An introduction to the HBase BlockCache, the internal system for caching data and speeding reads.

1. HBase
BlockCache
101
Nick
Dimiduk,
Hortonworks
@xefyr
n10k.com

2. Agenda
• What
is
a
BlockCache?
• Why
cache
blocks?
• Available
BlockCaches
• ConfiguraFons

3. RegionServer (HBase)
DataNode (Hadoop DFS)
HLog
(WAL)
HRegion
HStore
StoreFile
HFile
StoreFile
HFile
MemStore
...
...
HStore
BlockCache
HRegion
...
HStoreHStore
...
1 5
2
3
3
2
4
Read
Path
Refresher
BlockCache

4. What
is
a
block?
HFile
v2
Format
HFile
v2
format
figure
reproduced
from
MaMeo
Bertozzi,
"Apache
HBase
I/O
–
HFile”,
hMp://blog.cloudera.com/blog/2012/06/hbase-­‐io-­‐
hfile-­‐input-­‐output/

5. Why
cache
blocks?
Storage hierarchy: a different view
Dean/2009
Memory
is
100000x
faster
than
disk!
Disk
seek
=
10ms

6. LruBlockCache!
• On-­‐heap
ConcurrentHashMap!
• 3
block
prioriFes,
based
on
access
frequency
– Single
– MulF
– In-­‐memory
• Background
evicFon
thread
• HBASE-­‐1460,
0.20

7. LruBlockCache
ConfiguraFon
• Size
as
pct
of
heap
– hfile.block.cache.size
– default:
0.4
• Single
access
Priority
size
– hbase.lru.blockcache.single.percentage
– default:
0.25
• MulF-­‐access
priority
size
– hbase.lru.blockcache.multi.percentage
– default:
0.5
• In-­‐memory
priority
size
– hbase.lru.blockcache.memory.percentage
– default:
0.25

8. SlabCache!
• L2
cache;
fronted
by
LruBlockCache
(L1)
• DoubleBlockCache
caches
blocks
in
both
L1
and
L2
• Allocates
porFons
of
memory
with
target
block
size
• DirectByteBuffers
manage
off-­‐heap
memory
• Blocks
copied
on
and
off
heap
• HBASE-­‐4027,
0.92

9. SlabCache
ConfiguraFon
• SlabCache
size
– hbase.offheapcache.percentage!
• Target
Block
size
– hbase.offheapcache.minblocksize!
– default:
64k
• Slab
proporFons
– hbase.offheapcache.slab.proportions!
– default:
0.8,
0.2
• Slab
target
block
sizes
– hbase.offheapcache.slab.sizes!
– default:
64k,
128k

10. BucketCache!
• L2
cache;
fronted
by
LruBlockCache
(L1)
• CombinedBlockCache
caches
data
blocks
in
L2,
everything
else
in
L1
• Blocks
evicted
from
L1
are
promoted
to
L2
• Allocates
porFons
of
space
with
target
block
size
• Three
operaFng
modes:
– onheap
–
HeapByteBuffers
manage
on-­‐heap
memory
space
– offheap
–
DirectByteBuffers
manage
off-­‐heap
memory
space
– file
–
RandomAccessFile
manages
file-­‐backed
space
• Blocks
copied
on
and
off
heap
• HBASE-­‐7404,
0.96

11. BucketCache
ConfiguraFons
• OperaFng
mode
– hbase.bucketcache.ioengine!
• Total
cache
size
as
pct
of
heap
OR
size
in
MB:
– hbase.bucketcache.size!
• Size
of
L1
as
pct
of
total
cache
size:
– hbase.bucketcache.percentage.in.combinedcache!
– default:
0.9
• Target
Block
size
– hbase.offheapcache.minblocksize!
– default:
64k
• Bucket
target
block
sizes
– hbase.bucketcache.bucket.sizes!
– default:
14
sizes,
clustered
around
64k

12. BlockCache
Showdown
• Compare
performance:
– Total
cache
size
– Cache
size
vs.
Data
size
n10k.com/blog/
blockcache-­‐showdown/
L2
off-­‐heap
BucketCache
makes
a
strong
showing

13. Thanks!
M A N N I N G
Nick Dimiduk
Amandeep Khurana
FOREWORD BY
Michael Stack
hbaseinacFon.com
Nick
Dimiduk
github.com/ndimiduk
@xefyr
n10k.com
n10k.com/blog/blockcache-­‐101