Cisco Cloud Services provides an OpenStack platform to Cisco SaaS applications using a petabyte-scale Ceph cluster. The initial Ceph cluster design led to stability problems as usage grew past 50% capacity. Improvements such as client IO throttling, NVMe journaling, upgrading Ceph versions, and moving the MON levelDB to SSD stabilized the cluster and reduced recovery times from hardware failures. Lessons learned included the need for devops practices, knowledge sharing, performance modeling, and avoiding technical debt from shortcuts.

1. Yuming Ma , Architect
Cisco Cloud Services
Ceph Day, Portland Oregon, May 25th, 2016
Stabilizing Petabyte Ceph
Cluster in OpenStack Cloud

2. Highlights
1. What are we doing with Ceph?
2. What did we start with?
3. We need a bigger boat
4. Getting better and sleeping through the night
5. Lessons learned

3. Cisco Cloud Services provides an Openstack platform to Cisco SaaS
applications and tenants through a worldwide deployment of
datacenters.
Background
SaaS Cases
• Collaboration
• IoT
• Security
• Analytics
• “Unknown
Projects”
Swift
• Database (Trove)
Backups
• Static Content
• Cold/Offline data for
Hadoop
Cinder
• Generic/Magnetic
Volumes
• Low Performance

4. • Boot Volumes for all VM flavors except those with
Ephemeral (local) storage
• Glance Image store
• Generic Cinder Volume
• Swift Object store
• In production since March 2014
• 13 clusters in production in two years
• Each cluster is 1800TB raw over 45 nodes and 450
OSDs.
How Do We Use Ceph?
Cisco UCS
Ceph
High-Perf
Platform
Generic
Volume
Prov
IOPS
Cinder API
Object
Swift API

5. • Nice consistent growth…
• Your users will not warn
you before:
• “going live”
• Migrating out of S3
• Backing up a Hadoop
HDFS
• Stability problems started
after 50% used
Growth: It will happen, just not sure when

6. CCS Ceph 1.0
RACK3RACK2
1 2 10
LSI 9271 HBA
Data
partition
HDD
Journal
partition
…..
…..
XFS
…..
…..
OSD2 OSD10OSD1
…..
1211
OS on
RAID1
MIRROR
2x10Gb PRIVATE NETWORK
KEYSTONE
API
SWIFT
API
CINDER
API
GLANCE
API
NOVA
API
OPENSTACK
RADOS GATE WAY CEPH BLOCK DEVICE (RBD)
Libvirt/kv
m
2x10Gb PUBLIC NETWORK
monitors monitors monitors
15xC240
CEPH libRADOS API
RACK1
15xC240 15xC240
OSD: 45 x UCS C240 M3
• 2xE5 2690 V2, 40 HT/core
• 64GB RAM
• 2x10Gbs for public
• 2x10Gbs for cluster
• 3X replication
• LSI 9271 HBA
• 10 x 4TB HDD, 7200 RPM
• 10GB journal partition from
HDD
• RHEL3.10.0-
229.1.2.el7.x86_64
NOVA: UCS C220
• Ceph 0.94.1
• RHEL3.10.0-
229.4.2.el7.x86_64
MON/RGW: UCS C220 M3
• 2xE5 2680 V2, 40 HT/core
• 64GB RAM
• 2x10Gbs for public
• 4x3TB HDD, 7200 RPM
• RHEL3.10.0-
229.4.2.el7.x86_64
Started with Cuttlefish/Dumpling

7. • Get to MVP and keep costs down.
• High capacity, hence C240 M3 LFF for 4TB HDDs
• Tradeoff was that C240 M3 LFF could not also accommodate SSD 
• So Journal was collocated on OSD
• Monitors were on HDD based systems as well
Initial Design Considerations

8. Major Stability Problems: Monitors
Problem Impact
MON election storm
impacting client IO
Monmap changes due to flaky NIC or chatty messaging between MON and
client. Caused unstable quorum and an election storm between MON hosts
Results: blocked and slowed client IO requests
LevelDB inflation Level DB size grows to XXGB over time that prevents MON daemon from
serving OSD requests
Results: Blocked IO and slow request
DDOS due to chatty
client msg attack
Slow response from MON to client due to levelDB or election storm causing
message flood attack from client.
Results: failed client operation, e.g volume creation, RBD connection

