Re: [Lsf-pc] [LSF/MM TOPIC] atomic block device

[Alex Elsayed <eternaleye@gmail.com>]

Dave Chinner wrote:

(My apologies for snipping, but I only wanted to address a very small part 
of what you said)
> Similarly, things like ordered writes are great until you consider
> how they interact with journalling and cause priority inversion
> issues. The only way to make use of ordered writes is to design the
> filesystem around ordered writes from the ground up. i.e. the
> soft updates complexity problem. Unlike atomic writes, this can't
> easily be retrofitted to an exising filesystem, and once you have
> soft updates in place you are effectively fixing the format and
> features of the filesystem in stone because if you need to change a
> single operation or on disk structure you have to work out the
> dependency graph for the entire filesystem from the ground up again.

One thing that keeps coming to mind whenever ordering guarantees in 
filesystems come up is Featherstitch[1], which is (to quote the article) "a 
generalization of the soft updates system of write dependencies and rollback 
data" (since "not enough file systems geniuses exist in the world to write 
and maintain more than one instance of soft updates").

Aside from its relevance to your observations on soft updates, it had a 
userspace API that provided similar guarantees to Howard Chu's suggestion.

[1] article by Valerie Aurora: https://lwn.net/Articles/354861/