Month: July 2017

SSD – How much is too much?

Speed, SSD, fast, SAN, NAS
Image courtesy of pixabay

I wrote an article just a few days ago entitled “Where did the 15k disk drive go?”  It was a short piece, quickly done and meant to draw fairly obvious conclusions.  When given a choice between faster and fastest, for the same or close money, people will always choose fastest.  Little did I suspect the sheer amount of comments and emails that I would get from that article.  It appears that everyone has an opinion on storage technology and how storage vendors build out their appliances.  So, in the spirit of keeping the discussion going, I’ve decided to ask the flip side of most of the comment and email subjects.  “If 15k drives are dead, then how much SSD is too much?”  Let the games begin!

How much SSD?

I heard a Texan once say “How much is too much?  Well, if it’s money in the bank or cows on the ranch, you can never have too much!”  He was talking about things that directly affected his performance as a cattleman and his ability to perform his job or company function.  The same can be said for SSD disk in the ever-changing storage arena of business.  How much is too much SSD in a storage array or on a server?  I’m not talking about the sheer amount of physical space – that depends on the applications and data depositories that the application will require.  Plus a little bit for growth.  What I am talking about is a percentage.  Of 100% of the storage space on a given server or storage appliance, just how what percentage should be SSD – fast but expensive?

In my opinion, much will depend on a storage study.  How many IOPs does your environment need so that storage is not the bottleneck in your environment?  Is there too much latency in your SAN or NAS?  If you don’t know the answers to these questions, then a storage study should be your next step.  Check out my article here.  SSD tends to be the most expensive option in GB/$, but that ratio is coming down as manufacturing processes change and get more efficient.  But we all work in the here-and-now, so as of today, how much SSD is too much in your SAN, NAS, or hyperconverged appliance?

All Flash, or no Flash?

I have seen several examples of SSD ratios, all aided by software in one form or another.  These fall into two camps at either end of the spectrum.

To start, there is the storage appliances with no SSD.  These are fairly simple, and I don’t see them around much. If all you need is an array of large disks spinning merrily along, and your storage goals are met, do you really need SSD?  I have been in proof-of-concept trials where SSD would not make any difference is system performance, until the programmers changed the application code to make it more parallel.

Then there is the “all flash all-the-time” argument.  I am familiar with one storage array vendor that sells an all flash array with compression and de-duplication and claims that across the environment, the cost per used Gigabyte is cheaper than their hybrid array (which does not offer any compression type functionality).  Of course with de-duplication your milage may vary, but that makes a compelling argument for all flash.  There are certain industries where milliseconds matter, like stock market trading, or transaction processing.  Those industries go all flash without a second thought.

The middle ground?

So now we reach the middle ground, where the argument get heated.  Hybrid arrays replace the fastest tier of storage with SSD, or use large amounts of SSD as caches to buffer regular hard drives.  Manufacturers use SSD to take the place of those good ol’ 15K drives, as well as some of the 10k drives, too.  The larger and slower SATA drives remain the workhorses for storage.  Older, slower data goes there to die.  Or at least drive your backup software crazy.

So, where does all this leave us?  Should we go ahead and use all flash since it is the wave of the future?  Since I will be replacing my array as I outgrow it, should I buy affordable now, and look to put in all-flash when it is the standard?  Assuming that I am not a government agency with black-budget trillions to spend, how much SSD is too much SSD?  Looking forward to your comments.

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Where did the 15K disk drive go?

15k disk
Image courtesy of Suriya Kankliang at

Just a few years ago, everyone wanted disk drives that spun at 15,000 rpm, commonly known as “15k disk”.  Why did people want these?  Well, the faster the spindle turned, the shorter the seek times, the less latency and the faster the writes to that disk.  Since I never worked at any of the drive manufacturers, I can’t really speak to the truth of this, but I do take it on faith.  So when everything on a storage array was spinning disk, why did people want “15k spindles” in the line up?  And since SSD has become so popular, why don’t I really see them anymore?

Why do I want expensive, small disks?

The reason that everyone wanted 15k disk drives was pretty straightforward.  The disk themselves were fairly small in capacity (600GB being a standard size) and expensive on a GB/$ ratio.  But they were FAST.  If there was a target IOPs for a storage array, it was easier to balance out size and speed with a ratio of 15k disk, 10k disk, and standard 7.2k SATA drives.  Speed from the smaller drives and space from the slower drives.  While everything was acceptable ON AVERAGE, the laws of physics still applied to the different speeds of disk. There was a bit of balance that had to happen. You could put your fast access volumes on 15k, but you still needed the SATA drives for the larger storage requirements.  This solution worked, but was expensive – and a bit “hands-on”.

There were even a few manufacturers that started to offer ILM with these systems.  This means that “hot” or active data writes to the 15k disk drives since theoretically the write speed on these is fastest.  Your storage appliance now writes more across the aggregate of your SAN environment.  Once this data is written to the fastest disk on your SAN or NAS, it stays there for a bit.  This logic being that it also has the fastest read times and therefore the best performance when you wanted to recall that data.  These ILM vendors then move the data off of the fastest tier of disk to a slower tier as that data becomes less active or “ages”.  This allowed you to store older, less accessed data on the slower and less expensive tiers of storage.  Because the database has to run quickly, but who cares if it takes accounting a week to get the data for their year-end reporting, right?  Remember that the next time you need that expense report reimbursed!

The next step

Then SSD entered the market.  At an affordable price, that is.  Not only could manufacturers use SSD as caching, but they were large enough that manufacturers could also use them as the fastest tier of data storage in an ILM strategy.  And the form factor of SSD disks allows them to be used in the existing storage appliance enclosures – JUST LIKE spinning disk.  Now, instead of expensive 15k disks, you could put in units in the same form factor that would read and write several hundred times faster than disk.  With the speed and storage capability of SSD, it became unnecessary to use 15K disk in storage appliances for speed.

But I still see some 15k disk out there…

You will still see 15k disk used in local solutions.  A 15K SAS disk RAID 5 array is quite speedy when used in a local physical server.  Virtualization solutions, or database servers will often use 15K spindles for disk targets.  They need sizable storage capacities and quick access.  However, the cost of SSD is coming down.  This allows the justification for installation of SSD disk or arrays in physical servers.  Seagate has stopped development of new models for their 15k disk.  Previously storage technology leapt from Tape to HDD for large data storage like disaster recovery.  Now storage acceptance from high speed disk to SSD will likely accelerate.  Technology to increase access speed, reduce manufacturer costs, and increase storage capacity will accelerate this change.  So long 15k disk, we hardly knew ya!

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