Efficient usage of local drives in the cloud for big data processing

Cloud sounds like a perfect platform for the big data processing – you get as much processing power when you need it and release when you don’t. But why does a lot of big data processing happen outside of cloud? Lets try to find out:

The question came from following dilemma in big data processing in cloud :
Store data in S3 and process in EC2. It is elastic and economical per GB, you can resize you cluster as you wish, but you are limited by S3 bandwidth. EMR against S3 is popular example of this approach.

Or, you can also build HDFS or other distributed storage on top of local (ephemeral) drives. There appears to be a clear tradeoff: good bandwidth is available, but storage is going to be expensive and elasticity will suffer, because you can not remove nodes when their processing power is not needed. Redshift or hadoop with HDFS on local drives are the perfect examples of this approach.

Both solutions have drawbacks. Lets take a closer look.
Cloud storage, like s3, is built to store a lot of data in cheap and reliable way. Circa $30 per TB per month. It is also highly reliable: SLA with a lot of nines…
Local drives should be fast. Today it means SSD. This technology provides very good performance but price per GB is high.

The cost of HDD space is 20-25 times lower than on SSD. In Amazon cloud difference in cost of local drive space vs s3 space is even higher. For example 1TB of storage on c3.8xlarge instances costs aroud $2K per month. It is x60 (sixty times!!!) more expensive than to store data in s3.

What about throughput? The difference between access to local data and data on S3 is around 5 times. Moreover, bandwidth to S3 can be throttled by amazon, depending on the current load and other factors. As opposed to always reliable access to local drives.

There is possible counter-argument, that we do not need this storage bandwidth. Assuming that we process data in a speed matching the storage bandwidth – we do not need more of it. S3 can give us 50-100 MB/sec of data for big instance, like c3.8xlarge. If we process data using MapReduce or Hive – it is close to processing speed assuming MR processing to be about 5MB/Sec per core.
In case of more efficient engines – like Redshift or impala – the speed is about 100MB/sec per core or more…
So, we need faster access. To prove this point, you can pay attention that RedShift nodes has 2.4 GB/sec of disk IO. I can trust that AWS engineers know what they are doing.

Now lets recall that usually big data is a huge pile of cheap data. By cheap I mean – low value per GB. Should data be expensive (like financial transactions) it could happily live in Oracle + enterprise storage.

So, how do we utilize our resources more efficiently? On one hand we have a lot of slow and inexpensive storage, and on the other a bit of fast and expensive. The obvious solution is to use fast storage as cache. These days it is rather common: DRAM memory holds disk cache, SRAM memory inside CPU used as cache for DRAM.

In the above situation I suggest to use local SSD drives as a cache for cloud storage (s3).
So, what does it mean? Effective cache should meet the following requirements:
Store hot data set. It is main duty of the cache. Usual heuristics is LRU – last recently used. We assume that data recently used has good chance to be used again.

Prefetch: Predict what data will be needed and load it ahead of time. In case of disk cache – it’s read ahead (if we read first block of the file there’s a good chance we will need the next). In case of CPU – there are very advanced algorithms for pre-fetch. In case of usual data warehouse we can assume that recently added data has better chance to be of interest than old one…

To summarize: I believe that to be able to efficiently process big data in the cloud we need to use local drives as a cache of the dataset stored in the cloud storage. I also believe that other ways will not be efficient, as long as cloud storage is much slower and cheaper than local drives.

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