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https://issues.apache.org/jira/browse/HDDS-10411?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17855134#comment-17855134
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Wei-Chiu Chuang commented on HDDS-10411:
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This one doesn't look to be such a big deal now that the chunk size is reduced
from 4MB to 16KB. Let's put it in the backlog.
> Support incremental ChunkBuffer checksum calculation
> ----------------------------------------------------
>
> Key: HDDS-10411
> URL: https://issues.apache.org/jira/browse/HDDS-10411
> Project: Apache Ozone
> Issue Type: Sub-task
> Reporter: Siyao Meng
> Assignee: Siyao Meng
> Priority: Major
>
> h2. Goal
> Calculate ChunkBuffer (ByteBuffer) checksum incrementally rather than having
> to calculating it from scratch every single time in {{writeChunkToContainer}}.
> h2. Background
> Currently ChunkBuffer (ByteBuffer) checksum is always calculated from
> scratch. As can be seen here in checksum function initialization, which it
> always calls {{reset()}} before feeding any data with {{update()}}:
> {code:title=newChecksumByteBufferFunction:
> https://github.com/apache/ozone/blob/5f0925e190f1dbf2d4617daad8ad42401d5079e1/hadoop-hdds/common/src/main/java/org/apache/hadoop/ozone/common/Checksum.java#L67-L68}
> private static Function<ByteBuffer, ByteString>
> newChecksumByteBufferFunction(
> Supplier<ChecksumByteBuffer> constructor) {
> final ChecksumByteBuffer algorithm = constructor.get();
> return data -> {
> algorithm.reset();
> algorithm.update(data);
> return int2ByteString((int)algorithm.getValue());
> };
> }
> {code}
> Each ByteBuffer (4 MB by default) inside a block's ChunkBuffer gets its
> checksum calculated here:
> {code:title=https://github.com/apache/ozone/blob/5f0925e190f1dbf2d4617daad8ad42401d5079e1/hadoop-hdds/common/src/main/java/org/apache/hadoop/ozone/common/Checksum.java#L171-L177}
> // Checksum is computed for each bytesPerChecksum number of bytes of data
> // starting at offset 0. The last checksum might be computed for the
> // remaining data with length less than bytesPerChecksum.
> final List<ByteString> checksumList = new ArrayList<>();
> for (ByteBuffer b : data.iterate(bytesPerChecksum)) {
> checksumList.add(computeChecksum(b, function, bytesPerChecksum));
> }
> {code}
> which is called from
> [{{BlockOutputStream#writeChunkToContainer}}|https://github.com/apache/ozone/blob/f0b75b7e4ee93e89f9e4fc96cb30d59f78746eb5/hadoop-hdds/client/src/main/java/org/apache/hadoop/hdds/scm/storage/BlockOutputStream.java#L697].
> And when the function is applied in the inner {{computeChecksum}}, it always
> calls {{reset()}} first. So it calculates the whole ByteBuffer from offset 0.
> h2. Motivation
> While this may not be a big issue before Ozone {{hsync()}} is implemented (or
> in HDFS, where each chunk is much smaller, at 64 KB by default), it can now
> contribute to ~10% of hsync latency between client-DN if the client is only
> appending a few bytes between hsyncs, as can be seen from [~weichiu]'s flame
> graph.
> Estimated latency improvement is 0%~20% with this change, depending on the
> client write/hsync pattern.
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