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From "Walter Su (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-9040) Erasure coding: Refactor DFSStripedOutputStream (Move Namenode RPC Requests to Coordinator)
Date Tue, 15 Sep 2015 01:21:46 GMT

    [ https://issues.apache.org/jira/browse/HDFS-9040?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14744645#comment-14744645
] 

Walter Su commented on HDFS-9040:
---------------------------------

bq. Yeah, I can understand your concern. In the replication mechanism, the async implementation
matches the single write pipeline model, and the datastreamer can handle its failure perfectly.
But with 9 streamers in parallel, we need to 1) sync all the streamers when writing a new
block, and 2) stop all the streamers and assign them with new GS when failure happens. Thus
I think we'd better add some sync code in DFSStripedOutputStream. Also in this way it becomes
easier to calculate block length and set/reset external error state.
yeah. streamer synchronization is too slow. It can't be slower to do it in DFSStripedOutputStream.
I'll take some time to review the patch.

bq. With BlockGroupDataStreamer I can make 9 internal streamers to wait for error-handling
to be finished, until then I put empty_last_packet to all 9 internal streamers to let them
close blockStreams.
bq. I actually did similar thing: closeImpl() first let all the streamers to flush out all
the data packets, then call checkStreamerFailures to handle any failure during the data transfer,
and in the end to send out the last empty packet to close the packet. But the challenge here
is, we could not use the same way to handle the failure for the last empty packet, since successful
streamers may have closed the block already.
closeImpl() did well in handling last paritial blockGroup. What if the failure happens in
last stripe of full blockGroup? The first # streamers ends but one of the last streamers fails.
{noformat}
writeChunk(..) --> super.writeChunk(..) --> enqueueCurrentPacketFull() --> endBlock()
--> send empty_last_packet
{noformat}


> Erasure coding: Refactor DFSStripedOutputStream (Move Namenode RPC Requests to Coordinator)
> -------------------------------------------------------------------------------------------
>
>                 Key: HDFS-9040
>                 URL: https://issues.apache.org/jira/browse/HDFS-9040
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>            Reporter: Walter Su
>         Attachments: HDFS-9040-HDFS-7285.002.patch, HDFS-9040.00.patch, HDFS-9040.001.wip.patch,
HDFS-9040.02.bgstreamer.patch
>
>
> The general idea is to simplify error handling logic.
> Proposal 1:
> A BlockGroupDataStreamer to communicate with NN to allocate/update block, and StripedDataStreamer
s only have to stream blocks to DNs.
> Proposal 2:
> See below the [comment|https://issues.apache.org/jira/browse/HDFS-9040?focusedCommentId=14741388&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14741388]
from [~jingzhao].



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