Evgeniy Polyakov wrote:

Al Boldi (a1426z@gawab.com) wrote:

quoted

Look at ZFS; it illegally violates layering by combining md/dm/lvm with
the fs, but it does this based on a realistic understanding of the
problems involved, which enables it to improve performance, flexibility,
and functionality specific to its use case.

This implies that there are two distinct forces at work here:

  1. Layer components
  2. Use-Case composers

Layer components should technically not implement any use case (other
than providing a plumbing framework), as that would incur unnecessary
dependencies, which could reduce its generality and thus reusability.

Use-Case composers can now leverage layer components from across the
layering hierarchy, to yield a specific use case implementation.

DRBD is such a Use-Case composer, as is mdm / dm / lvm and any fs in
general, whereas aoe / nbd / loop and the VFS / FUSE are examples of
layer components.

It follows that Use-case composers, like DRBD, need common functionality
that should be factored out into layer components, and then recompose to
implement a specific use case.

Out of curiosity, did you try ndb+dm+raid1 compared to drbd and/or zfs
on top of distributed storage (which is a urprise to me, that holy zfs
suppors that)?

Actually, I may not have been very clear in my Use-Case composer description 
to mean internal in-kernel Use-Case composer as opposed to external Userland 
Use-Case composer.

So, nbd+dm+raid1 would be an external Userland Use-Case composition, which 
obviously could have some drastic performance issues.

DRBD and ZFS are examples of internal in-kernel Use-Case composers, which 
obviously could show some drastic performance improvements.  

Although you could allow in-kernel Use-Case composers to be run on top of 
Userland Use-Case composers, that wouldn't be the preferred mode of 
operation.  Instead, you would for example recompose ZFS to incorporate an 
in-kernel distributed storage layer component, like nbd.

All this boils down to refactoring Use-Case composers to produce layer 
components with both in-kernel and userland interfaces.  Once we have that, 
it becomes a matter of plug-and-play to produce something awesome like ZFS.


Thanks!

--
Al