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https://issues.apache.org/jira/browse/HDFS-16147?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17393310#comment-17393310
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Stephen O'Donnell commented on HDFS-16147:
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When the image is saved, there is a single stream written out serially. To
enable parallel load on the image, index entries are added for the sub-sections
as they are written out.
This means we have a single stream, with the position of the sub-sections saved.
That means, when we load the image, there are two choices:
1. We start at the beginning of a section and open a stream and read the entire
section.
2. We open several streams at by jumping to that position and read the given
length.
When you enabled compression too, this means the entire stream is compressed
from end to end as a single compressed stream. I wrongly thought there would be
many compressed streams within the image file, and that is why I though OIV etc
would have trouble reading this.
So it makes sense OIV can read the image serially, and the namenode can also
read the image with parallel disabled when compression is on. The surprise to
me, is that we can load the image in parallel, as that involves jumping into
the compressed stream somewhere in the middle and starting to read, which more
compression codecs do not support. It was my belief that gzip does not support
this.
However, looking at the existing code, before this change, I see that we jump
around in the stream already:
{code}
for (FileSummary.Section s : sections) {
channel.position(s.getOffset());
InputStream in = new BufferedInputStream(new LimitInputStream(fin,
s.getLength()));
in = FSImageUtil.wrapInputStreamForCompression(conf, summary.getCodec(), in);
{code}
So that must mean the compression codecs are splittable somehow, and they are
start decompressing from an random position in the stream. Due to this, if the
image is compressed, the existing parallel code can be mostly reused to load
the sub-sections within the compressed stream.
>From the above, could we allow parallel loading of compressed images by simply
>removing the code:
{code}
- if (loadInParallel) {
- if (compressionEnabled) {
- LOG.warn("Parallel Image loading and saving is not supported when {}" +
- " is set to true. Parallel will be disabled.",
- DFSConfigKeys.DFS_IMAGE_COMPRESS_KEY);
- loadInParallel = false;
- }
- }
{code}
Then let the image save compressed with the sub-sections indexed and try to
load it?
> load fsimage with parallelization and compression
> -------------------------------------------------
>
> Key: HDFS-16147
> URL: https://issues.apache.org/jira/browse/HDFS-16147
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: namanode
> Affects Versions: 3.3.0
> Reporter: liuyongpan
> Priority: Minor
> Attachments: HDFS-16147.001.patch, HDFS-16147.002.patch,
> subsection.svg
>
>
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