On 6/8/11 10:01 AM, Amir G. wrote:

On Wed, Jun 8, 2011 at 5:41 PM, Eric Sandeen [off-list ref] wrote:

quoted

On 6/8/11 9:04 AM, Amir G. wrote:

quoted

quoted

And one last note, I also think that the snapshot format change in the

quoted

future, when we'll have snpashots with 64bit feature compatible seems
just wrong to me. Adding some features or changing the implementation a
bit is ok, but format change is different. When the code is upstream and
stable it is just wrong.

What can I say, I understand why it looks bad, but is 64bit code
upstream and stable? Hell no! e2fsprogs 64bit is not out yet!
There is no reason to call it 'format change'.
It's going to be a new format used only for 64bit fs, which are not
even out there yet. And when they are finally out there, they won't
have
snapshots until the new format is implemented.

Well, the on-disk format for 64-bit (48-bit?) ext4 is there & fixed; it's
just that there is no released userspace which can properly handle it, right?

I don't know, you tell me.
Are there many users out there using 64bit feature, without the proper
user space tools?

No, but that doesn't mean the disk format has to change when the tools
come out... I just don't want to confuse "there are no tools" with
"the disk format is unstable" - Andreas et. al. have been using
that format for years.

quoted

I don't anticipate ext4 format changes for >16T, or am I missing something?

-Eric

Argh! I wish I hadn't missed the Monday call (it's
not in a good time for me).
This whole 'format change' has gone out of control
and I find it hard to present my case properly on scattered emails.

Sorry; I may have just misunderstood...

The message I am trying to get through is:
There is 32bit snapshot file format, which is implemented and well tested.
There is 64bit snapshot file format, which is not implemented yet, so
64bit and snapshot feature are mutually exclusive.
If and when 64bit snapshot file format will be implemented, it will be
a new type of extent mapped file (v2) with 48bit logical addresses.
Is this a 'format change'? Call it what you will, but it shouldn't
affect anything on existing structures. It should only affect the
non-existing structure of 64bit snapshot file.

Does this answer your question?

Yes, I guess I had misunderstood your point; I thought you were
implying that ext4's format had to change to support 64-bit, so why
not change snapshots along with it....

But you're just saying that you wish to push 32-bit snapshots which only
work with certain sizes of ext4 filesystems now, and later you will
release a new snapshot format which works with the larger filesystems.
Right?

(I don't actually know if we'll ever have 64-bit ext4, though, there
are still so many scaling issues beyond just being able to mkfs,
mount, growfs etc ... it's a serious game of catch-up with xfs
in that space, IMHO, which has been doing it well for years now...)

Still, pushing snapshots upstream which will have an on-disk format
more limited than the rest of the filesystem's on-disk format 
does strike me as suboptimal from a pure technical design POV.

What if we proposed, say, xattr code that could only apply xattrs
to files located in the first 16T?  I don't think it'd be accepted.

I understand that you have a history and a format and a business case,
but that really should not change whether we do it right the first time,
upstream, IMHO...  But I'm just the peanut gallery, here....  ;)

-Eric

Amir.
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