Re: [PATCH v1 0/8] VFS: In-kernel copy system call

On Tue, Sep 8, 2015 at 3:39 PM, Darrick J. Wong [off-list ref] wrote:

On Tue, Sep 08, 2015 at 02:45:39PM -0700, Andy Lutomirski wrote:

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On Tue, Sep 8, 2015 at 2:29 PM, Darrick J. Wong [off-list ref] wrote:

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On Tue, Sep 08, 2015 at 09:03:09PM +0100, Pádraig Brady wrote:

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On 08/09/15 20:10, Andy Lutomirski wrote:

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On Tue, Sep 8, 2015 at 11:23 AM, Anna Schumaker
[off-list ref] wrote:

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On 09/08/2015 11:21 AM, Pádraig Brady wrote:

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I see copy_file_range() is a reflink() on BTRFS?
That's a bit surprising, as it avoids the copy completely.
cp(1) for example considered doing a BTRFS clone by default,
but didn't due to expectations that users actually wanted
the data duplicated on disk for resilience reasons,
and for performance reasons so that write latencies were
restricted to the copy operation, rather than being
introduced at usage time as the dest file is CoW'd.

If reflink() is a possibility for copy_file_range()
then could it be done optionally with a flag?

The idea is that filesystems get to choose how to handle copies in the
default case.  BTRFS could do a reflink, but NFS could do a server side

Eww, different default behaviors depending on the filesystem. :)

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copy instead.  I can change the default behavior to only do a data copy
(unless the reflink flag is specified) instead, if that is desirable.

What does everybody think?

I think the best you could do is to have a hint asking politely for
the data to be deep-copied.  After all, some filesystems reserve the
right to transparently deduplicate.

Also, on a true COW filesystem (e.g. btrfs sometimes), there may be no
advantage to deep copying unless you actually want two copies for
locality reasons.

Agreed. The relink and server side copy are separate things.
There's no advantage to not doing a server side copy,
but as mentioned there may be advantages to doing deep copies on BTRFS
(another reason not previous mentioned in this thread, would be
to avoid ENOSPC errors at some time in the future).

So having control over the deep copy seems useful.
It's debatable whether ALLOW_REFLINK should be on/off by default
for copy_file_range().  I'd be inclined to have such a setting off by default,
but cp(1) at least will work with whatever is chosen.

So far it looks like people are interested in at least these "make data appear
in this other place" filesystem operations:

1. reflink
2. reflink, but only if the contents are the same (dedupe)

What I meant by this was: if you ask for "regular copy", you may end
up with a reflink anyway.  Anyway, how can you reflink a range and
have the contents *not* be the same?

reflink forcibly remaps fd_dest's range to fd_src's range.  If they didn't
match before, they will afterwards.

dedupe remaps fd_dest's range to fd_src's range only if they match, of course.

Perhaps I should have said "...if the contents are the same before the call"?

Oh, I see.

Can we have a clean way to figure out whether two file ranges are the
same in a way that allows false negatives?  I.e. return 1 if the
ranges are reflinks of each other and 0 if not?  Pretty please?  I've
implemented that in the past on btrfs by syncing the ranges and then
comparing FIEMAP output, but that's hideous.

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3. regular copy
4. regular copy, but make the hardware do it for us
5. regular copy, but require a second copy on the media (no-dedupe)

If this comes from me, I have no desire to ever use this as a flag.

I meant (5) as a "disable auto-dedupe for this operation" flag, not as
a "reallocate all the shared blocks now" op...

Hmm, interesting.  What effect does it have on systems that do
deferred auto-dedupe?

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I think we should focus on what the actual legit use cases might be.
Certainly we want to support a mode that's "reflink or fail".  We
could have these flags:

COPY_FILE_RANGE_ALLOW_REFLINK
COPY_FILE_RANGE_ALLOW_COPY

Setting neither gets -EINVAL.  Setting both works as is.  Setting just
ALLOW_REFLINK will fail if a reflink can't be supported.  Setting just
ALLOW_COPY will make a best-effort attempt not to reflink but
expressly permits reflinking in cases where either (a) plain old
write(2) might also result in a reflink or (b) there is no advantage
to not reflinking.

I don't agree with having a 'copy' flag that can reflink when we also have a
'reflink' flag.  I guess I just don't like having a flag with different
meanings depending on context.

Users should be able to get the default behavior by passing '0' for flags, so
provide FORBID_REFLINK and FORBID_COPY flags to turn off those behaviors, with
an admonishment that one should only use them if they have a goooood reason.
Passing neither gets you reflink-xor-copy, which is what I think we both want
in the general case.

FORBID_REFLINK = 1
FORBID_COPY = 2
CHECK_SAME = 4
HW_COPY = 8

DEDUPE = (FORBID_COPY | CHECK_SAME)

What do you say to that?

What does HW_COPY mean?

If we have enough weird combinations, maybe having a mode instead of
flags makes sense.

--Andy
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