On Ter, 2009-06-30 at 10:58 -0700, Jeremy Archer wrote:
> OK..
>
> > Once the file is deleted, ARC entry {dva 5 txg 5} is no longer valid.
> > So why do we need it?
>
> >>Let's suppose file is removed from the directory but is still open by
> >>some process...
I think both of you (Jeremy and Victor) are getting a bit confused (or
maybe it's me?).
Even if file A is removed from the directory but is still open, this
wouldn't cause a block of File B from getting assigned the same DVA as a
block from the (unlinked from the directory, but still existent on-disk)
file A, as in the example provided.
The blocks of the unlinked file A would only be freed when the file is
closed by the process (assuming no snapshot is present), at which point
there's no need to access its cached blocks anymore.
If a snapshot is taken after file A is written but before it is
unlinked, then the blocks of file B would never be assigned the same
DVAs as the blocks of file A, since the blocks of file A would only be
freed after the snapshot is deleted.
> On a traditional fle system, you would not be able to delete an open file.
> Which I believe is good.
Actually, I think you can do that on most UNIX filesystems.
> So, on ZFS you say, you can write to a file that you opened in txg 5
> that was already deleted in txg 10?
If by deleted you mean unlinked from the directory but still open by a
process, then yes (as in most UNIX filesystems).
> When txg 5 completes, you will have the updated content, and when txg 10
> finishes, you will have a deleted file.
The file will still be there in txg 10, but linked to a special ZAP,
hidden from the namespace.
As to why the ARC can cache two blocks with the same DVA allocated in
different txgs (which means one of them would be invalid?), that I don't
know..
Cheers,
Ricardo
