Messages in this thread | | | Date | Fri, 11 Apr 2003 13:40:37 +0200 | From | Helge Hafting <> | Subject | Re: kernel support for non-english user messages |
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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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