On Thu, Feb 18, 2021 at 12:36 AM Robin H. Johnson <robbat2@xxxxxxxxxx> wrote: > > On Wed, Feb 17, 2021 at 05:36:53PM +0100, Loïc Dachary wrote: > > Bonjour, > > > > TL;DR: Is it more advisable to work on Ceph internals to make it > > friendly to this particular workload or write something similar to > > EOS[0] (i.e Rocksdb + Xrootd + RBD)? > CERN's EOSPPC instance, which is one of the biggest from what I can > find, was up around 3.5B files in 2019; and you're proposing running 10B > files, so I don't know how EOS will handle that. Maybe Dan can chime in > on the scalability there. The EOS namespace is now QuarkDB https://github.com/gbitzes/QuarkDB But even with a clever namespace I don't think it is practical to manage a system with 10B tiny files. Enumerating them for a consistency check or migrating between hosts or recovering from failures is going to be painful. Pack them... -- Dan > > Please do keep on this important work! I've tried to do something > similar at a much smaller scale for Gentoo Linux's historical collection > of source code media (distfiles), but am significantly further behind > your effort. > > > Let say those 10 billions objects are stored in a single 4+2 erasure > > coded pool with bluestore compression set for objects that have a size > > 32KB and the smallest allocation size for bluestore set to 4KB[3]. > > The 750TB won't use the expected 350TB but about 30% more, i.e. > > ~450TB (see [4] for the maths). This space amplification is because > > storing a 1 byte object uses the same space as storing a 16KB object > > (see [5] to repeat the experience at home). In a 4+2 erasure coded > > pool, each of the 6 chunks will use no less than 4KB because that's > > the smallest allocation size for bluestore. That's 4 * 4KB = 16KB > > even when all that is needed is 1 byte. > I think you have an error here: with 4KB allocation size in 4+2 pool, > any object sized (0,16K] will take _6_ chunks: 20KB of storage. > Any object sized (16K,32K] will take _12_ chunks: 40K of storage. > > I'd attack this from another side entirely: > - how aggressively do you want to pack objects overall? e.g. if you have > a few thousand objects in the 4-5K range, do you want zero bytes > wasted between objects? > - how aggressively do you want to dudup objects that share common data, > esp if it's not aligned on some common byte margins? > - what are the data portability requirements to move/extract data from > this system at a later point? > - how complex of an index are you willing to maintain to > reconstruct/access data? > - What requirements are there about the ordering and accessibility of > the packs? How related do the pack objects need to be? e.g. are the > packed as they arrive in time order, to build up successive packs of > size, or are there many packs and you append the "correct" pack for a > given object? > > I'm normally distinctly in the camp that object storage systems should > natively expose all objects, but that also doesn't account for your > immutability/append-only nature. > > I see your discussion at https://forge.softwareheritage.org/T3054#58977 > as well, about the "full scale out" vs "scale up metadata & scale out > data" parts. > > To brainstorm parts of an idea, I'm wondering about Git's > still-in-development partial clone work, with the caveat that you intend > to NEVER checkout the entire repository at the same time. > > Ideally, using some manner of fuse filesystem (similar to Git Virtual > Filesystem) w/ an index-only clone, naive clients could access the > object they wanted, which would be fetched on demand from the git server > which has mostly git packs and a few sparse objects that are waiting for > packing. > > The write path on ingest clients would involve sending back the new > data, and git background processes on some regular interval packing the > loose objects into new packfiles. > > Running this on top of CephFS for now means that you get the ability to > move it to future storage systems more easily than any custom RBD/EOS > development you might do: bring up enough space, sync the files over, > profit. > > Git handles the deduplication, compression, access methods, and > generates large pack files, which Ceph can store more optimally than the > plethora of tiny objects. > > Overall, this isn't great, but there aren't a lot of alternatives as > your great research has noted. > > Being able to take a backup of the Git-on-CephFS is also made a lot > easier since it's