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Cost-effective Dynamic Replication Management (CDRM) Agenda Outline Introduction Problem Statement Cost-effective Dynamic Replication Management (CDRM) Evaluation Conclusion
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CDRM: CDRM: A Cost-effective Dynamic A Cost-effective Dynamic Replication Management Replication Management Scheme Scheme
for Cloud Storage for Cloud Storage ClusterCluster
The IEEE International Conference on Cluster Computing 2010
Qingsong Wei Data Storage Institute, A-STAR, Singapore
Bharadwaj Veeravalli, Bozhao GongNational University of Singapore
Lingfang Zeng, Dan FengHuazhong University of Science & Technology, China
Page 2 of 19
1. Introduction
2. Problem Statement
3. Cost-effective Dynamic Replication Management (CDRM)
4. Evaluation
5. Conclusion
OutlineAgenda
Page 3 of 19
Outline1, Introduction
HDFS Architecture
Network
Meta Data
Data Blocks
Data Nodes
Name Node
NodeDiskDisks
NodeDiskDisks
NodeDiskDisks
NodeDiskDisks
NodeDiskDisks
NodeDiskDisks
Clients
Control
1. Introduction
Page 4 of 19
Data Striping
B1 B2 … Bm
Node1 Node2 … Noden
In the HDFS, files are striped into date blocks across multiple data nodes to enable parallel access.
However, Block may be unaccessible due to date node unavailable. If one of the blocks is unavailable, so as the whole file.
Failure is normal instead of exception in large scale storage cloud system. Fault tolerance is required in such a system.
1. Introduction
Page 5 of 19
12
2 14 5 2
35
4 35 4
Clients
Replication is used in HDFS. When one data node fails, the data is still accessible from the
replicas and storage service need not be interrupted. Besides fault tolerance, replicas among data nodes can be used to
balance workload.
Data nodes
2. Problem Statement
Page 6 of 19
212
35
43
54 1
1 2 4 53
Current replication managements Treat all data as same: same replica number for all data Treat all storage nodes as same Fixed and Static
High cost & Poor load balance
2. Problem Statement
Page 7 of 19
Replica number is critical to management cost. More replica, more cost.
12
2 14 5 2
35 4 35 4The block 5
is modified
Update to maintain consistency
Because large number of blocks are stored in system, even a small increase of replica number can result in a significant increase of management cost in the overall system.
Then, how many minimal replica should be kept in the system to satisfy availability requirement?
2. Problem Statement
Page 8 of 19
Replica placement influences intra-request parallelism.
Client
Data Node1 Data Node2 Data Node3
B3 B2 B1
Sessionmax=3Sessionfree=1
Sessionmax=3Sessionfree=2
Sessionmax=2Sessionfree=0
B3
Requests
File (B1, B2, B3)
B2 B1
Blocked
B1
2. Problem Statement
Page 9 of 19
Replica placement also influences inter-request parallelism.
Client1 Client2
B3 B2 B1 B1
Sessionmax=3Sessionfree=0
Sessionmax=3Sessionfree=1
Sessionmax=2Sessionfree=0
Requests
How to place these replicas among Data nodes clusters in a balance way to improve access parallelism?
Data Node1 Data Node2 Data Node3
B1B2 B3B1
3. Cost-effective Dynamic Replication Management
Page 10 of 19
Node1 … Nodei … NodeN
Total arrival rate: λ
(p1, s1, r1, t1)
B1……
(pj, sj, rj, tj)
Bj……
(pM, sM, rM, tM)
BM
(λ1, τ1, f1, c1) (λi, τi, fi, ci) (λN, τN, fN, cN)
System Modelpj : popularitysj : sizerj : replica numbertj : access latency
requirement
λi : req. arr. rateτi : average ser. timefi : failure rateci : max sessions
Data has different attributes Data nodes are different
Availability Suppose file F is striped into m blocks {b1 , b2 ,…, bm}. To retrieve
whole file F, we must get all the m blocks. Availability is modeled as function of replica number.
Page 11 of 19
ect
m
j
jr
ii
jm
j AfCj
exp1 1
1 )()1(1
Minimum replicas can be calculated from above Eq. for a given expected availability.
Suppose the expected availability for file F is Aexpect, which defined by users. To satisfy the availability requirement for a given file, we get
m
j
jr
ii
jm
jj
fCFAP1 1
1 )()1(1)(
3. Cost-effective Dynamic Replication Management
1
0 !)(
!)(
ii c
k
kii
i
cii
i kcB
Replica placement policy: replica will be placed into data node with lowest blocking probability to dynamically maintain overall load balancing.
Page 12 of 19
Blocking Probability Blocking probability is used as criterion to place replicas among data
nodes to improve load balance . An data node Si is modeled as M/G/ci system with arrival rate λi and
service time τi, and accordingly, the blocking probability of data node Si can be given to be
3. Cost-effective Dynamic Replication Management
Page 13 of 19
3. Cost-effective Dynamic Replication Management
Framework of cost-effective dynamic replication management in HDFS
Client
Data Nodes
Name NodeB1B2…Bm
Calculate the replication factor and Search the Datanode B+Tree to obtain Datanode list.
2
4Flush and replicate blocks to selected Datanodes
Request to create a file with <Availability, Block Number>
1
Return replication policy <Bi, Replication factor, DataNode list>
3
Replication Pipelining
4. Evaluation
Setup Our test platform is built on a cluster with one name node and twenty
data nodes of commodity computer
The operating system is Red Hat AS4.4 with kernel 2.6.20.
Hadoop version is 0.16.1 and java version is 1.6.0.
AUSPEX file system trace is used
A synthesizer is developed to create workloads with different
characteristics, such as data sets of different sizes, varying data rates,
and different popularities. These characteristics reflect the differences
among various workloads to the cloud storage cluster.
Page 14 of 19
4. Evaluation
Cost effective Availability Initially, one replica per object. CDRM only maintain minimal replicas to satisfy availability. Higher failure rate, more replica required.
Page 15 of 19
Dynamic replication with Data node failure rate of 0.1 and 0.2 , Aexpect=0.8
0
1
2
3
4
5
0 5 10 15 20 25 30 35 40
Time(Min)
Rep
lica
Num
ber
Failure Rate=0.2Failure Rate=0.1
4. Evaluation Performance
CDRM vs. HDFS default Replication Management (HDRM) under different popularity and workload intensity.
Performance of CDRM is much better than that of HDRM when popularity is small. CDRM outperform HDRM under heavy workload.
Page 16 of 19
0510152025303540
10 20 30 40 50 60 70 80 90 100
Popularity(%)
Ave
rage
Lat
ency
(ms) HDRM λ=0.6 CDRM λ=0.6
HDRM λ=0.2 CDRM λ=0.2
Effect of popularity and access arrival rate, 20 data nodes
4. Evaluation
Load Balance The figure shows the difference of system utilization of each data node
comparing to the average system utilization of the cluster. CDRM can dynamically distribute workload among whole cluster.
Page 17 of 19
-50
-40
-30
-20
-10
0
10
20
30
40
50
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Data Node#
Diff
. of s
ys. u
tiliz
atio
n
CDRM HDRM
System utilization among data nodes, popularity=10%, λ=0.6
5. Conclusion
Page 18 of 19
Current replication management policies CDRM
Data is same Data is differentStorage node are same Storage nodes are different
Same replica number for all data Different replica number for different data
Static placement Dynamic placementHigh Cost Cost effectivePoor load balance Good balanceLow performance High performance