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https://issues.apache.org/jira/browse/HBASE-16973?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15622248#comment-15622248
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Heng Chen commented on HBASE-16973:
-----------------------------------
Notice that HBase-11544 has been only applied on branch-1.1.x, So the default
value for branch-1.2.x is still 100? Did our default value has some
compatibility rules (If not, should we have it)? It confused our users. And in
this case, i think we should keep the default value to be small as [~carp84]
mentioned, and respect all configurations about scanner.
> Revisiting default value for hbase.client.scanner.caching
> ---------------------------------------------------------
>
> Key: HBASE-16973
> URL: https://issues.apache.org/jira/browse/HBASE-16973
> Project: HBase
> Issue Type: Bug
> Reporter: Yu Li
> Assignee: Yu Li
> Attachments: Scan.next_p999.png
>
>
> We are observing below logs for a long-running scan:
> {noformat}
> 2016-10-30 08:51:41,692 WARN
> [B.defaultRpcServer.handler=50,queue=12,port=16020] ipc.RpcServer:
> (responseTooSlow-LongProcessTime): {"processingtimems":24329,
> "call":"Scan(org.apache.hadoop.hbase.protobuf.generated.ClientProtos$ScanRequest)",
> "client":"11.251.157.108:50415","scandetails":"table: ae_product_image
> region: ae_product_image,494:
> ,1476872321454.33171a04a683c4404717c43ea4eb8978.","param":"scanner_id:
> 5333521 number_of_rows: 2147483647
> close_scanner: false next_call_seq: 8 client_handles_partials: true
> client_handles_heartbeats: true",
> "starttimems":1477788677363,"queuetimems":0,"class":"HRegionServer","responsesize":818,"method":"Scan"}
> {noformat}
> From which we found the "number_of_rows" is as big as {{Integer.MAX_VALUE}}
> And we also observed a long filter list on the customized scan. After
> checking application code we confirmed that there's no {{Scan.setCaching}} or
> {{hbase.client.scanner.caching}} setting on client side, so it turns out
> using the default value the caching for Scan will be Integer.MAX_VALUE, which
> is really a big surprise.
> After checking code and commit history, I found it's HBASE-11544 which
> changes {{HConstants.DEFAULT_HBASE_CLIENT_SCANNER_CACHING}} from 100 to
> Integer.MAX_VALUE, and from the release note there I could see below notation:
> {noformat}
> Scan caching default has been changed to Integer.Max_Value
> This value works together with the new maxResultSize value from HBASE-12976
> (defaults to 2MB)
> Results returned from server on basis of size rather than number of rows
> Provides better use of network since row size varies amongst tables
> {noformat}
> And I'm afraid this lacks of consideration of the case of scan with filters,
> which may involve many rows but only return with a small result.
> What's more, we still have below comment/code in {{Scan.java}}
> {code}
> /*
> * -1 means no caching
> */
> private int caching = -1;
> {code}
> But actually the implementation does not follow (instead of no caching, we
> are caching {{Integer.MAX_VALUE}}...).
> So here I'd like to bring up two points:
> 1. Change back the default value of
> HConstants.DEFAULT_HBASE_CLIENT_SCANNER_CACHING to some small value like 128
> 2. Reenforce the semantic of "no caching"
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