Cassandra troubleshooting:
out of the shadows
Benjamin Black, b@b3k.us

Introducing: This Guy

The Allegory of the Cave

Most people start
troubleshooting problems
interpreting shadows on the
wall.

Common shadows.

Paths out of the cave.

Combination of
basic system tools
&
nodetool/JMX

I’m using RP.
My ring is very unbalanced.

I’m using RP.
My ring is very unbalanced.
WTF?

nodetool ring

Address Status Load Range Ring
148873535527910577765226390751398592511
10.248.54.192 Up 5.59 GB 0 |<--|
10.248.254.15 Up 10.58 GB 42535295865117307932921825928971026431 | ^
10.248.135.239Up 11.01 GB 85070591730234615865843651857942052863 v |
10.248.223.191Up 5.42 GB 106338239662793269832304564822427566079 | ^
10.248.122.240Up 5.51 GB 127605887595351923798765477786913079295 v |
10.248.34.80 Up 5.45 GB 148873535527910577765226390751398592511 |-->|

Address Status Load Range Ring
148873535527910577765226390751398592511
10.248.54.192 Up 5.59 GB 0 |<--|
10.248.254.15 Up 10.58 GB 42535295865117307932921825928971026431 | ^
10.248.135.239Up 11.01 GB 85070591730234615865843651857942052863 v |
10.248.223.191Up 5.42 GB 106338239662793269832304564822427566079 | ^
10.248.122.240Up 5.51 GB 127605887595351923798765477786913079295 v |
10.248.34.80 Up 5.45 GB 148873535527910577765226390751398592511 |-->|

Autobootstrap
+
Automatic token assignment

Automatic token algorithm:
Assign a token that will give me
half the range of
the most loaded node.

32
16 16
8 8 16
8888
44888
444488

Address Status Load Range Ring
148873535527910577765226390751398592511
10.248.54.192 Up 5.59 GB 0 |<--|
10.248.254.15 Up 10.58 GB 42535295865117307932921825928971026431 | ^
10.248.135.239Up 11.01 GB 85070591730234615865843651857942052863 v |
10.248.223.191Up 5.42 GB 106338239662793269832304564822427566079 | ^
10.248.122.240Up 5.51 GB 127605887595351923798765477786913079295 v |
10.248.34.80 Up 5.45 GB 148873535527910577765226390751398592511 |-->|

nodetool move
+
Manual token assignment

0-(2**127 - 1)
def tokens(nodes)
0.upto(nodes - 1) do |n|
p (n * (2**127 - 1) / nodes)
end
end

=> tokens(6)
0
283568639100782052886145506193140
17621
567137278201564105772291012386280
35242
850705917302346158658436518579420
52863
113427455640312821154458202477256
070484
141784319550391026443072753096570
088105

YES:
This means you need to change tokens on
most of the nodes in your cluster whenever
you add a node.

Writes are fast.
Reads keep getting slower.

Writes are fast.
Reads keep getting slower.
WTF?

iostat -x
look at %util

nodetool tpstats

Pool Name Active Pending Completed
STREAM-STAGE 0 0 0
RESPONSE-STAGE 0 0 516280
ROW-READ-STAGE 8 4096 1164326
LB-OPERATIONS 0 0 0
MESSAGE-DESERIALIZER-POOL 1 682008 1818682
GMFD 0 0 6467
LB-TARGET 0 0 0
CONSISTENCY-MANAGER 0 0 661477
ROW-MUTATION-STAGE 0 0 998780
MESSAGE-STREAMING-POOL 0 0 0
LOAD-BALANCER-STAGE 0 0 0
FLUSH-SORTER-POOL 0 0 0
MEMTABLE-POST-FLUSHER 0 0 4
FLUSH-WRITER-POOL 0 0 4
AE-SERVICE-STAGE 0 0 0
HINTED-HANDOFF-POOL 0 0 3

