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Playing with NetApp … After Rightsizing

It has been a tough week for me and that’s why I haven’t been writing much this week. So, right now, right after dinner, I am back on keyboard again, continuing where I have left off with NetApp’s usable capacity.

A blog and a half ago, I wrote about the journey of getting NetApp’s usable capacity and stopping up to the point of the disk capacity after rightsizing. We ended with the table below.

Manufacturer Marketing Capacity NetApp Rightsized Capacity
36GB 34.0/34.5GB*
72GB 68GB
144GB 136GB
300GB 272GB
600GB 560GB
1TB 847GB
2TB 1.69TB
3TB 2.48TB

* 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.

At this stage, the next variable to consider is RAID group sizing. NetApp’s ONTAP employs 2 types of RAID level – RAID-4 and the default RAID-DP (a unique implementation of RAID-6, employing 2 dedicated disks as double parity).

Before all the physical hard disk drives (HDDs) are pooled into a logical construct called an aggregate (which is what ONTAP’s FlexVol is about), the HDDs are grouped into a RAID group. A RAID group is also a logical construct, in which it combines all HDDs into data or parity disks. The RAID group is the building block of the Aggregate.

So why a RAID group? Well, first of all, (although likely possible), it is not prudent to group a large number of HDDs into a single group with only 2 parity drives supporting the RAID. Even though one can maximize the allowable, aggregated capacity from the HDDs, the data reconstruction or data resilvering operation following a HDD failure (disks are supposed to fail once in a while, remember?) would very much slow the RAID operations to a trickle because of the large number of HDDs the operation has to address. Therefore, it is best to spread them out into multiple RAID groups with a recommended fixed number of HDDs per RAID group.

RAID group is important because it is used to balance a few considerations

  • Performance in recovery if there is a disk reconstruction or resilvering
  • Combined RAID performance and availability through a Mean Time Between Data Loss (MTBDL) formula

Different ONTAP versions (and also different disk types) have different number of HDDs to constitute a RAID group. For ONTAP 8.0.1, the table below are its recommendation.

So, given a large pool of HDDs, the NetApp storage administrator has to figure out the best layout and the optimal number of HDDs to get to the capacity he/she wants. And there is also a best practice to set aside 2 HDDs for a RAID-DP configuration with every 30 or so HDDs. Also, it is best practice to take the default recommended RAID group size most of the time.

I would presume that this is all getting very confusing, so let me show that with an example. Let’s use the common 2TB SATA HDD and let’s assume the customer has just bought a 100 HDDs FAS6000. From the table above, the default (and recommended) RAID group size is 14. The customer wants to have maximum usable capacity as well. In a step-by-step guide,

  1. Consider the hot sparing best practice. The customer wants to ensure that there will always be enough spares, so using the rule-of-thumb of 2 HDDs per 30 HDDs, 6 disks are set aside as hot spares. That leaves 94 HDDs from the initial 100 HDDs.
  2. There is a root volume, rootvol, and it is recommended to put this into an aggregate of its own so that it gets maximum performance and availability. To standardize, the storage administrator configures 3 HDDs as 1 RAID group to create the rootvol aggregate, aggr0. Even though the total capacity used by the rootvol is just a few hundred GBs, it is not recommended to place data into rootvol. Of course, this situation cannot be avoided in most of the FAS2000 series, where a smaller HDDs count are sold and implemented. With 3 HDDs used up as rootvol, the customer now has 91 HDDs.
  3. With 91 HDDs, and using the default RAID group size of 14, for the next aggregate of aggr1, the storage administrator can configure 6 x full RAID group of 14 HDDs (6 x 14 = 84) and 1 x partial RAID group of 7. (91/14 = 6 remainder 7). And 84 + 7 = 91 HDDs.
  4. RAID-DP requires 2 disks per RAID group to be used as parity disks. Since there are a total of 7 RAID groups from the 91 HDDs, 14 HDDs are parity disks, leaving 77 HDDs as data disks.

This is where the rightsized capacity comes back into play again. 77 x 2TB HDDs is really 77 x 1.69TB = 130.13TB from an initial of 100 x 2TB = 200TB.

If you intend to create more aggregates (in our example here, we have only 2 aggregates – aggr0 and aggr1), there will be more consideration for RAID group sizing and parity disks, further reducing the usable capacity.

This is just part 2 of our “Playing with NetApp Capacity” series. We have not arrived at the final usable capacity yet and I will further share that with you over the weekend.

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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.