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.

ONTAP vs ZFS

I have to get this off my chest. Oracle’s Solaris ZFS is better than NetApp’s ONTAP WAFL! There! I said it!

I have been studying both similar Copy-on-Write (COW) file systems at the data structure level for a while now and I strongly believe ZFS is a better implementation of the COW file systems (also known as “shadow-paging” file system) than WAFL. How are both similar and how are both different? The angle we are looking at is not performance but about resiliency and reliability.

(Note: btrfs or “Butter File System” is another up-and-coming COW file system under GPL license and is likely to be the default file system for the coming Fedora 16)

In Computer Science, COW file system are tree-like data structures as shown below. They are different than the traditional Berkeley Fast File System data structure as shown below:

As some of you may know, Berkeley Fast File System is the foundation of some modern day file systems such as Windows NTFS, Linux ext2/3/4, and Veritas VxFS.

COW file system is another school of thought and this type of file system is designed in a tree-like data structure.

In a COW file system or more rightly named shadow-paging file system, the original node of the data block is never modified. Instead, a copy of the node is created and that copy is modified, i.e. a shadow of the original node is created and modified. Since the node is linked to a parent node and that parent node is linked to a higher parent node and so on all the way to the top-most root node, each parent and higher-parent nodes are modified as it traverses through the tree ending at the root node.

The diagram below shows the shadow-paging process in action as modifications of the node copy and its respective parent node copies traverse to the top of the tree data structure. The diagram is from ZFS but the same process applies to WAFL as well.

 

As each data block of either the leaf node (the last node in the tree) or the parent nodes are being modified, pointers to either the original data blocks or the copied data blocks are modified accordingly relative to the original tree structure, until the last root node at the top of the shadow tree is modified. Then, the COW file system commit is considered complete. Take note that the entire process of changing pointers and modifying copies of the nodes of the data blocks is done is a single I/O.

The root at the top for ZFS is called uberblock and called fsinfo in WAFL. Because an exact shadow of the tree-like file system is created when the data blocks are modified, this also gives birth to how snapshots are created in a COW file system. It’s all about pointers, baby!

Here’s how it looks like with the original data tree and the snapshot data tree once the shadow paging modifications are complete.

 

However, there are a few key features from the data integrity and reliability point of view where ZFS is better than WAFL. Let me share that with you.

In a nutshell, ZFS is a layered architecture that looks like this

The Data Management Unit (DMU) layer is one implementation that ensures stronger data integrity. The DMU maintains a checksum on the data in each data block by storing the checksum in the parent’s blocks. Thus if something is messed up in the data block (possibly by Silent Data Corruption), the checksum in the parent’s block will be able to detect it and also repair the data corruption if there is sufficient data redundancy information in the data tree.

WAFL will not be able to detect such data corruptions because the checksum is applied at the disk block level and the parity derived during the RAID-DP write does not flag this such discrepancy. An old set of slides I found portrayed this comparison as shown below.

 

Another cool feature that addresses data resiliency is the implementation of ditto blocks. Ditto blocks stores 3 copies of the metadata and this allows the recovery of lost metadata even if 2 copies of the metadata are deleted.

Therefore, the ability of ZFS to survive data corruption, metadata deletion is stronger when compared to WAFL .This is not discredit NetApp’s WAFL. It is just that ZFS was built with stronger features to address the issues we have with storing data in modern day file systems.

There are many other features within ZFS that have improved upon NetApp’s WAFL. One such feature is the implementation of RAID-Z/Z2/Z3. RAID-Z is a superset implementation of the traditional RAID-5 but with a different twist. Instead of using fixed stripe width like RAID-4 or RAID-DP, RAID-Z/Z2 uses a dynamic variable stripe width. This addressed the parity RAID-4/5 “write hole” flaw, where incomplete or partial stripes will result in a “hole” that leads to file system fragmentation. RAID-Z/Z2 address this by filling up all blocks with variable stripe width. A parity can be calculated and assigned with any striped width, as shown below.

