Adding to this, On Tue, 2015-10-20 at 05:34 -0700, Sage Weil wrote: > On Mon, 19 Oct 2015, James (Fei) Liu-SSI wrote: > > 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. > > Are you talking about open channel SSDs? Or something else? Everything > I'm familiar with that is currently shipping is exposing a vanilla block > interface (conventional SSDs) that hides all of that or NVMe (which isn't > much better). > > If there is a low-level KV interface we can consume that would be > great--especially if we can glue it to our KeyValueDB abstract API. Even > so, we need to make sure that the object *data* also has an efficient API > we can utilize that efficiently handles block-sized/aligned data.
If there's a way to efficiently utilize more generic NVRAM-based block devices for quick metadata ops such that payload data can fly without much delay, I'd be quite happy. Also, a current concern of mine is backups in some fashion of the metadata, given risk for (human configuration error||device malfunction)&&(cluster wide power outage). Some type of flushing to underlying consistent media, and/or snapshot-like backups. As long as the constructs aren't too exotic, perhaps this could be addressed using standard Linux FS or device mapper code (bcache, or other) Not sure how popular journals on NVRAM is. But here's one user at least. /M > sage > > > > 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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