During the normal loading NN startup process, fall back on a different image or
EditLog if we see one that is corrupt
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Key: HDFS-3049
URL: https://issues.apache.org/jira/browse/HDFS-3049
Project: Hadoop HDFS
Issue Type: New Feature
Components: name-node
Reporter: Colin Patrick McCabe
Assignee: Colin Patrick McCabe
Priority: Minor
Fix For: 0.24.0
During the NameNode startup process, we load an image, and then apply edit logs
to it until we believe that we have all the latest changes. Unfortunately, if
there is an I/O error while reading any of these files, in most cases, we
simply abort the startup process. We should try harder to locate a readable
edit log and/or image file.
*There are three main use cases for this feature:*
1. If the operating system does not honor fsync (usually due to a
misconfiguration), a file may end up in an inconsistent state.
2. In certain older releases where we did not use fallocate() or similar to
pre-reserve blocks, a disk full condition may cause a truncated log in one edit
directory.
3. There may be a bug in HDFS which results in some of the data directories
receiving corrupt data, but not all. This is the least likely use case.
*Proposed changes to normal NN startup*
* We should try a different FSImage if we can't load the first one we try.
* We should examine other FSEditLogs if we can't load the first one(s) we try.
* We should fail if we can't find EditLogs that would bring us up to what we
believe is the latest transaction ID.
Proposed changes to recovery mode NN startup:
we should list out all the available storage directories and allow the operator
to select which one he wants to use.
Something like this:
{code}
Multiple storage directories found.
1. /foo/bar
edits__curent__XYZ size:213421345 md5:2345345
image size:213421345 md5:2345345
2. /foo/baz
edits__curent__XYZ size:213421345 md5:2345345345
image size:213421345 md5:2345345
Which one would you like to use? (1/2)
{code}
As usual in recovery mode, we want to be flexible about error handling. In
this case, this means that we should NOT fail if we can't find EditLogs that
would bring us up to what we believe is the latest transaction ID.
*Not addressed by this feature*
This feature will not address the case where an attempt to access the NameNode
name directory or directories hangs because of an I/O error. This may happen,
for example, when trying to load an image from a hard-mounted NFS directory,
when the NFS server has gone away. Just as now, the operator will have to
notice this problem and take steps to correct it.
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