spark-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From kant kodali <kanth...@gmail.com>
Subject Re: Are RDD's ever persisted to disk?
Date Tue, 23 Aug 2016 19:26:01 GMT
ok now that I understand RDD can be stored to the disk. My last question on this
topic would be this.
Storing RDD to disk is nothing but storing JVM byte code to disk (in case of
Java or Scala). am I correct?





On Tue, Aug 23, 2016 12:19 PM, RK Aduri rkaduri@collectivei.com wrote:
On an other note, if you have a streaming app, you checkpoint the RDDs so that
they can be accessed in case of a failure. And yes, RDDs are persisted to DISK.
You can access spark’s UI and see it listed under Storage tab.
If RDDs are persisted in memory, you avoid any disk I/Os so that any lookups
will be cheap. RDDs are reconstructed based on a graph (DAG - available in Spark
UI )
On Aug 23, 2016, at 12:10 PM, < srikanth.jella@gmail.com > < srikanth.jella@gmail.com
> wrote:
RAM or Virtual memory is finite, so data size needs to be considered before
persist. Please see below documentation when to choose the persistency level. 
http://spark.apache.org/docs/latest/programming-guide.html#which-storage-level-to-choose Thanks,
Sreekanth Jella From: kant kodali
Sent: Tuesday, August 23, 2016 2:42 PM
To: srikanth.jella@gmail.com
Cc: user@spark.apache.org
Subject: Re: Are RDD's ever persisted to disk? so when do we ever need to persist RDD on disk?
given that we don't need to
worry about RAM(memory) as virtual memory will just push pages to the disk when
memory becomes scarce.



On Tue, Aug 23, 2016 11:23 AM, srikanth.jella@gmail.com wrote:Hi Kant Kodali,



Based on the input parameter to persist() method either it will be cached on
memory or persisted to disk. In case of failures Spark will reconstruct the RDD
on a different executor based on the DAG. That is how failures are handled.
Spark Core does not replicate the RDDs as they can be reconstructed from the
source (let’s say HDFS, Hive or S3 etc.) but not from memory (which is lost
already).



Thanks,
Sreekanth Jella



From: kant kodali
Sent: Tuesday, August 23, 2016 2:12 PM
To: user@spark.apache.org
Subject: Are RDD's ever persisted to disk?



I am new to spark and I keep hearing that RDD's can be persisted to memory or
disk after each checkpoint. I wonder why RDD's are persisted in memory? In case
of node failure how would you access memory to reconstruct the RDD? persisting
to disk make sense because its like persisting to a Network file system (in case
of HDFS) where a each block will have multiple copies across nodes so if a node
goes down RDD's can still be reconstructed by the reading the required block
from other nodes and recomputing it but my biggest question is Are RDD's ever persisted to
disk?





Collective[i] dramatically improves sales and marketing performance using
technology, applications and a revolutionary network designed to provide next
generation analytics and decision-support directly to business users. Our goal
is to maximize human potential and minimize mistakes. In most cases, the results
are astounding. We cannot, however, stop emails from sometimes being sent to the
wrong person. If you are not the intended recipient, please notify us by
replying to this email's sender and deleting it (and any attachments)
permanently from your system. If you are, please respect the confidentiality of
this communication's contents.
Mime
View raw message