Delay second Block Reports until after cluster finishes startup, to improve
startup times
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Key: HDFS-1767
URL: https://issues.apache.org/jira/browse/HDFS-1767
Project: Hadoop HDFS
Issue Type: Improvement
Components: data-node
Affects Versions: 0.22.0
Reporter: Matt Foley
Assignee: Matt Foley
Fix For: 0.23.0
Consider a large cluster that takes 40 minutes to start up. The datanodes
compete to register and send their Initial Block Reports (IBRs) as fast as they
can after startup (subject to a small sub-two-minute random delay, which isn't
relevant to this discussion).
As each datanode succeeds in sending its IBR, it schedules the starting time
for its regular cycle of reports, every hour (or other configured value of
dfs.blockreport.intervalMsec). In order to spread the reports evenly across the
block report interval, each datanode picks a random fraction of that interval,
for the starting point of its regular report cycle. For example, if a
particular datanode ends up randomly selecting 18 minutes after the hour, then
that datanode will send a Block Report at 18 minutes after the hour every hour
as long as it remains up. Other datanodes will start their cycles at other
randomly selected times. This code is in DataNode.blockReport() and
DataNode.scheduleBlockReport().
The "second Block Report" (2BR), is the start of these hourly reports. The
problem is that some of these 2BRs get scheduled sooner rather than later, and
actually occur within the startup period. For example, if the cluster takes 40
minutes (2/3 of an hour) to start up, then out of the datanodes that succeed in
sending their IBRs during the first 10 minutes, between 1/2 and 2/3 of them
will send their 2BR before the 40-minute startup time has completed!
2BRs sent within the startup time actually compete with the remaining IBRs, and
thereby slow down the overall startup process. This can be seen in the
following data, which shows the startup process for a 3700-node cluster that
took about 17 minutes to finish startup:
<code>
time starts sum regs sum IBR sum 2nd_BR sum total_BRs/min
0 1299799498 3042 3042 1969 1969 151 151 0 151
1 1299799558 665 3707 1470 3439 248 399 0 248
2 1299799618 3707 224 3663 270 669 0 270
3 1299799678 3707 14 3677 261 930 3 3 264
4 1299799738 3707 23 3700 288 1218 1 4 289
5 1299799798 3707 7 3707 258 1476 3 7 261
6 1299799858 3707 3707 317 1793 4 11 321
7 1299799918 3707 3707 292 2085 6 17 298
8 1299799978 3707 3707 292 2377 8 25 300
9 1299800038 3707 3707 272 2649 25 272
10 1299800098 3707 3707 280 2929 15 40 295
11 1299800158 3707 3707 223 3152 14 54 237
12 1299800218 3707 3707 143 3295 54 143
13 1299800278 3707 3707 141 3436 20 74 161
14 1299800338 3707 3707 195 3631 78 152 273
15 1299800398 3707 3707 51 3682 209 361 260
16 1299800458 3707 3707 25 3707 369 730 394
17 1299800518 3707 3707 3707 166 896 166
18 1299800578 3707 3707 3707 72 968 72
19 1299800638 3707 3707 3707 67 1035 67
20 1299800698 3707 3707 3707 75 1110 75
21 1299800758 3707 3707 3707 71 1181 71
22 1299800818 3707 3707 3707 67 1248 67
23 1299800878 3707 3707 3707 62 1310 62
24 1299800938 3707 3707 3707 56 1366 56
25 1299800998 3707 3707 3707 60 1426 60
</code>
This data was harvested from the startup logs of all the datanodes, and
correlated into one-minute buckets. Each row of the table represents the
progress during one elapsed minute of clock time. It seems that every cluster
startup is different, but this one showed the effect fairly well.
The "starts" column shows that all the nodes started up within the first 2
minutes, and the "regs" column shows that all succeeded in registering by
minute 6. The IBR column shows a sustained rate of Initial Block Report
processing of 250-300/minute for the first 10 minutes.
The question is why, during minutes 11 through 16, the rate of IBR processing
slowed down. Why didn't the startup just finish? In the "2nd_BR" column, we
see the rate of 2BRs ramping up as more datanodes complete their IBRs. As the
rate increases, they become more effective at competing with the IBRs, and slow
down the IBR processing even more. After the IBRs finally finish in minute 16,
the rate of 2BRs settles down to a steady ~60-70/minute.
In order to decrease competition for locks and other resources, to speed up IBR
processing during startup, we propose to delay 2BRs until later into the cycle.
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