 

Other really cool stuff are Hybrid Storage Pool and the ability to create software-based caching using fast disk drives such as SSDs. This approach of creating ReadZilla (read caching) and LogZilla (write caching) eliminates the need for proprietary NVRAM as implemented in NetApp’s WAFL.

The only problem is, despite the super cool features of ZFS, most Oracle (not Sun) sales does not have much clue how to sell ZFS storage. NetApp, with its well trained and tuned, sales force is beating Oracle to pulp.

Copy-on-Write and SSDs – A better match than other file systems?

We have been taught that file systems are like folders, sub-folders and eventually files. The criteria in designing file systems is to ensure that there are few key features

  • Ease of storing, retrieving and organizing files (sounds like a fridge, doesn’t it?)
  • Simple naming convention for files
  • Performance in storing and retrieving files – hence our write and read I/Os
  • Resilience in restoring full or part of a file when there are discrepancies

In file systems performance design, one of the most important factors is locality. By locality, I mean that data blocks of a particular file should be as nearby as possible. Hence, in most file systems designs originated from the Berkeley Fast File System (BFFS), requires the file system to seek the data block to be modified to ensure locality, i.e. you try not to split up the contiguity of the data blocks. The seek time to find the require data block takes time, but you are compensate with faster reads because the read-ahead feature allows you to read extra blocks ahead in anticipation that the data blocks are related.

In Copy-on-Write file systems (also known as shadow-paging file systems), the seek portion is usually not present because the new modified block is written somewhere else, not the present location of the original block. This is the foundation of Copy-on-Write file systems such as NetApp’s WAFL and Oracle Solaris ZFS. Because the new data blocks are written somewhere else, the storing (write operation) portion is faster. It eliminated the seek time and it also skipped the read-modify-write action to the original location of the data block. Therefore, write is likely to be faster.

However, the read portion will be slower because if you want to read a file, the file system has to go around looking for the data blocks because it lacks the locality. Therefore, as the COW file system ages, it tends to have higher file system fragmentation. I wrote about this in my previous blog. It is a case of ENJOY-FIRST/SUFFER-LATER. I am not writing this to say that COW file systems are bad. Obviously, NetApp and Oracle have done enough homework to make the file systems one of the better storage file systems in the market.

So, that’s Copy-on-Write file systems. But what about SSDs?

Solid State Drives (SSDs) will make enemies with file systems that tend prefer locality. Remember that some file systems prefer its data blocks to be contiguous? Well, SSDs employ “wear-leveling” and required writes to be spread out as much as possible across the SSDs device to prolong the life of the SSD device to reduce “wear-and-tear”. That’s not good news because SSDs just told the file systems, “I don’t like locality and I will spread out the data blocks“.

NAND Flash SSDs (the common ones we find in the market and not DRAM-based SSDs) are funny creatures. When you write to SSDs, you must ERASE first, WRITE AGAIN to the SSDs. This is the part that is creating the wear-and tear of the device. When I mean ERASE first, WRITE AGAIN, I describe it below

  • Writing 1 –> 0 (OK, no problem)
  • Writing 0 –> 1 (not OK, because NAND Flash can’t do that)

So, what does the SSD do? It ERASES everything, writing the entire data blocks on the device to 1s, and then converting some of them to 0s. Crazy, isn’t it? The firmware in the SSDs controller will also spread out the erase-and-then write operations across the entire SSD device to avoid concentrating the operations on a small location or dataset. This is the “wear-leveling” we often hear about.

Since SSDs shun locality and avoid the data blocks to be nearby, and Copy-on-Write file systems are already doing this because its nature to write new data blocks somewhere else, the combination of both COW file system and SSDs seems like a very good fit. It even looks symbiotic because it is a case of “I help you; and you help me“.

From this perspective, the benefits of COW file systems and SSDs extends beyond resiliency of the SSD device but also in performance. Since the data blocks are spread out at different locations in the SSD device, the effect of parallelism will inadvertently help with COW’s performance. Make sense, doesn’t it?

I have not learned about other file systems and how they behave with SSDs, but it is pretty clear that Copy-on-Write file systems works well with Solid State Devices. Have a good week ahead :-)!