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== Bluestore vs Filestore ==
Bluestore is Ceph’s «new» storage layer. It’s reasonable to expect better performance and basically better everything from something «new».
And yes, Bluestore is really 2x faster than Filestore for linear write workloads, because it really has double-write eliminated — big blocks are written only once, not twice (first to the journal and then to the data device) as it was with Filestore.
Bluestore is also more feature-rich: it has checksums, compression, erasure-coded overwrites and virtual clones (which make snapshots faster).
However, the random write performance varies:
* Bluestore is 2x faster than Filestore in HDD-only (or bad-SSD-only) configurations, because it can do 1 commit per write, at least if you remove the handbrake from here: https://github.com/ceph/ceph/pull/26909 or if you wait for the backport. In fact it’s OK to say that Bluestore’s deferred-write algorithm is really optimal for HDD-only transactional write.
* In HDD+SDD configurations, Bluestore performance is very consistent, but it’s worse than the peak performance of Filestore for the same hardware. This is because Bluestore’s deferred write queue doesn’t act as a «temporary buffer» that can smooth burst random write loads. In other words, Bluestore OSD refuses to do random writes faster than the HDD can do on average.
*: So with Filestore you can easily get temporary 1000—2000 iops with single write thread — or even more.
== RAID WRITE HOLE ==
