Re: barriers vs. reads

[Jamie Lokier <jamie@shareable.org>]

Werner Almesberger wrote:
>  i) write requests may never cross a barrier that separates
>     them from other write requests
>  ii) a read request R and a write request W may change their
>      order (with respect to each other), unless they overlap
>      AND are separated by at least one barrier
>  iii) in all other cases, requests are free to move about
> 
> Rule ii) seems a little tricky. Is there actually a means for
> user space to send a barrier, e.g. when doing direct IO ? If
> not, the I think only user of ii) would be "direct IO" that
> comes from unbuffered file system meta-data and such.

It should be possible to write a decent filesystem in userspace --
even if it's just for prototyping -- so barriers from userspace should
be offered, eventually.

Note that unbuffered file system meta-data can theoretically overlap
page cache, although it usually doesn't, and a filesystem could easily
set a flag to say that it doesn't ever.

Think of the experimental modifications to ext3 to allow inodes to be
allocated in a file.  That means blocks are changing their meaning
from being file blocks to inode blocks or vice versa -- and both I/Os
could conceivably be in flight at the same time.

-- Jamie