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

[Anna Schumaker]

On 09/09/2015 05:16 PM, Darrick J. Wong wrote:
> On Wed, Sep 09, 2015 at 02:52:08PM -0400, Anna Schumaker wrote:
>> On 09/08/2015 06:39 PM, Darrick J. Wong wrote:
>>> On Tue, Sep 08, 2015 at 02:45:39PM -0700, Andy Lutomirski wrote:
>>>> 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?
>>>
>>> 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"?
>>>
>>>>
>>>>> 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...
>>>
>>>> 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.
>>>
>>> "Unshare these blocks" sounds more like something fallocate could do.
>>>
>>> So far in my XFS reflink playground, it seems that using the defrag tool to
>>> un-cow a file makes most sense.  AFAICT the XFS and ext4 defraggers copy a
>>> fragmented file's data to a second file and use a 'swap extents' operation,
>>> after which the donor file is unlinked.
>>>
>>> Hey, if this syscall turns into a more generic "do something involving two
>>> (fd:off:len) (fd:off:len) tuples" call, I guess we could throw in "swap
>>> extents" as a 7th operation, to refactor the ioctls.  <smirk>
>>>
>>>>
>>>>> 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.
>>>
>>> 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.
>>
>> I agree here that 0 for flags should do something useful, and I wanted to
>> double check if reflink-xor-copy is a good default behavior.
> 
> Ok.
> 
>>>
>>> FORBID_REFLINK = 1
>>> FORBID_COPY = 2
>>
>> I don't like the idea of using flags to forbid behavior.  I think it would be
>> more straightforward to have flags like REFLINK_ONLY or COPY_ONLY so users
>> can tell us what they want, instead of what they don't want.
> 
> Seems fine to me.
> 
>> While I'm thinking about flags, COPY_FILE_RANGE_REFLINK_ONLY would be a bit
>> of a mouthful.  Does anybody have suggestions for ways that I could make this
>> shorter?
> 
> CFR_REFLINK_ONLY?

That could work!  Although I might do as Austin suggests and drop the _ONLY 
part, and then make the man page clear about what's going on.

Would you expect to trigger a NFS server side copy by passing the pagecache 
copy flag?  Or would that only happen if I pass flags=0?

Anna

> 
> --D
> 
>>
>> Thanks,
>> Anna
>>
>>> CHECK_SAME = 4
>>> HW_COPY = 8
>>>
>>> DEDUPE = (FORBID_COPY | CHECK_SAME)
>>>
>>> What do you say to that?
>>>
>>>> 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.
>>>
>>> That's more or less where I was going too. :)
>>>
>>> --D
>>>
>>
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