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SubjectRe: [RFC] Generic deferred file writing


On Saturday, December 30, 2000 06:28:39 PM -0800 Linus Torvalds
<torvalds@transmeta.com> wrote:

> There are only two real advantages to deferred writing:
>
> - not having to do get_block() at all for temp-files, as we never have to
> do the allocation if we end up removing the file.
>
> NOTE NOTE NOTE! The overhead for trying to get ENOSPC and quota errors
> right is quite possibly big enough that this advantage is possibly very
> questionable. It's very possible that people could speed things up
> using this approach, but I also suspect that it is equally (if not
> more) possible to speed things up by just making sure that the
> low-level FS has a fast get_block().
>
> - Using "global" access patterns to do a better job of "get_block()", ie
> taking advantage of issues with journalling etc and deferring the write
> in order to get a bigger journal.
>
> The second point is completely different, and THIS is where I think there
> are potentially real advantages.

Absolutely. I wrote reiserfs delayed allocation code back in october, and
kind of left it alone until the VM had the callbacks needed to make it
clean (err, less ugly). I included bunches of optimizations to
reiserfs_get_block, and the most effective one was a cache of block
pointers in the inode to avoid consecutive tree searches. This was a
locking and an i/o win, for both reading and writing (reiserfs needs this
more than ext2 does)

For growing the file, delayed allocation was a huge bonus. For all the
reasons you've already discussed, and because writing a file went from this:

(reiserfs_get_block is starting/stopping the transaction)
while(bytes_to_write)
start_transaction
allocate block
insert block pointer
end_transaction
end

To this:

while(bytes_to_write)
update counters
end

(delayed alloc routine is starting/stopping trans)
start_transaction
allocate X blocks
insert X block pointers
update counters
end_transaction

A big fat transaction is a happy one ;-)

Anyway, I'll return to the optimizations once things have settled down a
bit, and might give the generic delayed allocation (instead of reiserfs
only code) a try.

-chris

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