Re: [RFD] Layering: Use-Case Composers (was: DRBD - what is it, anyways? [compare with e.g. NBD + MD raid])

[Evgeniy Polyakov <johnpol@2ka.mipt.ru>]

On Sun, Aug 12, 2007 at 01:35:17PM +0300, Al Boldi (a1426z@gawab.com) wrote:
> Lars Ellenberg wrote:
> > meanwhile, please, anyone interessted,
> > the drbd paper for LinuxConf Eu 2007 is finalized.
> > http://www.drbd.org/fileadmin/drbd/publications/
> > drbd8.linux-conf.eu.2007.pdf
> >
> > it does not give too much implementation detail (would be inapropriate
> > for conference proceedings, imo; some paper commenting on the source
> > code should follow).
> >
> > but it does give a good overview about what DRBD actually is,
> > what exact problems it tries to solve,
> > and what developments to expect in the near future.
> >
> > so you can make up your mind about
> >  "Do we need it?", and
> >  "Why DRBD? Why not NBD + MD-RAID?"
> 
> Ok, conceptually your driver sounds really interresting, but when I read the 
> pdf I got completely turned off.  The problem is that the concepts are not 
> clearly implemented, when in fact the concepts are really simple:
> 
>   Allow shared access to remote block storage with fault tolerance.
> 
> The first thing to tackle here would be write serialization.  Then start 
> thinking about fault tolerance.
> 
> Now, shared remote block access should theoretically be handled, as does 
> DRBD, by a block layer driver, but realistically it may be more appropriate 
> to let it be handled by the combining end user, like OCFS or GFS.
> 
> The idea here is to simplify lower layer implementations while removing any 
> preconceived dependencies, and let upper layers reign free without incurring 
> redundant overhead.
> 
> 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)?
 
> Thanks!
> 
> --
> Al
> 
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-- 
	Evgeniy Polyakov