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From "Alex Ivanov (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-9841) FileDiff's skipped by hdfs snapshotDiff
Date Sun, 21 Feb 2016 23:14:18 GMT
Alex Ivanov created HDFS-9841:
---------------------------------

             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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