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 <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? 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 >>> >> >> -- >> 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
