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 <darrick.wong@xxxxxxxxxx> 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 > >>>>> <Anna.Schumaker@xxxxxxxxxx> 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? --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 > > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-api" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
