The document discusses the future developments of the RADOS Block Device (RBD) within the Ceph storage system, highlighting new features like deep-copy of images, live image migration, and enhancements in image cloning. It also mentions improvements to mirroring capabilities, support for active-active configurations, and ongoing performance optimizations. Additionally, it outlines upcoming features and work-in-progress aimed at enhancing usability and performance in Ceph deployments.

1. RBD: What Will the Future Bring?
Jason Dillaman
RBD Project Technical Lead

2. Introductons
Jason Dillaman
●
Ceph developer since 2014
●
Lead for RADOS Block Device (RBD)
●
Principal Software Engineer at Red Hat
●
dillaman@redhat.com

3. Ceph Components
RGW
A web services gateway for
object storage, compatible with
S3 and Swift
LIBRADOS
A library allowing apps to directly access RADOS (C, C++, Java, Python, Ruby, PHP)
RADOS
A software-based, reliable, autonomous, distributed object store comprised of
self-healing, self-managing, intelligent storage nodes and lightweight monitors
RBD
A reliable, fully-distributed block
device with cloud platform
integration
CEPHFS
A distributed fle system with
POSIX semantics and scale-out
metadata management
OBJECT BLOCK FILE

4. RADOS Block Device
● Block device abstracton
● Striped over fied-size objects
● Highlights
– Broad integraton
– Thinly provisioned
– Snapshots
– Copy-on-write clones

5. RADOS Block Device
LIBRADOS
LIBRBD
M M
M
KRBD
User-space Client Linux Host
LIBCEPH
IMAGE IO

6. New Features in Mimic

7. Deep-Copy of Images
● Generic version of mirroring’s “image sync” logic
– Preserves clone parent linkage and snapshots
– Sparse copy of backing objects
● Useful for copying between pools or changing fied
image format parameters
● Optonally laten copies of cloned images
● Usage:
– “rbd deep copy <src-snap-spec> <dest-image-spec>”
– Similar to “rbd eiport --eiport-format 2 <src-snap-spec> -
| rbd import --eiport-format 2 <dest-image-spec> -”

8. Live Image Migraton
● Live “deep-copy” of image
– Safe to use with concurrent IO against image
● Reads follow hierarchy chain
● Writes only afect destnaton
– May require “copy-up” from hierarchy chain
LIBRADOS
LIBRBD
User-space Client READSWRITES
DESTINATIONSOURCEPARENT
(OPTIONAL)

9. Live Image Migraton, cont
● Useful for copying between pools or changing fied image
format parameters
– Cannot live-migrate between clusters (use mirroring)
● Alternatve to bolt-on solutons
– QEMU drive-mirror
– Kernel sofware RAID “repair”
● Usage:
– “rbd migraton prepare <src-image> <dst-image>”
– “rbd migraton eiecute”
– “rbd migraton commit”

10. Image Cloning (v2)
● Simplifed operatons for eiistng clone feature
– No change to data low handling
– Supported in krbd (new feature bit whitelisted)
● Parent snapshots no longer need to be protected
– Atomic reference countng in the OSD
– OSD “profle rbd[-read-only]” caps whitelist support
● Snapshots with linked children can be “deleted”
– In reality: moved to “trash” snapshot namespace
– Trashed snapshot auto-removed

11. Image Cloning (v2), cont
● Support enabled automatcally when Mimic clients
required
– “ceph osd set-require-min-compat-client mimic”
– Confg override: “rbd default clone format = [1, 2, auto]”
● Usage:
– “rbd snap create <parent-image>@<snap>”
– “rbd clone <parent-image>@<snap> <child image>”
– “rbd snap rm <parent-image>@<snap>”
– “rbd snap ls --all parent”
SNAPID NAME SIZE TIMESTAMP NAMESPACE
4 29da36c8-9ff6-46b7-9888-9e9804c22525 1024 kB Sun Mar 18 20:41:13 2018 trash

12. Image Groups
● Associate related-images together
● Perform snapshots concurrently against grouped images
● Future work:
– Need to add support for group cloning
– Need to add support for group journaling (consistency
group)
● Usage:
– “rbd group <group-name>”
– “rbd group image add <image-name>”
– “rbd group snap create <group-name>@<snap-name>”

