space reservations being collapsed into their _real_ space
usage, which is much less.  So writeout reclaims space in the filesystem
and prepare_write() can proceed.

That worked fine on ext2.  But on ext3 we have a transaction open in
prepare_write(), and the forced writeback will cause arbitrary amounts of
unexpected metadata to be pumped into the current transaction, causing the
fs to explode.

At least, I _think_ that was the problem.  All is hazy.



Alex Tomas has current patches which do delalloc, but I don't know if they
do all the reservation stuff yet.

We would still face layout problems on SMP - two or more CPUs allocating
blocks in parallel.  Could be solved by serialising writeback in some
manner - the fs-writeback.c code does that to some extent already.