Hello, On Tue, 2 Jan 2018 01:23:45 +0100 Ronny Aasen wrote:
> On 30.12.2017 15:41, Milanov, Radoslav Nikiforov wrote: > > Performance as well - in my testing FileStore was much quicker than > > BlueStore. > > > with filestore you often have a ssd journal in front, this will often > mask/hide slow spinning disk write performance, until the journal size > becomes the bottleneck. > The journal size basically never becomes the bottleneck, as Ceph will start flushing very quickly with default settings and then hits the effective HDD speed as well. The journal can deal nicely with short/small bursts, though. > with bluestore only metadata db and wal is on ssd. so there is no > doublewrite, and there is no journal bottleneck. but write latency will > be the speed of the disk, and not the speed of the ssd journal. this > will feel like a write performance regression. > Small writes with bluestore also will go to the DB (SSD), as the Ceph developers noted that latencies were rather bad otherwise. >From where I'm standing, Bluestore is rather wet behind the ears, with probably some bugs lurking (a file system, even as simple as this one isn't trivial) and more importantly space for performance improvements. > you can use bcache in front of bluestore to regain the "journal+ > doublewrite" write characteristic of filestore+journal. > I'm using bcache and tested LVM cache in a non-Ceph (DRBD) setup. LVM cache is hilariously complex, poorly (outdated) documented and performs under normal/typical workloads far worse than bcache. OTOH while bcache will give you nice improvements (also on reads) it isn't bug free (and I'm not even thinking about the 4.14 data corruption issue) and when pushed hard (when it wants/needs to flush to HDD) it will overload things and doesn't honor I/O priorities as others have mentioned here. I'm using bcache for now because in my use case the issues above won't show up, but I'd be wary to use it with Ceph in a cluster where I don't control/know the IO patterns. Regards, Christian -- Christian Balzer Network/Systems Engineer ch...@gol.com Rakuten Communications _______________________________________________ ceph-users mailing list ceph-users@lists.ceph.com http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
