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Tanel Poder

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Linux, Oracle, Hadoop.
Updated: 3 hours 17 min ago

RAM is the new disk – and how to measure its performance – Part 1 – Introduction

Sun, 2015-08-09 17:26

RAM is the new disk, at least in the In-Memory computing world.

No, I am not talking about Flash here, but Random Access Memory – RAM as in SDRAM. I’m by far not the first one to say it. Jim Gray wrote this in 2006: “Tape is dead, disk is tape, flash is disk, RAM locality is king” (presentation)

Also, I’m not going to talk about how RAM is faster than disk (everybody knows that), but in fact how RAM is the slow component of an in-memory processing engine.

I will use Oracle’s In-Memory column store and the hardware performance counters in modern CPUs for drilling down into the low-level hardware performance metrics about CPU efficiency and memory access.

But let’s first get started by looking a few years into past into the old-school disk IO and index based SQL performance bottlenecks :)

Have you ever optimized a SQL statement by adding all the columns it needs into a single index and then letting the database do a fast full scan on the “skinny” index as opposed to a full table scan on the “fat” table? The entire purpose of this optimization was to reduce disk IO and SAN interconnect traffic for your critical query (where the amount of data read would have made index range scans inefficient).

This special-purpose approach would have benefitted your full scan in two ways:

  1. In data warehouses, a fact table may contain hundreds of columns, so an index with “only” 10 columns would be much smaller. Full “table” scanning the entire skinny index would still generate much less IO traffic than the table scan, so it became a viable alternative to wide index range scans and some full table scans (and bitmap indexes with star transformations indeed benefitted from the “skinniness” of these indexes too).
  2. As the 10-column index segment is potentially 50x smaller than the 500-column fact table, it might even fit entirely into buffer cache, should you decide so.

This is all thanks to physically changing the on-disk data structure, to store a copy of only the data I need in one place (column pre-projection?) and store these elements close to each other (locality).

Note that I am not advocating the use of this as a tuning technique here, but just explaining what was sometimes used to make a handful critical queries fast at the expense of the disk space, DML, redo and buffer cache usage overhead of having another index – and why it worked.

Now, why would I worry about this at all in a properly warmed up inmemory database, where the disk IO is not at the critical path of data retrieval at all? Well, now that we have removed the disk IO bottleneck, we inevitably hit the next slowest component as a bottleneck and this is … RAM.

Sequentially scanning RAM is slow. Randomly accessing RAM lines is even slower! Of course this slowness is all relative to the modern CPUs that are capable of processing billions of instructions per core every second.

Back to Oracle’s In-Memory column store example: Despite all the marketing talk about loop vectorization with CPU SIMD processing extensions, the most fundamental change required for “extreme performance” is simply about reducing the data traffic between RAM and CPUs.

This is why I said “SIMD would be useless if you waited on main memory all the time” at the Oracle Database In-Memory in Action presentation at Oracle OpenWorld (Oct 2014):

Oracle In-Memory in Action presentation

The “secret sauce” of Oracle’s in-memory scanning engine is the columnar storage of data, the ability to (de)compress it cheaply and accessing only the filtered columns’ memory first, before even touching any of the other projected columns required by the query. This greatly reduces the slow RAM traffic, just like building that skinny index reduced disk I/O traffic back in the on-disk database days. The SIMD instruction set extensions are just icing on the columnar cake.

So far this is just my opinion, but in the next part I will show you some numbers too!


Related Posts

The Hybrid World is Coming

Mon, 2015-06-29 17:14

Here’s the video of E4 keynote we delivered together with Kerry Osborne a few weeks ago.

It explains what we see is coming, at a high level, from long time Oracle database professionals’ viewpoint and using database terminology (as the E4 audience is all Oracle users like us).

However, this change is not really about Oracle database world, it’s about a much wider shift in enterprise computing: modern Hadoop data lakes and clouds are here to stay. They are already taking over many workloads traditionally executed on in-house RDBMS systems on SAN storage arrays – especially all kinds of reporting and analytics. Oracle is just one of the many vendors affected by all this and they’ve also jumped onto the Hadoop bandwagon.

However, it would be naive to to think that Hadoop would somehow replace all your transactional or ERP systems or existing application code with thousands of complex SQL reports. Many of the traditional systems aren’t going away any time soon.

But the hybrid world is coming. It’s been a very good idea for Oracle DBAs to additionally learn Linux over the last 5-10 years, now is pretty much the right time to start learning Hadoop too. More about this in a future article ;-)

Check out the keynote video here:

Enjoy :-)

Update 1: There’s a recently published good WSJ article which pretty much echoes how we see things happening in the hybrid world.

Update 2: On a more technical note, here’s an article about a Barclays use case in Cloudera blog that illustrates what I meant by pushing your complex (and custom) logic down into Hadoop stack with Spark as opposed to using some separate application server system that pulls in datasets for further processing via SQL.

Old ventures and new adventures

Fri, 2015-06-19 00:08

I have some news, two items actually.

First, today (it’s still 18th June in California) is my blog’s 8th anniversary!

I wrote my first blog post, about Advanced Oracle Troubleshooting, exactly 8 years ago, on 18th June 2007 and have written 229 blog posts since. I had started writing and accumulating my TPT script collection a couple of years earlier and now it has over 1000 files in it! And no, I don’t remember what all of them do and even why I had written them. Also I haven’t yet created an index/documentation for all of them (maybe on the 10th anniversary? ;)

Thanks everyone for your support, reading, commenting and the ideas we’ve exchanged over all these years, it’s been awesome to learn something new every single day!

You may have noticed that I haven’t been too active in online forums nor blogging much in the last couple of years, which brings me to the second news item(s):

I’ve been heavily focusing on Hadoop. It is the future. It will win, for the same reasons Linux won. I moved to US over a year ago and am currently in San Francisco. The big data hype is the biggest here. Except it’s not hype anymore; and Hadoop is getting enterprise-ready.

I am working on a new startup. I am the CEO who still occasionally troubleshoots stuff (must learn something new every day!). We officially incorporated some months ago, but our first developers in Dallas and London have been busy in the background for over a year. By now we are beta testing with our most progressive customers ;-) We are going to be close partners with old and new friends in modern data management space and especially the awesome folks in Accenture Enkitec Group.

The name is Gluent. We glue together the old and new worlds in enterprise IT. Relational databases vs. Hadoop. Legacy ETL vs. Spark. SAN storage vs. the cloud. Jungles of data feeds vs. a data lake. I’m not going to tell you any more as we are still in stealth mode ;-)

Now, where does this leave Oracle technology? Well, I think it still kicks ass and it ain’t going away! In fact we are betting on it. Hadoop is here to stay, but your existing systems aren’t going away any time soon.

I wouldn’t want to run my critical ERP or complex transactional systems on anything other than Oracle. Want real time in-memory reporting on your existing Oracle OLTP system – with immediate consistency, not a multi-second lag: Oracle. Oracle is the king of complex OLTP and I don’t see it changing soon.

So, thanks for reading all the way to the end – and expect to hear much more about Gluent in the future! You can follow @GluentInc Twitter handle to be the first to hear any further news :-)