Xiao Chen updated HDFS-11009:
    Attachment: HDFS-11009.04.patch

Thanks Andrew for explaining and more reviews.

I see your point now... added more text to make this scarier and more accurate. 
How does it feel now? The 'if' text is also fixed
cmd line output looks like this:
xiao$ bin/hdfs debug computeMeta
computeMeta -block <block-file> -out <output-metadata-file>
  Compute HDFS metadata from the specified block file, and save it to
  the specified output metadata file.

**NOTE: Use at your own risk!
 If the block file is corrupt and you overwrite it's meta file, 
 it will show up as good in HDFS, but you can't read the data.
 Only use as a last measure, and when you are 100% certain the block file is 
The assumption here is that when people is running the {{debug}} command, they 
know what they're doing. (At least need to figure out which block file from 
which HDFS file is of interest, and where that block file locates etc.)

bq. Could we fix the verifyMeta help text while we're here to also remove the 
"[]"? The "-block" flag looks required to me.
Not really, so if you {{verify}} a meta file, it checks it can be read as a 
meta. If you also give a block file, it then verifies checksum matches. :)

> Add a tool to reconstruct block meta file from CLI
> --------------------------------------------------
>                 Key: HDFS-11009
>                 URL: https://issues.apache.org/jira/browse/HDFS-11009
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: datanode
>            Reporter: Xiao Chen
>            Assignee: Xiao Chen
>         Attachments: HDFS-11009.01.patch, HDFS-11009.02.patch, 
> HDFS-11009.03.patch, HDFS-11009.04.patch
> If the block file on local disk presents, but the meta file is missing, it's 
> theoretically possible to manually restore the meta file and have the block 
> readable.
> This jira is to propose to add such a tool to do that.

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