Hi Robin, On 18/02/2021 00:35, Robin H. Johnson 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. This is an essential piece of information I was missing. It also makes sense that there are much larger objects in the context of the CERN. > > 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. Thanks for the encouragements! These are very preliminary stages but I'm enthusiastic about what will follow because I'll have the opportunity to work on it until a solution is implemented and deployed. > >> 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 should have mentioned that my calculations were ignoring the replication overhead (parity chunks or copies). Good catch :-) > > 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? 50% of the objects have a size <4KB, that is ~5billions currently and growing. *But* they account for only 1% of the total size. So maybe not very agressively but not passively either. > - how aggressively do you want to dudup objects that share common data, > esp if it's not aligned on some common byte margins? Objects/files are addressed by the SHA256 of their content and that takes care of deduplication. > - what are the data portability requirements to move/extract data from > this system at a later point? The data portability is ensured by using Free Software only and open standards where possible. And by distributing the software in a way that can be conveniently installed by a third party. Does that answer your question? The durability of the software/format couple used to store data is something I'm not worried about but may I should. > - how complex of an index are you willing to maintain to > reconstruct/access data? I don't envision the index being more complex than SHA256 => content (roughly). > - 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? There are no ordering requirements. > > 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, [snip] I did not know about "partial clone" and will explore this in https://forge.softwareheritage.org/T3065. Although it is probably not a good fit for a 2021 solution, it sounds like a great source of inspiration. > 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). I'm tempted to overlook lessons from the past. In part because I'm afraid I'll loose myself :-) In part because I assume the world changed a lot since. If however you think (have a hunch) that it might be useful, I'll give it a try. Thanks for the great feedback! -- Loïc Dachary, Artisan Logiciel Libre
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