Getting exception _t.si (No such file or directory) [LUCENE-6883]

We are getting following exception when we are trying to update cache. Following are two scenario when we get this error

scenario 1:

2015-11-03 06:45:18,213 [main] ERROR java.io.FileNotFoundException: /app/cache/index-persecurity/PERSECURITY_INDEX-QCH/_mb.si (No such file or directory)
at java.io.RandomAccessFile.open(Native Method)
at java.io.RandomAccessFile.<init>(RandomAccessFile.java:241)
at org.apache.lucene.store.MMapDirectory.openInput(MMapDirectory.java:193)
at org.apache.lucene.codecs.lucene40.Lucene40SegmentInfoReader.read(Lucene40SegmentInfoReader.java:50)
at org.apache.lucene.index.SegmentInfos.read(SegmentInfos.java:301)
at org.apache.lucene.index.StandardDirectoryReader$1.doBody(StandardDirectoryReader.java:56)
at org.apache.lucene.index.SegmentInfos$FindSegmentsFile.run(SegmentInfos.java:783)
at org.apache.lucene.index.StandardDirectoryReader.open(StandardDirectoryReader.java:52)
at org.apache.lucene.index.DirectoryReader.open(DirectoryReader.java:65)
.....

scenario 2:

java.io.FileNotFoundException: /app/1.0.5_loadtest/index-persecurity/PERSECURITY_INDEX-ITQ/_t.si (No such file or directory)
at java.io.RandomAccessFile.open(Native Method)
at java.io.RandomAccessFile.<init>(RandomAccessFile.java:241)
at org.apache.lucene.store.MMapDirectory.openInput(MMapDirectory.java:193)
at org.apache.lucene.codecs.lucene40.Lucene40SegmentInfoReader.read(Lucene40SegmentInfoReader.java:50)
at org.apache.lucene.index.SegmentInfos.read(SegmentInfos.java:301)
at org.apache.lucene.index.SegmentInfos$1.doBody(SegmentInfos.java:347)
at org.apache.lucene.index.SegmentInfos$FindSegmentsFile.run(SegmentInfos.java:783)
at org.apache.lucene.index.SegmentInfos$FindSegmentsFile.run(SegmentInfos.java:630)
at org.apache.lucene.index.SegmentInfos.read(SegmentInfos.java:343)
at org.apache.lucene.index.StandardDirectoryReader.isCurrent(StandardDirectoryReader.java:326)
at org.apache.lucene.index.StandardDirectoryReader.doOpenNoWriter(StandardDirectoryReader.java:284)
at org.apache.lucene.index.StandardDirectoryReader.doOpenIfChanged(StandardDirectoryReader.java:247)
at org.apache.lucene.index.StandardDirectoryReader.doOpenIfChanged(StandardDirectoryReader.java:235)
at org.apache.lucene.index.DirectoryReader.openIfChanged(DirectoryReader.java:169)
......

What might be the possible reasons for this?

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Migrated from [LUCENE-6883](https://issues.apache.org/jira/browse/LUCENE-6883) by Tejas Jethva, updated Nov 25 2015

Start by tracing the reported cache-update paths through MMapDirectory.openInput, Lucene40SegmentInfoReader.read, SegmentInfos.read, and DirectoryReader.openIfChanged. Reproduce both missing _mb.si and _t.si scenarios and inspect when those segment files disappear or become unavailable. Done means identifying and documenting a reproducible root cause and an actionable fix or mitigation.

Written by the indexing model from the issue text.