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https://issues.apache.org/jira/browse/HDFS-8182?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14511936#comment-14511936
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Gera Shegalov commented on HDFS-8182:
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Thanks for comments [~andrew.wang] and [~jlowe]
bq. We might get some hotspotting, since the first reader on a rack will
localize everything. Probably still random enough though?
I agree there is a hotspotting risk, hopefully indeed low. But if turns out to
be big deal we can introduce random node picking in the rack.
Agree that we need to clarify flag names and their semantics (RE: DONTNEED)
And the issue of quota/accounting is something I haven't thought of yet either.
Thanks for bringing it up.
> Implement topology-aware CDN-style caching
> ------------------------------------------
>
> Key: HDFS-8182
> URL: https://issues.apache.org/jira/browse/HDFS-8182
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: hdfs-client, namenode
> Affects Versions: 2.6.0
> Reporter: Gera Shegalov
>
> To scale reads of hot blocks in large clusters, it would be beneficial if we
> could read a block across the ToR switches only once. Example scenarios are
> localization of binaries, MR distributed cache files for map-side joins and
> similar. There are multiple layers where this could be implemented (YARN
> service or individual apps such as MR) but I believe it is best done in HDFS
> or even common FileSystem to support as many use cases as possible.
> The life cycle could look like this e.g. for the YARN localization scenario:
> 1. inputStream = fs.open(path, ..., CACHE_IN_RACK)
> 2. instead of reading from a remote DN directly, NN tells the client to read
> via the local DN1 and the DN1 creates a replica of each block.
> When the next localizer on DN2 in the same rack starts it will learn from NN
> about the replica in DN1 and the client will read from DN1 using the
> conventional path.
> When the application ends the AM or NM's can instruct the NN in a fadvise
> DONTNEED style, it can start telling DN's to discard extraneous replica.
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