device-specific qid.version behaviour (was Re: [9fans] QTCTL?)

["roger peppe" <rogpeppe@gmail.com>]

On 11/1/07, erik quanstrom <quanstro@quanstro.net> wrote:
> i think it makes sense to use the medium changes (not connections, if
> possible) to determine the version.  the marvell and aoe driver
> consider a device changed if the serial# or number of sectors
> change.that is something most io clients are interested in.

this kind of thing is fine until you want to run software that
uses the original notion of "version". for instance, i suppose it's
not inconceivable that you could run something like cfs over
such a device.

turning things around, by providing device-specific versioning behavior,
you can end up with clients that won't work well when run over normal
files. if a client expects the version number to change only when the
media changes, then it's quite likely to be fairly surprised if
the version number changes every time it writes some data.

that's why i think that preserving standard semantics is a Good Thing.
it preserves filesystem transparency, one of the nicest
things about plan 9.

> it does not seem too much of a stretch.  the stat.size field isn't the
> "file size" of a stream (whatever that means).  so i think this is
> well within the tradition.

in what filesystems does stat.size hold anything other than zero
(don't know) or the
number of bytes that are currently readable from the file?