Category: Cloud

I have a post which uses Polybase to insert into Azure Blob Storage:

One additional question I have involves whether the process for loading data is round-robin on a row-by-row basis.  My conjecture is that it is not (particularly given that our first example had 4 files with zero records in them!), but I figured I’d create a new table and test.  In this case, I’m using three fixed-width data types and loading 10 million identical records.  I chose to use identical record values to make sure that the text length of the columns in this line were exactly the same; the reason is that we’re taking data out of SQL Server (where an int is stored in a 4-byte block) and converting that int to a string (where each numeric value in the int is stored as a one-byte character).  I chose 10 million because I now that’s well above the cutoff point for data to go into each of the eight files, so if there’s special logic to handle tiny row counts, I’d get past it.

Read on for the exciting(?) conclusion.

I look at an execution plan and packet capture of a Polybase query which reads from Azure Blob Storage:

In this case, all of those packets were 1514 bytes, so it’s an easy multiplication problem to see that we downloaded approximately 113 MB.  The 2008.csv.bz2 file itself is 108 MB, so factoring in TCP packet overhead and that there were additional, smaller packets in the stream, I think that’s enough to show that we did in fact download the entire file.  Just like in the Hadoop scenario without MapReduce, the Polybase engine needs to take all of the data and load it into a temp table (or set of temp tables if you’re using a Polybase scale-out cluster) before it can pull out the relevant rows based on our query.

The upshot is that Polybase behaves very similarly on Azure Blob Storage as it does with on-prem Hadoop for non-MapReduce queries.

I look at using Polybase to read data from Azure Blob Storage:

To this point, I have focused my Polybase series on interactions with on-premises Hadoop, as it’s the use case most apropos to me.  I want to start expanding that out to include other interaction mechanisms, and I’m going to start with one of the easiest:  Azure Blob Storage.

Ayman El-Ghazali has a great blog post the topic, which he turned into a full-length talk.  As such, this post will fill in the gaps rather than start from scratch.  In today’s post, my intention is to retrieve data from Azure Blob Storage and get an idea of what’s happening.  From there, we’ll spend a couple more posts on Azure Blob Storage, looking a bit deeper into the process.  That said, my expectation going into this series is that much of what we do with Azure Blob Storage will mimic what we did with Hadoop, as there are no Polybase core concepts unique to Azure Blob Storage, at least any of which I am aware.

Spoilers:  I’m still not aware of any core concepts unique to Azure Blob Storage.