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SubjectRe: kernel support for non-english user messages
Valdis.Kletnieks@vt.edu wrote:

> 1) What happens when *multiple* programs have a file open at the same time?
> If you only handle one level of versioning, somebody is going to lose. Handling
Sure -which is why this isn't so useful.

> multiple levels is of course more "fun". And you get to worry about race
> conditions - does your code DTRT if multiple processes do an open() on alternate
> CPUs at the same time. Does it DTRT if a process open()s a file, and then fork()s,
> and both parent and child start scribbling on the file descriptor?
>
> 2) For that matter, should new versions be created at open() or at the first write()?
> Doing it at write() allows not creating a new version if no changes have
> actually happened - but this has its own issues.
>
Simple solution - duplicate at open() but remove on close() if unchanged.

> 3) Version a 500 megabyte file. Change one block. Do it a few more times.
> Are you better off copying the whole file (which bloats your disk usage and
> kills your I/O bandwidth), or keeping deltas (the list of allocated blocks could be
> almost identical except for the replaced/rewritten blocks). However, this DOES
> make doing an fsck() a *lot* more interesting - is a block allocated to multiple
> files in error or not?
>
If you want this, you want a copy-on-write fs. Of course it needs a
different fsck.
> 4) What happens if you rename() a file?
It is renamed - nothing special there. Renaming onto an existing file
makes the renamed file the most recent version of that file.
"mv foo;2 foo" turns foo;2 into the current version - this is one
way of restoring an old version without deleting the newer ones.

> Can you open() a previous version for
> writing? If so, does it get versioned? How does a backup program restore a
> previous version?

VMS let you open() any previous version for writing. I don't remember
exactly,
but I believe this creates a new version with a version number higher
than the
highest existing version of the file. Either that, or you modify the
old file.

Backup is not a problem. You don't want to create new versions while
restoring,
so either remove existing files before recreating from backup (You
can then explicitly create "file.txt;45") or implement some open flag
that means "use exactly this filename - no messing with versioning" I
think the
first approach is better it keeps the interface smaller.

> 5) Let's say we use VMS-style filenames to version. foo, foo;2, foo;3, etc.
> Now, is open("foo;2",...) a reference to the previous version of foo or to a new
> file that happens to be called foo;2? What happens if some other file happens
> to be called foo;2 and you create a version of foo?
>
I remember trying this. If foo;3 exists (but not foo;2 and lower) then
the numbering of new versions go upwards and simply skips the existing ones.

> 6) OK, since anything besides \0 and / is legal in a filename, we can't use ;N to
> version.
We can if we want to - by redefining whats legal. This isn't a problem
at all
if we take the approach that opening "foo" does the versioning automatically
while explicitly opening "foo;x" opens that particular version. Users
creating
a "fake" version of a file isn't a problem - they simply created a new
version
that happened to not be based on the contents of the previous one.

Helge Hafting

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