Dave Marion created HDFS-16864:
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Summary: HDFS advisory caching should drop cache behind block when
block closed
Key: HDFS-16864
URL: https://issues.apache.org/jira/browse/HDFS-16864
Project: Hadoop HDFS
Issue Type: Improvement
Components: hdfs
Affects Versions: 3.3.4
Reporter: Dave Marion
One of the comments in HDFS-4817 describes the behavior in
BlockReceiver.manageWriterOsCache:
"The general idea is that there isn't much point in calling {{sync_file_pages}}
twice on the same offsets, since the sync process has presumably already begun.
On the other hand, calling {{fadvise(FADV_DONTNEED)}} again and again will tend
to purge more and more bytes from the cache. The reason is because dirty pages
(those containing un-written-out-data) cannot be purged using
{{{}FADV_DONTNEED{}}}. And we can't know exactly when the pages we wrote will
be flushed to disk. But we do know that calling {{FADV_DONTNEED}} on very
recently written bytes is a waste, since they will almost certainly not have
been written out to disk. That is why it purges between 0 and
{{{}lastCacheManagementOffset - CACHE_WINDOW_SIZE{}}}, rather than simply 0 to
pos."
Looking at the code, I'm wondering if at least the last 8MB (size of
CACHE_WINDOW_SIZE) of a block might be left without an associated
FADVISE_DONT_NEED call. We're having a
[discussion|https://the-asf.slack.com/archives/CERNB8NDC/p1669399302264189] in
#accumulo about the file caching feature and I found some interesting
[results|https://gist.github.com/dlmarion/1835f387b0fa8fb9dbf849a0c87b6d04] in
a test that we wrote. Specifically, that for a multi-block file using
setDropBehind with either hsync or CreateFlag.SYNC_BLOCK, parts of each block
remained in the cache instead of parts of the last block.
I'm wondering if there is a reason not to call fadvise(FADV_DONTNEED) on the
entire block in close
[here|https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/BlockReceiver.java#L371]
when dropCacheBehindWrites is true.
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