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From Josh Elser <josh.el...@gmail.com>
Subject Re: Accumulo on s3
Date Tue, 26 Apr 2016 14:59:29 GMT
Shawn -- you win the gold star for the day from me. This is exactly the 
fear I had, but had an inability put it into words correctly :)

Valerio/chutium -- The common scenario I have run into is that 
processing jobs (your use of Spark) can read data from S3 and ingest it 
into the database (Accumulo here, but commonly Hive or HBase for others).

One thought would be that you could use Spark to create Accumulo RFiles, 
store those in s3 and then get load them into Accumulo running not on 
s3. You could use ec2 to run Accumulo instead and bulk load your 
pre-created Accumulo RFiles from s3 (this should be fine, but I haven't 
tried it myself). It isn't quite the same as what you were hoping to get 
via S3, but I think it could be very close (easy to reprovision ec2 and 
re-import all of your current data to a new Accumulo instance).

I don't think S3 provides enough of a "real filesystem" implementation 
to run Accumulo natively over -- Shawn's points really drive the "why" 
home. This isn't something we can "fix Accumulo" to do as it would 
change the entire characteristics of the ssytem. Accumulo needs to be 
able to append data to a file and sync it to make it durable -- 
otherwise, Accumulo *will* eventually lose data. You may not see it in 
trivial testing, but I guarantee you 100% that you will run into data 
loss issues.

Does that make sense?

Shawn Walker wrote:
> RFiles (Accumulo's primary data storage mechanism) are immutable and lazily
> deleted (by the GC process).  Read-after-write consistency for new files
> should be sufficient for them.  I suspect the only real gotchas would be:
> NativeS3FileSystem has a 5G max file size, and NativeS3FileSystem is very
> slow when renaming files.  One might consider using the Hadoop S3 block
> filesystem instead, for better rename performance.
>
> On the other hand, write-ahead logs simply can't function as expected atop
> the NativeS3FileSystem:  A write-ahead log is an incrementally built file,
> and S3 doesn't support a compatible concept of incremental writes to a
> stored object.  Neither hflush()'ing or hsync()'ing a FSDataOutputStream
> sourced from a NativeS3FileSystem actually makes any data available outside
> the process.  The S3 block filesystem improves matters slightly, but not
> enough to
>
> This means that a tablet server death (e.g. caused by a lost Zookeeper
> lock) can (almost certainly will) lead to lost mutations.  It strikes me
> that this would be particularly bad for mutations against the root tablet
> or metadata tablets, and can leave Accumulo in an inconsistent state.
>
> Working around that limitation in Accumulo would likely prove somewhere
> between difficult and impossible.  At the least, it might mean redesigning
> the entire WAL concept.
>
> --
> Shawn Walker
>
> On Tue, Apr 26, 2016 at 5:12 AM, chutium<tengqiu@gmail.com>  wrote:
>
>> Hi Josh,
>>
>> about the guarantees of s3, according to this doc from amazon:
>>
>> https://docs.aws.amazon.com/ElasticMapReduce/latest/ReleaseGuide/emr-plan-consistent-view.html
>>
>>> Amazon S3 buckets in xxx, xxx regions provide read-after-write
>> consistency
>>> for put requests of new objects and eventual consistency for overwrite
>> put
>>> and delete requests.
>> so may be accumulo will get problem with consistency only by major
>> compactions right? it seems no other operation is overwriting or deleting
>> files on HDFS.
>>
>> let me describe our usage of accumulo on s3, basically, we want to combine
>> the unlimited storage feature of s3 and the fine grained access control
>> provided by accumulo.
>>
>> we are using "accumulo on s3" as a secured storage behind data processing
>> engine (spark), data are ingested into accumulo regularly, not in real time
>> (no single put, batch ingestion each X hours), most of data access use
>> cases
>> are batch processing, so no realtime read or write.
>>
>> then consistency or sync will still be a problem or not?
>>
>> I added some thoughts of mine in that stackoverflow thread:
>> http://stackoverflow.com/a/36845743/5630352 ,  I really want to know is
>> this
>> possible to solve the s3 problem for our use case? because it seems until
>> now, no other tools can provide such a powerful access control framework
>> like accumulo.
>>
>> Thanks!
>>
>>
>>
>> --
>> View this message in context:
>> http://apache-accumulo.1065345.n5.nabble.com/Accumulo-on-s3-tp16737p16764.html
>> Sent from the Developers mailing list archive at Nabble.com.
>>
>

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