Pool Name Active Pending Completed
STREAM-STAGE 0 0 0
RESPONSE-STAGE 0 0 516280
ROW-READ-STAGE 8 4096 1164326
LB-OPERATIONS 0 0 0
MESSAGE-DESERIALIZER-POOL 1 682008 1818682
GMFD 0 0 6467
LB-TARGET 0 0 0
CONSISTENCY-MANAGER 0 0 661477
ROW-MUTATION-STAGE 0 0 998780
MESSAGE-STREAMING-POOL 0 0 0
LOAD-BALANCER-STAGE 0 0 0
FLUSH-SORTER-POOL 0 0 0
MEMTABLE-POST-FLUSHER 0 0 4
FLUSH-WRITER-POOL 0 0 4
AE-SERVICE-STAGE 0 0 0
HINTED-HANDOFF-POOL 0 0 3

Pool Name Active Pending Completed
STREAM-STAGE 0 0 0
RESPONSE-STAGE 0 0 516280
ROW-READ-STAGE 8 4096 1164326
LB-OPERATIONS 0 0 0
MESSAGE-DESERIALIZER-POOL 1 682008 1818682
GMFD 0 0 6467
LB-TARGET 0 0 0
CONSISTENCY-MANAGER 0 0 661477
ROW-MUTATION-STAGE 0 0 998780
MESSAGE-STREAMING-POOL 0 0 0
LOAD-BALANCER-STAGE 0 0 0
FLUSH-SORTER-POOL 0 0 0
MEMTABLE-POST-FLUSHER 0 0 4
FLUSH-WRITER-POOL 0 0 4
AE-SERVICE-STAGE 0 0 0
HINTED-HANDOFF-POOL 0 0 3

Pool Name Active Pending Completed
STREAM-STAGE 0 0 0
RESPONSE-STAGE 0 0 516280
ROW-READ-STAGE 8 4096 1164326
LB-OPERATIONS 0 0 0
MESSAGE-DESERIALIZER-POOL 1 682008 1818682
GMFD 0 0 6467
LB-TARGET 0 0 0
CONSISTENCY-MANAGER 0 0 661477
ROW-MUTATION-STAGE 0 0 998780
MESSAGE-STREAMING-POOL 0 0 0
LOAD-BALANCER-STAGE 0 0 0
FLUSH-SORTER-POOL 0 0 0
MEMTABLE-POST-FLUSHER 0 0 4
FLUSH-WRITER-POOL 0 0 4
AE-SERVICE-STAGE 0 0 0
HINTED-HANDOFF-POOL 0 0 3

YOU ARE OUT OF
DISK BANDWIDTH

You can:

Throttle reads at clients

Adjust memtable settings
(size/ops/time)

Less frequent memtable flush
Less frequent compaction
Less disk bandwidth demand

Add more nodes

Add more spindles per node

Switch to SSDs

I inserted a bunch of data.
Now my nodes are flapping.

I inserted a bunch of data.
Now my nodes are flapping.
WTF?

iostat -x
look at %util

vmstat
look at swap

INFO 13:27:35,309
DiskAccessMode 'auto' determined to be mmap,
indexAccessMode is mmap

mmap() in Cassandra
consumes up to 2GB.

mmap() in Cassandra
consumes up to 2GB.
Per segment.

NOT tracked as JVM heap.
*See: https://issues.apache.org/jira/browse/CASSANDRA-1214

NOT tracked as JVM heap.
JVM heap not locked in
memory.
*See: https://issues.apache.org/jira/browse/CASSANDRA-1214

When your data set exceeds
memory,
this is likely.

Swapping can delay gossip
long enough to cause a
node to be marked down.

<DiskAccessMode>mmap_index_only</
DiskAccessMode>
or
disk_access_mode: mmap_index_only

On Linux: swappiness=0

INFO 13:27:35,309
DiskAccessMode isstandard,
indexAccessMode is mmap

Most people start
troubleshooting problems
interpreting shadows on the
wall.

You can now see the path
and the sunlight outside.

YOU CAN HELP!

What things have confused
you?

What problems have you
solved?

What tools have you used to
solve them?

GET INVOLVED!

http://wiki.apache.org/cassandra

#cassandra on freenode