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

[Andy Lutomirski]

On Tue, Sep 8, 2015 at 2:29 PM, Darrick J. Wong <address@hidden> wrote:
> On Tue, Sep 08, 2015 at 09:03:09PM +0100, Pádraig Brady wrote:
>> On 08/09/15 20:10, Andy Lutomirski wrote:
>> > On Tue, Sep 8, 2015 at 11:23 AM, Anna Schumaker
>> > <address@hidden> wrote:
>> >> On 09/08/2015 11:21 AM, Pádraig Brady wrote:
>> >>> 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. :)
>
>> >> 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?

> 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.
If someone wants to use chattr or some new operation to say "make this
range of this file belong just to me for purpose of optimizing future
writes", then sure, go for it, with the understanding that there are
plenty of filesystems for which that doesn't even make sense.

> 6. regular copy, but don't CoW (eatmyothercopies) (joke)
>
> (Please add whatever ops I missed.)
>
> I think I can see a case for letting (4) fall back to (3) since (4) is an
> optimization of (3).
>
> However, I particularly don't like the idea of (1) falling back to (3-5).
> Either the kernel can satisfy a request or it can't, but let's not just
> assume that we should transmogrify one type of request into another.  
> Userspace
> should decide if a reflink failure should turn into one of the copy variants,
> depending on whether the user wants to spread allocation costs over rewrites 
> or
> pay it all up front.  Also, if we allow reflink to fall back to copy, how do
> programs find out what actually took place?  Or do we simply not allow them to
> find out?
>
> Also, programs that expect reflink either to finish or fail quickly might be
> surprised if it's possible for reflink to take a longer time than usual and
> with the side effect that a deep(er) copy was made.
>
> I guess if someone asks for both (1) and (3) we can do the fallback in the
> kernel, like how we handle it right now.
>

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.

An example of (b) would be a filesystem backed by deduped
thinly-provisioned storage that can't do anything about ENOSPC because
it doesn't control it in the first place.

Another option would be to split up the copy case into "I expect to
overwrite a lot of the target file soon, so (c) try to commit space
for that or (d) try to make it time-efficient".  Of course, (d) is
irrelevant on filesystems with no random access (nvdimms, for
example).

I guess the tl;dr is that I'm highly skeptical of any use for
disallowing reflinking other than forcibly committing space in cases
where committing space actually means something.