Olaf van der Spek wrote:
On Fri, Jan 7, 2011 at 8:29 PM, Thomas Bellman <bellman@xxxxxxxxxx> wrote:
What is the visibility of the changes for other processes supposed
to be in the meantime? I.e., if things happen in this order:
Should be atomic too, at close time.
1. Process A does fda = open("foo.txt", O_TRUNC|O_ATOMIC)
2. Process B does fdb = open("foo.txt", O_RDONLY)
3. B does read(fdb, buf, 4096)
4. A does write(fda, "NEW DATA\n", 9)
5. Process C comes in and does fdc = open("foo.txt", O_RDONLY)
6. C does read(fdc, buf, 4096)
7. A calls close(fda)
Does B see an empty file, or does it see the old contents of
the file?
Old file, otherwise A wouldn't be atomic.
Does C see "NEW DATA\n", or does it see the old
contents of the file, or perhaps an empty file?
Old file again, as the 'transaction' isn't finished until close.
So, basically database transactions with an isolation level of
"committed read", for file operations. That's something I have
wanted for a long time, especially if I also get a rollback()
operation, but have never heard of any Unix that implemented it.
A separate commit() operation would be better than conflating it
with close(). And as I said, we want a rollback() as well. And
a process that terminates without committing the transaction that
it is performing, should have the transaction automatically rolled
back.
I only have a very shallow knowledge about the internals of the
Linux kernel in regards to filesystems, but I suspect that this
could be implemented almost entirely within the VFS, and not need
to touch the actual filesystems, as long as you are satisfied
with a limited amount of transaction space (what fits in RAM +
swap).
I'm looking forward to your implementation. :-) Even though I
suspect that it would be a rather large undertaking to implement...
/Bellman
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