Playing with NetApp … (Capacity) BR

Much has been said about usable disk storage capacity and unfortunately, many of us take the marketing capacity number given by the manufacturer in verbatim. For example, 1TB does not really equate to 1TB in usable terms and that is something you engineers out there should be informing to the customers.

NetApp, ever since the beginning, has been subjected to the scrutiny of the customers and competitors alike about their usable capacity and I intend to correct this misconception. And the key of this misconception is to understand what is the capacity before rightsizing (BR) and after rightsizing (AR).

(Note: Rightsizing in the NetApp world is well documented and widely accepted with different views. It is part of how WAFL uses the disks but one has to be aware that not many other storage vendors publish their rightsizing process, if any)

Before Rightsizing (BR)

First of all, we have to know that there are 2 systems when it comes to system of unit prefixes. These 2 systems can be easily said as

  • Base-10 (decimal) – fit for human understanding
  • Base-2 (binary) – fit for computer understanding

So according the International Systems of Units, the SI prefixes for Base-10 are

Text Factor Unit
kilo 103 1,000
mega 106 1,000,000
giga 109 1,000,000,000
tera 1012 1,000,000,000,000

In computer context, where the binary, Base-2 system is relevant, that SI prefixes for Base-2 are

Text Factor Unit
kilo-byte 210 1,024
mega-byte 220 1,048,576
giga-byte 230 1,073,741,824
tera-byte 240 1,099,511,627,776

And we must know that the storage capacity is in Base-2 rather than in Base-10. Computers are not humans.

With that in mind, the next issue are the disk manufacturers. We should have an axe to grind with them for misrepresenting the actual capacity. When they say their HDD is 1TB, they are using the Base-10 system i.e. 1TB = 1,000,000,000,000 bytes. THIS IS WRONG!

Let’s see how that 1TB works out to be in Gigabytes in the Base-2 system:

1,000,000,000/1,073,741,824 = 931.3225746154785 Gigabytes

Note: 230 =1,073,741,824

That result of 1TB, when rounded, is only about 931GB! So, the disk manufacturers aren’t exactly giving you what they have advertised.

Thirdly, and also the most important factor in the BR (Before Rightsizing) phase is how WAFL handles the actual capacity before the disk is produced to WAFL/ONTAP operations. Note that this is all done before all the logical structures of aggregates, volumes and LUNs are created.

In this third point, WAFL formats the actual disks (just like NTFS formats new disks) and this reduces the usable capacity even further. As a starting point, WAFL uses 4K (4,096 bytes) per block

For Fibre Channel disks, WAFL formats them with a 520 byte per sector. Therefore, for each block, 8 sectors (520 x 8 = 4160 bytes) fill 1 x 4K block, with remainder of 64 bytes (4,160 – 4,096 = 64 bytes) for the checksum of the 1 x 4K block. This additional 64 bytes per block checksum is not displayed by WAFL or ONTAP and not accounted for in its usable capacity.

512 bytes per sector are used for formatting SATA/SAS disks and it consumes 9 sectors (9 x 512 = 4,608 bytes). 8 sectors will be used for WAFL’s 4K per block (4,096/512 = 8 sectors), the remainder of 1 sector (the 9th sector) of 512 bytes is used partially for its 64 bytes checksum. Again, this 448 bytes (512 – 64 = 448 bytes) is not displayed and not part of the usable capacity of WAFL and ONTAP.

And WAFL also compensates for the ever-so-slightly irregularities of the hard disk drives even though they are labelled with similar marketing capacities. That is to say that 1TB from Seagate and 1TB from Hitachi will be different in terms actual capacity. In fact, 1TB Seagate HDD with firmware 1.0a (for ease of clarification) and 1TB Seagate HDD with firmware 1.0b (note ‘a’ and ‘b’) could be different in actual capacity even when both are shipped with a 1.0TB marketing capacity label.

So, with all these things in mind, WAFL does what it needs to do – Right Size – to ensure that nothing get screwed up when WAFL uses the HDDs in its aggregates and volumes. All for the right reason – Data Integrity – but often criticized for their “wrongdoing”. Think of WAFL as your vigilante superhero, wanted by the law for doing good for the people.

In the end, what you are likely to get Before Rightsizing (BR) from NetApp for each particular disk capacity would be:

Manufacturer Marketing Capacity NetApp Rightsized Capacity Percentage Difference
36GB 34.0/34.5GB* 5%
72GB 68GB 5.55%
144GB 136GB 5.55%
300GB 272GB 9.33%
600GB 560GB 6.66%
1TB 847GB 11.3%
2TB 1.69TB 15.5%
3TB 2.48TB 17.3%

* The size of 34.5GB was for the Fibre Channel Zone Checksum mechanism employed prior to ONTAP version 6.5 of 512 bytes per sector. After ONTAP 6.5, block checksum of 520 bytes per sector was employed for greater data integrity protection and resiliency.

From the table, the percentage of “lost” capacity is shown and to the uninformed, this could look significant. But since the percentage value is relative to the Manufacturer’s Marketing Capacity, this is highly inaccurate. Therefore, competitors should not use these figures as FUD and NetApp should use these as a way to properly inform their customers.

You have been informed about NetApp capacity before Right Sizing.

I will follow on another day with what happens next after Right Sizing and the final actual usable capacity to the users and operations. This will be called After Rightsizing (AR). Till then, I am going out for an appointment.

About cfheoh

I am a technology blogger with 20+ years of IT experience. I write heavily on technologies related to storage networking and data management because that is my area of interest and expertise. I introduce technologies with the objectives to get readers to *know the facts*, and use that knowledge to cut through the marketing hypes, FUD (fear, uncertainty and doubt) and other fancy stuff. Only then, there will be progress. I am involved in SNIA (Storage Networking Industry Association) and as of October 2013, I have been appointed as SNIA South Asia & SNIA Malaysia non-voting representation to SNIA Technical Council. I was previously the Chairman of SNIA Malaysia until Dec 2012. I have recently joined Hitachi Data Systems as an Industry Manager for Oil & Gas in Asia Pacific. The position does not require me to be super-technical (which is what I love) but it helps develop another facet of my career, which is building communities and partnership. I think this is crucial and more wholesome than just being technical alone. Given my present position, I am not obligated to write about HDS and its technology, but I am indeed subjected to Social Media Guidelines of the company. Therefore, I would like to make a disclaimer that what I write is my personal opinion, and mine alone. Therefore, I am responsible for what I say and write and this statement indemnify my employer from any damages.
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5 Responses to Playing with NetApp … (Capacity) BR

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