Robert Evans 2012-05-11, 14:29
That makes perfect sense to me. Especially because it really is a new implementation of shuffle that is optimized for very large jobs. I am happy to see anything go in that is going to improve the performance of hadoop, and I look forward to running some benchmarks on the changes. I am not super familiar with sailfish, but from what I remember from a while ago it is the modified version of KFS that is in reality doing the sorting. The maps will output data to "chunks" aka blocks that when each chunk is full it is sorted. When the sorting is finished for a chunk the reducers are now free to pull the sorted data from the chunks and run. I have a few concerns with it though.
1. How do we securely handle different comparators? Currently comparators run as the user that launched the job, not as a privileged user. Sailfish seems to require that comparators run as a privileged user, or we only support pure bitwise sorting of keys.
2. How does this work in a mixed environment? Sailfish, as I understand it, is optimized for large map/reduce jobs, and can be slower on small jobs than the current implementation. How do we make it so that large jobs are able to run faster, but not negatively impact the more common small jobs? We could run both in parallel and switch between them depending on the size of the job's input, or a config key of some sort, but then the RAM needed to make these big jobs run fast would not be available for smaller jobs to use when no really big job is running.
On 5/11/12 1:32 AM, "Todd Lipcon" <[EMAIL PROTECTED]> wrote:
We discussed this before, but for the benefit of the wider audience: :)
It seems like the requirements imposed on KFS by Sailfish are in most
ways much simplier than the requirements of a full distributed
filesystem. The one thing we need is atomic record append -- but we
don't need anything else, like filesystem metadata/naming,
replication, corrupt data scanning, etc. All of the data is
transient/short-lived and at replication count 1.
So I think building something specific to this use case would be
pretty practical - and my guess is it might even have some benefits
over trying to use a full DFS.
In the MR2 architecture, I'd probably try to build this as a service
plugin in the NodeManager (similar to the way that the ShuffleHandler
in the current implementation works)
On Thu, May 10, 2012 at 11:01 PM, Sriram Rao <[EMAIL PROTECTED]> wrote:
> Sailfish is builds upon record append (a feature not present in HDFS).
> The software that is currently released is based on Hadoop-0.20.2. You use
> the Sailfish version of Hadoop-0.20.2, KFS for the intermediate data, and
> then HDFS (or KFS) for storing the job/input. Since the changes are all in
> the handling of map output/reduce input, it is transparent to existing jobs.
> What is being proposed below is to bolt all the starting/stopping of the
> related deamons into YARN as a first step. There are other approaches that
> are possible, which have a similar effect.
> Hope this helps.
> On Thu, May 10, 2012 at 10:50 PM, M. C. Srivas <[EMAIL PROTECTED]> wrote:
>> Sriram, Sailfish depends on append. I just noticed the HDFS disabled
>> append. How does one use this with Hadoop?
>> On Wed, May 9, 2012 at 9:00 AM, Otis Gospodnetic <
>> [EMAIL PROTECTED]
>> > wrote:
>> > Hi Sriram,
>> > >> The I-file concept could possibly be implemented here in a fairly self
>> > contained way. One
>> > >> could even colocate/embed a KFS filesystem with such an alternate
>> > >> shuffle, like how MR task temporary space is usually colocated with
>> > >> HDFS storage.
>> > > Exactly.
>> > >> Does this seem reasonable in any way?
>> > > Great. Where do go from here? How do we get a colloborative effort
>> > going?
>> > Sounds like a JIRA issue should be opened, the approach briefly
Software Engineer, Cloudera