Thanks Ted for clarifying. So the sync is to just flush the current buffers to datanode and persist the block info in namenode once per block, isn't it?
Regarding reader able to see the unflushed data, I faced an issue in the following scneario: 1. a writer is writing a 10MB file(block size 2 MB) 2. wrote the file upto 4MB (2 finalized blocks in current and nothing in blocksBeingWritten directory in DN) . So 2 blocks are written 3. client calls addBlock for the 3rd block on namenode and not yet created outputstream to DN(or written anything to DN). At this point of time, the namenode knows about the 3rd block but the datanode doesn't. 4. at point 3, a reader is trying to read the file and he is getting exception and not able to read the file as the datanode's getBlockInfo returns null to the client(of course DN doesn't know about the 3rd block yet) In this situation the reader cannot see the file. But when the block writing is in progress , the read is successful. Is this a bug that needs to be handled in append branch? >> -----Original Message----- >> From: Konstantin Boudnik [mailto:c...@boudnik.org] >> Sent: Friday, February 11, 2011 4:09 AM >>To: common-u...@hadoop.apache.org >> Subject: Re: hadoop 0.20 append - some clarifications >> You might also want to check append design doc published at HDFS-265 I was asking about the hadoop 0.20 append branch. I suppose HDFS-265's design doc won't apply to it. _____ From: Ted Dunning [mailto:tdunn...@maprtech.com] Sent: Thursday, February 10, 2011 9:29 PM To: common-u...@hadoop.apache.org; gok...@huawei.com Cc: hdfs-user@hadoop.apache.org Subject: Re: hadoop 0.20 append - some clarifications Correct is a strong word here. There is actually an HDFS unit test that checks to see if partially written and unflushed data is visible. The basic rule of thumb is that you need to synchronize readers and writers outside of HDFS. There is no guarantee that data is visible or invisible after writing, but there is a guarantee that it will become visible after sync or close. On Thu, Feb 10, 2011 at 7:11 AM, Gokulakannan M <gok...@huawei.com> wrote: Is this the correct behavior or my understanding is wrong?
