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From Gokulakannan M <gok...@huawei.com>
Subject RE: hadoop 0.20 append - some clarifications
Date Tue, 15 Feb 2011 04:21:18 GMT
I agree that HDFS doesn't strongly follow POSIX semantics. But it would have
been better if this issue is fixed.

 

  _____  

From: Ted Dunning [mailto:tdunning@maprtech.com] 
Sent: Monday, February 14, 2011 10:18 PM
To: gokulm@huawei.com
Cc: common-user@hadoop.apache.org; hdfs-user@hadoop.apache.org;
dhruba@gmail.com
Subject: Re: hadoop 0.20 append - some clarifications

 

HDFS definitely doesn't follow anything like POSIX file semantics.

 

They may be a vague inspiration for what HDFS does, but generally the
behavior of HDFS is not tightly specified.  Even the unit tests have some
real surprising behavior.

On Mon, Feb 14, 2011 at 7:21 AM, Gokulakannan M <gokulm@huawei.com> wrote:

 

>> I think that in general, the behavior of any program reading data from an
HDFS file before hsync or close is called is pretty much undefined.

 

In Unix, users can parallelly read a file when another user is writing a
file. And I suppose the sync feature design is based on that.

So at any point of time during the file write, parallel users should be able
to read the file.

 

https://issues.apache.org/jira/browse/HDFS-142?focusedCommentId=12663958
<https://issues.apache.org/jira/browse/HDFS-142?focusedCommentId=12663958&pa
ge=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-
12663958>
&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comme
nt-12663958

  _____  

From: Ted Dunning [mailto:tdunning@maprtech.com] 
Sent: Friday, February 11, 2011 2:14 PM
To: common-user@hadoop.apache.org; gokulm@huawei.com
Cc: hdfs-user@hadoop.apache.org; dhruba@gmail.com
Subject: Re: hadoop 0.20 append - some clarifications

 

I think that in general, the behavior of any program reading data from an
HDFS file before hsync or close is called is pretty much undefined.

 

If you don't wait until some point were part of the file is defined, you
can't expect any particular behavior.

On Fri, Feb 11, 2011 at 12:31 AM, Gokulakannan M <gokulm@huawei.com> wrote:

I am not concerned about the sync behavior.

The thing is the reader reading non-flushed(non-synced) data from HDFS as
you have explained in previous post.(in hadoop 0.20 append branch)

I identified one specific scenario where the above statement is not holding
true.

Following is how you can reproduce the problem.

1. add debug point at createBlockOutputStream() method in DFSClient and run
your HDFS write client in debug mode

2. allow client to write 1 block to HDFS

3. for the 2nd block, the flow will come to the debug point mentioned in
1(do not execute the createBlockOutputStream() method). hold here.

4. parallely, try to read the file from another client

Now you will get an error saying that file cannot be read.



 _____

From: Ted Dunning [mailto:tdunning@maprtech.com]
Sent: Friday, February 11, 2011 11:04 AM
To: gokulm@huawei.com
Cc: common-user@hadoop.apache.org; hdfs-user@hadoop.apache.org;
cos@boudnik.org
Subject: Re: hadoop 0.20 append - some clarifications



It is a bit confusing.



SequenceFile.Writer#sync isn't really sync.



There is SequenceFile.Writer#syncFs which is more what you might expect to
be sync.



Then there is HADOOP-6313 which specifies hflush and hsync.  Generally, if
you want portable code, you have to reflect a bit to figure out what can be
done.

On Thu, Feb 10, 2011 at 8:38 PM, Gokulakannan M <gokulm@huawei.com> wrote:

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:cos@boudnik.org]
>> Sent: Friday, February 11, 2011 4:09 AM
>>To: common-user@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:tdunning@maprtech.com]
Sent: Thursday, February 10, 2011 9:29 PM
To: common-user@hadoop.apache.org; gokulm@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 <gokulm@huawei.com> wrote:

Is this the correct behavior or my understanding is wrong?





 

 


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