13. Actve/Actve Mirroring
● Actve/Passive HA support added in Luminous
– Single daemon-per-pool elected to leader
– Failure of leader will result in new leader electon
● Actve/Actve leader assigns images to followers
– Simple “equal # of images” policy
– Smarter load-based policies possible in future
rbd-mirror
LIBRBD
SITE A
rbd-mirror
LIBRBD
rbd-mirror
LIBRBD
FOLLOWERFOLLOWERLEADER
Acquire/Release Image

14. Additonal Mirroring Improvements
● Deferred Image Deletons
– Propagated deletes from primary move non-primary
image to RBD trash
– Confg override: “rbd mirroring delete delay =
<seconds>”
– Restore via “rbd trash restore <image-id-spec>”
● Clone non-primary images
– Helpful for OpenStack Glance-style workloads

15. Minor Improvements
● IOPS throtling
– Client-side rate limiter
– Confg override: “rbd qos iops limit = X”
● Trash purge CLI
– Human-readable “--eipired-before X” optonal
– Pool-usage “--threshold <percent>” optonal
● Creaton of image format v1 disabled
– Deprecated since Jewel

16. Out-of-Tree Features /
Works-in-Progress

17. Linui Kernel RBD Driver (krbd)
● Support for “fancy” image striping
– Already supported simple striping (stripe unit ==
object size, stripe count == 1)
– Might be preferable in small, sequental IO
workloads
– (Hopefully) included in 4.17
● Support for object-map/fast-dif in-progress
● Support for deep-laten is planned
● New operatons feature whitelisted in 4.16 rc1

18. Linui IO (LIO) Target
● LIO + tcmu-runner provide SCSI/iSCSI interface to RBD
● Live LUN resize support
● Persistent Group Reservatons (PGRs)
– Required for Windows Clustering
– Required for VMware Certfcaton
● Graceful failover for shared LUNs
– Avoid actve/optmized path ping-pong when failure
visible from only a subset of initators
● Integrate more storage admin worklows into tools

19. Works-in-Progress

20. QoS Reservatons
● Utlizes the dmClock work scheduler within the OSDs
– Conditonal on its completon
● Minimum IOPS reservaton vs throtling
– Reduce impact from noisy neighbors
● Interface / CLI changes stll TBD

21. Persistent Read-Only Cache
● Client-side, hot-spot cache for read-only image parent
objects
● Ofoad cluster for heavily re-used parents
– VDI infrastructure / common “golden” images
Client
LIBRBD
rbd-cache
Read/Promote Object
Object Hit Query
Demote Object

22. Persistent Writeback Cache
Goal: reduce tail latency

23. Persistent Writeback Cache, cont
● Replicated Write Log (RWL) plug-in for librbd cache
● All writes are streamed to persistent storage
– Writethrough mode: safely locally persisted on each
write
– Writeback mode: safely locally persisted on lush
● Uses pmem.io library for atomic operatons
– Optmized for persistent memory (i.e. NVDIMM)
– Optonally supports librpmem for RDMA streaming to
secondary host

24. Persistent Writeback Cache, cont
Client
LIBRBD
WRITES
Sync Point
Batched
Writeback
Log Append
Ceph Cluster
PMEM

25. Performance Improvements
● librbd uses more CPU per IOP than krbd
● Minor tweaks to librbd/librados can narrow the gap
4K rand-read 4K rand-write
20000
25000
30000
35000
40000
45000
50000
55000
60000
65000
0
50
100
150
200
250
librbd (current)
librbd (tweaked)
krbd
librbd (current)
librbd (tweaked)
IOPS
IOPS/%CPU

26. Performance Improvements
● Current client libraries
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27. Performance Improvements, cont
● Tweaked client libraries
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28. Feature Backlog

29. ● librbd
– Improved image/pool
confguraton overrides
– Client-side encrypton
– Improve performance
● Mirroring
– Deep scrub and repair
– QoS throtles
– Improved stats
– Improve performance

30. Questions?

31. Thank You!