On Tue, 20 Oct 2015, Chen, Xiaoxi wrote: > +1, nowadays K-V DB care more about very small key-value pairs, say > several bytes to a few KB, but in SSD case we only care about 4KB or > 8KB. In this way, NVMKV is a good design and seems some of the SSD > vendor are also trying to build this kind of interface, we had a NVM-L > library but still under development.
Do you have an NVMKV link? I see a paper and a stale github repo.. not sure if I'm looking at the right thing. My concern with using a key/value interface for the object data is that you end up with lots of key/value pairs (e.g., $inode_$offset = $4kb_of_data) that is pretty inefficient to store and (depending on the implementation) tends to break alignment. I don't think these interfaces are targetted toward block-sized/aligned payloads. Storing just the metadata (block allocation map) w/ the kv api and storing the data directly on a block/page interface makes more sense to me. sage > > -----Original Message----- > > From: ceph-devel-ow...@vger.kernel.org [mailto:ceph-devel- > > ow...@vger.kernel.org] On Behalf Of James (Fei) Liu-SSI > > Sent: Tuesday, October 20, 2015 6:21 AM > > To: Sage Weil; Somnath Roy > > Cc: ceph-devel@vger.kernel.org > > Subject: RE: newstore direction > > > > Hi Sage and Somnath, > > In my humble opinion, There is another more aggressive solution than raw > > block device base keyvalue store as backend for objectstore. The new key > > value SSD device with transaction support would be ideal to solve the > > issues. > > First of all, it is raw SSD device. Secondly , It provides key value > > interface > > directly from SSD. Thirdly, it can provide transaction support, consistency > > will > > be guaranteed by hardware device. It pretty much satisfied all of > > objectstore > > needs without any extra overhead since there is not any extra layer in > > between device and objectstore. > > Either way, I strongly support to have CEPH own data format instead of > > relying on filesystem. > > > > Regards, > > James > > > > -----Original Message----- > > From: ceph-devel-ow...@vger.kernel.org [mailto:ceph-devel- > > ow...@vger.kernel.org] On Behalf Of Sage Weil > > Sent: Monday, October 19, 2015 1:55 PM > > To: Somnath Roy > > Cc: ceph-devel@vger.kernel.org > > Subject: RE: newstore direction > > > > On Mon, 19 Oct 2015, Somnath Roy wrote: > > > Sage, > > > I fully support that. If we want to saturate SSDs , we need to get > > > rid of this filesystem overhead (which I am in process of measuring). > > > Also, it will be good if we can eliminate the dependency on the k/v > > > dbs (for storing allocators and all). The reason is the unknown write > > > amps they causes. > > > > My hope is to keep behing the KeyValueDB interface (and/more change it as > > appropriate) so that other backends can be easily swapped in (e.g. a btree- > > based one for high-end flash). > > > > sage > > > > > > > > > > Thanks & Regards > > > Somnath > > > > > > > > > -----Original Message----- > > > From: ceph-devel-ow...@vger.kernel.org > > > [mailto:ceph-devel-ow...@vger.kernel.org] On Behalf Of Sage Weil > > > Sent: Monday, October 19, 2015 12:49 PM > > > To: ceph-devel@vger.kernel.org > > > Subject: newstore direction > > > > > > The current design is based on two simple ideas: > > > > > > 1) a key/value interface is better way to manage all of our internal > > > metadata (object metadata, attrs, layout, collection membership, > > > write-ahead logging, overlay data, etc.) > > > > > > 2) a file system is well suited for storage object data (as files). > > > > > > So far 1 is working out well, but I'm questioning the wisdom of #2. A > > > few > > > things: > > > > > > - We currently write the data to the file, fsync, then commit the kv > > > transaction. That's at least 3 IOs: one for the data, one for the fs > > > journal, one for the kv txn to commit (at least once my rocksdb > > > changes land... the kv commit is currently 2-3). So two people are > > > managing metadata, here: the fs managing the file metadata (with its > > > own > > > journal) and the kv backend (with its journal). > > > > > > - On read we have to open files by name, which means traversing the fs > > namespace. Newstore tries to keep it as flat and simple as possible, but > > at a > > minimum it is a couple btree lookups. We'd love to use open by handle > > (which would reduce this to 1 btree traversal), but running the daemon as > > ceph and not root makes that hard... > > > > > > - ...and file systems insist on updating mtime on writes, even when it > > > is a > > overwrite with no allocation changes. (We don't care about mtime.) > > O_NOCMTIME patches exist but it is hard to get these past the kernel > > brainfreeze. > > > > > > - XFS is (probably) never going going to give us data checksums, which we > > want desperately. > > > > > > But what's the alternative? My thought is to just bite the bullet and > > consume a raw block device directly. Write an allocator, hopefully keep it > > pretty simple, and manage it in kv store along with all of our other > > metadata. > > > > > > Wins: > > > > > > - 2 IOs for most: one to write the data to unused space in the block > > > device, > > one to commit our transaction (vs 4+ before). For overwrites, we'd have one > > io to do our write-ahead log (kv journal), then do the overwrite async (vs > > 4+ > > before). > > > > > > - No concern about mtime getting in the way > > > > > > - Faster reads (no fs lookup) > > > > > > - Similarly sized metadata for most objects. If we assume most objects > > > are > > not fragmented, then the metadata to store the block offsets is about the > > same size as the metadata to store the filenames we have now. > > > > > > Problems: > > > > > > - We have to size the kv backend storage (probably still an XFS > > > partition) vs the block storage. Maybe we do this anyway (put > > > metadata on > > > SSD!) so it won't matter. But what happens when we are storing gobs of > > rgw index data or cephfs metadata? Suddenly we are pulling storage out of a > > different pool and those aren't currently fungible. > > > > > > - We have to write and maintain an allocator. I'm still optimistic this > > > can be > > reasonbly simple, especially for the flash case (where fragmentation isn't > > such an issue as long as our blocks are reasonbly sized). For disk we may > > beed to be moderately clever. > > > > > > - We'll need a fsck to ensure our internal metadata is consistent. The > > > good > > news is it'll just need to validate what we have stored in the kv store. > > > > > > Other thoughts: > > > > > > - We might want to consider whether dm-thin or bcache or other block > > layers might help us with elasticity of file vs block areas. > > > > > > - Rocksdb can push colder data to a second directory, so we could > > > have a fast ssd primary area (for wal and most metadata) and a second > > > hdd directory for stuff it has to push off. Then have a conservative > > > amount of file space on the hdd. If our block fills up, use the > > > existing file mechanism to put data there too. (But then we have to > > > maintain both the current kv + file approach and not go all-in on kv + > > > block.) > > > > > > Thoughts? > > > sage > > > -- > > > To unsubscribe from this list: send the line "unsubscribe ceph-devel" > > > in the body of a message to majord...@vger.kernel.org More > > majordomo > > > info at http://vger.kernel.org/majordomo-info.html > > > > > > ________________________________ > > > > > > PLEASE NOTE: The information contained in this electronic mail message is > > intended only for the use of the designated recipient(s) named above. If the > > reader of this message is not the intended recipient, you are hereby > > notified > > that you have received this message in error and that any review, > > dissemination, distribution, or copying of this message is strictly > > prohibited. 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