9. Major Stability Problems: Cluster
Problem Impact
Backfill & Recovery
impacting client IO
Osdmap changes due to loss of disk, resulting in PG peering and backfilling
Results: Clients receive blocked and slow IO.
Unbalanced data
distribution
Data on OSDs isn’t evenly distributed. Cluster may be 50% full, but some
OSDs are at 90%
Results: Backfill isn’t always able to complete.
Slow disk impacting
client IO
A single slow (sick, not dead) OSD can severely impact many clients until it’s
ejected from the cluster.
Results: Client have slow or blocked IO.

10. Stability Improvement Strategy
Strategy Improvement
Client IO throttling* Rate limit IOPS at Nova host to 250 IOPS per volume.
Backfill and recovery
throttling
Reduced IO consumption by backfill and recovery processes to yield to
client IO over
Retrofit with NVME (PCIe)
journals
Increased overall IOPS of the cluster
Upgrade to 1.2.3/1.3.2 Overall stability and hardened MONs preventing election storm
LevelDB on SSD
(replaced entire mon node)
Faster cluster map query
Re-weight by utilization Balance data distribution
*Client is the RBD client not the tenant

11. • Limit max/cap IO consumption at
qemu layer:
• iops ( IOPS read and write ) 250
• bps (Bits per second read and
write ) 100 MB/s
• Predictable and controlled IOPS
capacity
• NO min/guaranteed IOPS ->
future Ceph feature
• NO burst map -> qemu feature:
• iops_max 500
• bpx_max 120 MB/s
Client IO throttling
Swing ~ 100%
Swing ~ 12%

12. • Problem
• Blocked IO during peering
• Slow requests during backfill
• Both could cause client IO stall
and vCPU soft lockup
• Solution
• Throttling backfill and recovery
osd recovery max active = 3 （default : 15)
osd recovery op priority = 3 (default : 10)
osd max backfills = 1 (default : 10)
Backfill and Recovery Throttling

13. Rack-1
6 nodes
osd
1
osd
10…
Rack-2
5 nodes
osd
1
osd
10…
Rack-3
6 nodes
osd
1
osd
10…
nova1
vm
1
vm
20
…
nova10
vm
1
vm
20
…
nova2
vm
1
…
…..
NVMe Journaling: Performance Testing Setup
Partition starts at 4MB(s1024), 10GB each and
4MB offset in between
1 2 10
LSI 9271 HBA
1 2 10
RAID0
1DISK
1 2 10NVME
…..
…..
XFS
…..
…..
OSD
2
OSD10OSD
1
…..
300GB Free
1211
OS on RAID1
MIRROR OSD: C240 M3
• 2xE5 2690 V2, 40 HT/core
• 64GB RAM
• 2x10Gbs for public
• 2x10Gbs for cluster
• 3X replication
• Intel P3700 400GB NVMe
• LSI 9271 HBA
• 10x4TB, 7200 RPM
Nova C220
• 2xE5 2680 V2, 40 HT/core
• 380GB RAM
• 2x10Gbs for public
• 3.10.0-229.4.2.el7.x86_64
vm
20

14. NVMe Journaling: Performance Tuning
OSD host iostat:
• Both nvme and hdd disk %util and low most of the time, and spikes
every ~45s.
• Both nvme and hdd have very low queue size (iodepth) while frontend
VM pushes 16 qdepth to FIO.
• CPU %used is reasonable, converge at <%30. But the iowait is low
which corresponding to low disk activity

15. NVMe Journaling: Performance Tuning
Tuning Directions: increase disk %util:
• Disk thread: 4, 16, 32
• Filestore max sync interval: (0.1, 0.2, 0.5, 1 5, 10 20)