a filesystem: "just" write out the 350TB to 20x LTO-9 > tapes > > Thinking back to older systems, like SGI's hierarchal storage modules > for XFS, the packing overhead starts to become significant for your > objects: some of the underlying mechanisms in the XFS HSM DMAPI, if they > ended up packing immutable objects to tape still had tar & tar-like > headers (at least 512 bytes per object), your 10B objects would take at > least 4TB of extra space (before compression). > > > > It was suggested[6] to have two different pools: one with a 4+2 erasure pool and compression for all objects with a size > 32KB that are expected to compress to 16KB. And another with 3 replicas for the smaller objects to reduce space amplification to a minimum without compromising on durability. A client looking for the object could make two simultaneous requests to the two pools. They would get 404 from one of them and the object from the other. > > > > Another workaround, is best described in the "Finding a needle in Haystack: Facebook’s photo storage"[9] paper and essentially boils down to using a database to store a map between the object name and its location. That does not scale out (writing the database index is the bottleneck) but it's simple enough and is successfully implemented in EOS[0] with >200PB worth of data and in seaweedfs[10], another promising object store software based on the same idea. > > > > Instead of working around the problem, maybe Ceph could be modified to make better use of the immutability of these objects[7], a hint that is apparently only used to figure out how to best compress it and for checksum calculation[8]. I honestly have not clue how difficult it would be. All I know is that it's not easy otherwise it would have been done already: there seem to be a general need for efficiently (space wise and performance wise) storing large quantities of objects smaller than 4KB. > > > > Is it more advisable to: > > > > * work on Ceph internals to make it friendly to this particular workload or, > > * write another implementation of "Finding a needle in Haystack: Facebook’s photo storage"[9] based on RBD[11]? > > > > I'm currently leaning toward working on Ceph internals but there are pros and cons to both approaches[12]. And since all this is still very new to me, there also is the possibility that I'm missing something. Maybe it's *super* difficult to improve Ceph in this way. I should try to figure that out sooner rather than later. > > > > I realize it's a lot to take in and unless you're facing the exact same problem there is very little chance you read that far :-) But if you did... I'm *really* interested to hear what yout think. In any case I'll report back to this thread once a decision has been made. > > > > Cheers > > > > [0] https://eos-web.web.cern.ch/eos-web/ > > [1] https://lists.ceph.io/hyperkitty/list/ceph-users@xxxxxxx/thread/AEMW6O7WVJFMUIX7QGI2KM7HKDSTNIYT/ https://lists.ceph.io/hyperkitty/list/ceph-users@xxxxxxx/thread/RHQ5ZCHJISXIXOJSH3TU7DLYVYHRGTAT/ > > [2] https://forge.softwareheritage.org/T3054 > > [3] https://github.com/ceph/ceph/blob/3f5e778ad6f055296022e8edabf701b6958fb602/src/common/options.cc#L4326-L4330 > > [4] https://forge.softwareheritage.org/T3052#58864 > > [5] https://forge.softwareheritage.org/T3052#58917 > > [6] https://forge.softwareheritage.org/T3052#58876 > > [7] https://docs.ceph.com/en/latest/rados/api/librados/#c.@3.LIBRADOS_ALLOC_HINT_FLAG_IMMUTABLE > > [8] https://forge.softwareheritage.org/T3055 > > [9] https://www.usenix.org/legacy/event/osdi10/tech/full_papers/Beaver.pdf > > [10] https://github.com/chrislusf/seaweedfs/wiki/Components > > [11] https://forge.softwareheritage.org/T3049 > > [12] https://forge.softwareheritage.org/T3054#58977 > > > > -- > > Loïc Dachary, Artisan Logiciel Libre > > > > > > > > > > _______________________________________________ > > ceph-users mailing list -- ceph-users@xxxxxxx > > To unsubscribe send an email to ceph-users-leave@xxxxxxx > > > -- > Robin Hugh Johnson > Gentoo Linux: Dev, Infra Lead, Foundation Treasurer > E-Mail : robbat2@xxxxxxxxxx > GnuPG FP : 11ACBA4F 4778E3F6 E4EDF38E B27B944E 34884E85 > GnuPG FP : 7D0B3CEB E9B85B1F 825BCECF EE05E6F6 A48F6136 > _______________________________________________ > ceph-users mailing list -- ceph-users@xxxxxxx > To unsubscribe send an email to ceph-users-leave@xxxxxxx _______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx
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