Sebastien Han, eNovance

1. Ceph performance
CephDays Frankfurt 2014

2. Whoami
💥 Sébastien Han
💥 French Cloud Engineer working for eNovance
💥 Daily job focused on Ceph and OpenStack
💥 Blogger
Personal blog: http://www.sebastien-han.fr/blog/
Company blog: http://techs.enovance.com/
Last Cephdays presentation

3. How does Ceph perform?
42*
*The Hitchhiker's Guide to the Galaxy

4. The Good
Ceph IO pattern

5. CRUSH: deterministic object
placement
As soon as a client writes into Ceph, the operation is computed
and the client decides to which OSD the object should belong

6. Aggregation: cluster level
As soon as you write into Ceph, all the objects get equally spread across the entire
Cluster, understanding machines and disks..

7. Aggregation: OSD level
As soon as an IO goes into an OSD, no matter how the original pattern was,
it becomes sequential.

8. The Bad
Ceph IO pattern

9. Journaling
As soon as an IO goes into an OSD, it gets written twice.

10. Journal and OSD data on the same
disk
Journal penalty on the disk
Since we write twice, if the journal is stored on the same disk as
the OSD data this will result in the following:
Device: wMB/s
sdb1 - journal 50.11
sdb2 - osd_data 40.25

11. Filesystem fragmentation
• Objects are stored as files on the OSD filesystem
• Several IO patterns with different block sizes increase filesystem
fragmentation
• Possible root cause: image sparseness
• One year old cluster ends up with (see allocsize options for
XFS):
$ sudo xfs_db -c frag -r /dev/sdd
actual 196334, ideal 122582, fragmentation factor 37.56%

12. No parallelized reads
• Ceph will always serve the read request from the primary OSD
• Room for Nx times speed up where N is the replica count
Blueprint from Sage for the Giant release

13. Scrubbing impact
• Consistent object check at the PG level
• Compare replicas versions between each others (Fsck for objects)
• Light scrubbing (daily) checks the object size and attributes.
• Deep scrubbing (weekly) reads the data and uses checksums to ensure
data integrity.
• Corruption exists – ECC memory (10^15 for enterprise disk)
~113TB
• No pain No gain

14. The Ugly
Ceph IO pattern

15. IOs to the OSD disk
One IO into Ceph leads to 2 writes, well… the second write is the worst!

16. The problem
• Several objects map to the same physical disks
• Sequential streams get mixed all together
• Result: The disk seeks like hell

17. Even worse with erasure coding?
This is just an assumption!
•Since erasure coding does chunks of chunks we can possibly
have this phenomena amplified

18. CLUSTER
How to build it?

19. How to start?
Things that you must consider:
•Use case
• IO profile: Bandwidth? IOPS? Mixed?
• How many IOPS or Bandwidth per client do I want to deliver?
• Do I use Ceph in standalone or is it combined with a software solution?
•Amount of data (usable not RAW)
• Replica count
• Do I have a data growth planning?
•Leftover
• How much data am I willing to lose if a node fails? (%)
• Am I ready to be annoyed by the scrubbing process?
•

20. Things that you must not do
• Don't put a RAID underneath your OSD
• Ceph already manages the replication
• Degraded RAID breaks performances
• Reduce usable space on the cluster
• Don't build high density nodes with a tiny cluster
• Failure consideration and data to re-balance
• Potential full cluster
• Don't run Ceph on your hypervisors (unless you're broke)
• Well maybe…

21. Firefly: Interesting things
going on

22. Object store multi-backend
• ObjectStore is born
• Aims to support several backends:
• levelDB (default)
• RocksDB
• Fusionio NVMKV
• Seagate Kinetic
• Yours!

23. Why is it so good?
• No more journal! Yay!
• Object backends have built-in atomic functions

24. Firefly leveldb
• Relatively new
• Need to be tested with your workload first
• Tend to be more efficient with small objects

25. Many thanks!
Questions?
Contact: sebastien@enovance.com
Twitter: @sebastien_han
IRC: leseb