16. • These two tunings showed no impact:
filestore_wbthrottle_xfs_ios_start_flusher: default 500 vs 10
filestore_wbthrottle_xfs_inodes_start_flusher: default 500 vs 10
• Final Config:
osd_journal_size = 10240 (default :
journal_max_write_entries= 1000 (default : 100)
journal_max_write_bytes=1048576000 (default :10485760)
journal_queue_max_bytes=1048576000 (default :10485760)
filestore_queue_max_bytes=1048576000 （(default :10485760)
filestore_queue_committing_max_bytes=1048576000 （(default :10485760)
filestore_wbthrottle_xfs_bytes_start_flusher = 4194304 （(default :10485760)
NVMe Performance Tuning
Linear tuning filestore_wbthrottle_xfs_bytes_start_flusher:
filestore_wbthrottle_xfs_inodes_start_flusher
filestore_wbthrottle_xfs_ios_start_flusher

17. NVMe Stability Improvement Analysis
One Disk (70% of
3TB) failure MTTR
One Host (70% of
30TB) Failure MTTR
Colo 11 hrs, 7 mins. 6 secs 19 hrs, 2 mins, 2 secs
NVME 1 hr, 35 mins, 20 secs 16 hr, 46 mins, 3 secs
Disk failure (70% of
3TB) impact to client
IOPS
Host failure (70% of
30TB) impact to client
IOPS
Colo 232.991 vs 210.08
(Drop: 9.83%)
NVME 231.66 vs 194.13
(Drop: 16.20%)
231.66 vs 211.36 (Drop:
8.76%)
Backfill and recovery config:
osd recovery max active = 3 （default : 15)
osd max backfills = 1 (default : 10)
osd recovery op priority = 3 (default : 10)
Server impact:
• Shorter recovery time
Client impact
• <10% impact (tested without IO
throttling, should be less with throttling)

18. LevelDB :
• Key-value store for cluster metadata, e.g.
osdmap, pgmap, monmap, clientID,
authID etc
• Not in data path
• Impactful to IO operation: IO blocked by
the DB query
• Larger size, longer query time, hence
longer IO wait -> slow requests
• Solution:
• Level DB on SSD in increase disk IO rate
• Upgrade to Hammer to reduce DB size
MON Level DB Issues

19. MON Level DB on SSD
New BOM:
• UCS C220 M4 with 120GB SSD
Write wait time
with levelDB on HDD
Write wait time
with levelDB on SSD

20. • Problem
• Election Storm & LevelDB inflation
• Solutions
• Upgrade to 1.2.3 to fix election storm
• Upgrade to 1.3.2 to fix levelDB inflation
• Configuration change
MON Cluster Hardening
[mon]
mon_lease = 20 (default = 5)
mon_lease_renew_interval = 12 (default 3)
mon_lease_ack_timeout = 40 (default 10)
mon_accept_timeout = 40 (default 10)
[client]
mon_client_hunt_interval = 40 (defaiult 3)

21. • Problem
• High skew of %used of disks that is
preventing data intake even cluster capacity
allows
• Impact:
• Unbalanced PG distribution impacts
performance
• Rebalancing is impactful as well
• Solution:
• Upgrade to Hammer 1.3.2+patch
• Re-weight by utilization: >10% delta
Data Distribution and Balance
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
22
43
64
85
106
127
148
169
190
211
232
253
274
295
316
337
358
379
400
421
442
us-internal-1 disk % used
% used
Cluster: 67.9% full
OSDs:
• Min: 47.2&
• Max: 83.5%
• Mean: %69.6
• Stddev: 6.5

22. • Problem
• RBD image data distributed to
all disk and single slow disk
can impact critical data IO
• Solution: proactively detect
slow disks
Proactive Detection of Slow Disks

23. • Set Clear Stability Goals
• You can plan for everything except how tenants will use it
• Monitor Everything, but not “everything”
• Turn down logging… It is possible to send 900k logs in 30minutes
• Look for issues in services that consume storage
• Had 50TB of “deleted volumes” that weren’t
Lessons Learned

24. • DevOps
• It’s not just technology, it’s how your team operates as a team
• Share knowledge
• Manage your backlog and manage your management
• Consistent performance and stability modeling
• Rigorous testing
• Determine Requires and architect to them
• Balance performance, cost and time
• Automate builds and rebuilds
• Shortcuts create Technical Debt
Last Lesson…

25. Yuming Ma: yumima@cisco.com
Thank You
Seth Mason: setmason@cisco.com