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https://issues.apache.org/jira/browse/HDFS-4489?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13615749#comment-13615749
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Todd Lipcon commented on HDFS-4489:
-----------------------------------
// The _length field is not declared in C++. It is allocated after the
// declared nonstatic fields in arrayOopDesc if not compressed, otherwise
// it occupies the second half of the _klass field in oopDesc.
static int length_offset_in_bytes() {
return UseCompressedOops ? klass_gap_offset_in_bytes() :
sizeof(arrayOopDesc);
}
Basically if CompressedOops are on, then klassids are only 32-bits, but there's
already a 64-bit field for it, so it just uses the latter 4 bytes for the array
length. Otherwise it's an extra 4 bytes that comes after the standard oop
header (oopDesc). So, without compressed oops, arrays take 20 bytes base. With
them (on by default on heaps <32GB since 6u18 I believe), the array header is
the same size as normal objects (16 bytes).
Will take a look at loading a big image with that patch now.
> Use InodeID as as an identifier of a file in HDFS protocols and APIs
> --------------------------------------------------------------------
>
> Key: HDFS-4489
> URL: https://issues.apache.org/jira/browse/HDFS-4489
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: namenode
> Reporter: Brandon Li
> Assignee: Brandon Li
>
> The benefit of using InodeID to uniquely identify a file can be multiple
> folds. Here are a few of them:
> 1. uniquely identify a file cross rename, related JIRAs include HDFS-4258,
> HDFS-4437.
> 2. modification checks in tools like distcp. Since a file could have been
> replaced or renamed to, the file name and size combination is no t reliable,
> but the combination of file id and size is unique.
> 3. id based protocol support (e.g., NFS)
> 4. to make the pluggable block placement policy use fileid instead of
> filename (HDFS-385).
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