Alex Ivanov created HDFS-9841:
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Summary: FileDiff's skipped by hdfs snapshotDiff
Key: HDFS-9841
URL: https://issues.apache.org/jira/browse/HDFS-9841
Project: Hadoop HDFS
Issue Type: Bug
Components: snapshots
Affects Versions: 2.3.0, 2.4.0
Reporter: Alex Ivanov
Summary
When a file in HDFS is read, its corresponding inode's accessTime field is
updated. If the file is present in the last snapshot, the accessTime change
causes a FileDiff to be added to the SnapshotDiff of the last snapshot.
This behavior has the following problems:
- Since FileDiff's reside in memory on the namenodes, snapshots become
progressively more memory-heavy with increasing volume of data in hdfs. On a
system with frequent updates, e.g. hourly, this becomes a big problem since
for, say 2000 snapshots, one can have 2000 FileDiff's per file pointing to the
same inode.
- FSImage grows in size tremendously, and upload operation from standby to
active namenode takes much longer.
-The generated FileDiff does not contain any useful information that I can see.
Since all FileDiff's for that file are pointing to the same inode, the
accessTime they see is the same.-
- I was wrong about the last point. Each FileDiff includes a SnapshotCopy
attribute, which contains the updated accessTime. This may be a feature, but
I'd question the value of having it enabled by default.
Configuration:
CDH 5.0.5 (Hadoop 2.3 / 2.4)
We are NOT overwriting the default parameter:
DFS_NAMENODE_ACCESSTIME_PRECISION_DEFAULT = 3600000;
Note that it determines the allowed frequency of accessTime field updates -
every hour by default.
How to reproduce:
{code}
[root@node1076]# hdfs dfs -ls /data/tenants/testenv.testtenant/wddata
Found 3 items
drwxr-xr-x - hdfs hadoop 0 2015-10-04 10:52
/data/tenants/testenv.testtenant/wddata/folder1
-rw-r--r-- 3 hdfs hadoop 38 2015-10-05 03:13
/data/tenants/testenv.testtenant/wddata/testfile1
-rw-r--r-- 3 hdfs hadoop 21 2015-10-04 10:45
/data/tenants/testenv.testtenant/wddata/testfile2
[root@node1076]# hdfs dfs -ls /data/tenants/testenv.testtenant/wddata/.snapshot
Found 8 items
drwxr-xr-x - hdfs hadoop 0 2015-10-04 10:47
/data/tenants/testenv.testtenant/wddata/.snapshot/sn1
drwxr-xr-x - hdfs hadoop 0 2015-10-04 10:47
/data/tenants/testenv.testtenant/wddata/.snapshot/sn2
drwxr-xr-x - hdfs hadoop 0 2015-10-04 10:52
/data/tenants/testenv.testtenant/wddata/.snapshot/sn3
drwxr-xr-x - hdfs hadoop 0 2015-10-04 10:53
/data/tenants/testenv.testtenant/wddata/.snapshot/sn4
drwxr-xr-x - hdfs hadoop 0 2015-10-04 10:57
/data/tenants/testenv.testtenant/wddata/.snapshot/sn5
drwxr-xr-x - hdfs hadoop 0 2015-10-04 10:58
/data/tenants/testenv.testtenant/wddata/.snapshot/sn6
drwxr-xr-x - hdfs hadoop 0 2015-10-05 03:13
/data/tenants/testenv.testtenant/wddata/.snapshot/sn7
drwxr-xr-x - hdfs hadoop 0 2015-10-05 04:20
/data/tenants/testenv.testtenant/wddata/.snapshot/sn8
[root@node1076]# hdfs dfs -createSnapshot
/data/tenants/testenv.testtenant/wddata sn9
Created snapshot /data/tenants/testenv.testtenant/wddata/.snapshot/sn9
[root@node1076]# hdfs snapshotDiff /data/tenants/testenv.testtenant/wddata sn8
sn9
Difference between snapshot sn8 and snapshot sn9 under directory
/data/tenants/testenv.testtenant/wddata:
################
## IMPORTANT: testfile1 was put into HDFS more than 1 hour ago, which triggers
the accessTime update.
################
[root@node1076]# hdfs dfs -cat /data/tenants/testenv.testtenant/wddata/testfile1
This is test file 1, but now it's 11.
[root@node1076]# hdfs dfs -createSnapshot
/data/tenants/testenv.testtenant/wddata sn10
Created snapshot /data/tenants/testenv.testtenant/wddata/.snapshot/sn10
[root@node1076]# hdfs snapshotDiff /data/tenants/testenv.testtenant/wddata sn9
sn10
Difference between snapshot sn9 and snapshot sn10 under directory
/data/tenants/testenv.testtenant/wddata:
M ./testfile1
{code}
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