Tag Archives: DWPD

Do I need a Bigger Write Buffer?

Even since VMware published this article on cache sizing guidelines for all-flash I still get asked two questions, the first one is around the amount of cache required per node? The second is about when vSAN will have a larger write buffer?

The amount of cache required per node in all-flash is not dependant on the amount of usable space like it was in Hybrid configurations, the amount of cache per node is based purely on the endurance of the cache SSDs, which typically fall into four categories:

  • Up to 2 drive writes per day
  • 3 drive writes per day
  • 10 drive writes per day
  • 30 drive writes per day

With the birth of the P5800X from Intel having an endurance capability of 100 drive writes per day, I would expect a 5th category will appear soon too.

If we look at the amount of DWPD a drive is capable of we can see whether it would be good in a cache tier or not, for example a device with 0.4-2 DWPD is likely to be certified for the vSAN capacity tier and not the cache tier.

Since the cache tier is where 100% of the writes happen, this is where you need the higher endurance devices, the higher the writes in your environment means you need to look at the endurance as this will be the biggest factor in the amount of cache you need.

If we look at the 3 DWPD category, this is normally categorised by the vendors as “Mixed-Use”, and is the most economically priced cache device for vSAN, but because of the lower endurance, you actually need more cache. I have looked at a lot of Live Optics reports over the past few months to gather information on what is the average % of writes in a customer environment, and the number that came out was 37%, yes higher than the 30% normally envisaged.

So based on 3DWPD and >30% Random Writes, the VMware Article states you need 3.6TB of cache per node based on an AF-8 Config, so this would result in a likely configuration of 3x 1.6TB Devices:

The next category of 10 DWPD, these are usually classed as “Write Intensive” by the vendors, again according to the VMware table you would need 1.2TB of cache per node, again based on an AF-8 Config with >30% Random writes:

Then we come to the final category of 30 DWPD, these devices are usually categorised as “Write Intensive Express Flash”, and this is usually Intel Optane SSD Devices such as the P4800X, for the same workload, the VMware recommendation is to have 400GB of cache per node:

As you can see, the amount of cache you require is based on the endurance of the devices when it comes to vSAN all-flash.

To address the second question about vSAN ever having a larger write buffer, this has been mentioned for a long time, but my opinion here is that you do not need to have a larger write buffer if you are using high endurance devices, and with the new Intel P5800X having an endurance factor of 100 DWPD, I expect that the amount of cache per node would be lower still, so I would not expect a big emphasis on the write buffer from a vSAN perspective.

As SSDs become faster, more higher endurance, it mitigates the need to have larger write buffers, especially in Full NVMe configurations for example where the storage is sat on the PCIe BUS directly, rather than sat behind a disk controller. And in my experience with Intel Optane SSDs, the 375GB (P4800X) and 400GB (P5800X) serve very well even in write intensive environments.

Why a NAND based SSD may not be the right choice for vSAN cache Tier

As we all know storage media has evolved very quickly over the past few years, the decline of the spinning disk and the move to flash based storage devices, but also the shift from SAS/SATA protocol based drives to NVMe protocol based drives in order to address the performance limitations of older protocols that were designed for spinning disks and not for SSDs.

A question I get asked regularly is what type of SSD is best for the vSAN Cache tier, there are vSAN ready nodes out there that contain NAND based SSDs for both the cache tier and for the capacity tier, but then there are other technologies like Intel Optane™ SSDs being used for the cache tier, so let’s talk about the two, for the purpose of this comparison I am going to use the most common 3D NAND based NVMe in vSAN Ready node configurations, the 1.6TB Intel P4610 NVMe drive, and the 375GB P4800X Intel Optane™ SSD, both of these SSDs are NVMe based devices.

Let’s compare the two devices:

1.6TB Intel P4610375GB P4800X Intel Optane™ P4800X
Sequential Read (up to)3200 MB/Sec2400 MB/Sec
Sequential Write (up to)2080 MB/Sec2000 MB/Sec
Random Read (100% Span)643000 IOPS550000 IOPS
Random Write (100% Span)199000 IOPS500000 IOPS
Latency Read77 µs10 µs
Latency Write18 µs10 µs
Endurance Rating12.25PBW or around 3.5 DWPDUp to 60 DWPD or around 41PBW
Data obtained from ark.intel.com

As you can see from the above table there are some major differences between the two different SSD’s notably the Random Write performance which is critical in the cache tier in a vSAN environment as all the incoming writes are random in nature and are absorbed by the cache tier, the NAND based SSD does not have as much capability around the random writes versus the Optane™ SSD, but the biggest impact to a vSAN Cache tier is the Drive Writes Per Day (DWPD), if you look at the specifications in detail, the P4610 can handle around 3.5 DWPD which equates to around 5.6TB of data written daily, whereas the 375GB Optane™ SSD can handle up to 60 DWPD which equates to 15TB of data written daily, remember that the Optane™ SSD is also less than a quarter of the capacity of the P4610, so in a vSAN environment cache tier, the Optane™ SSD wins hands down from an endurance perspective as well as the abaility to handle the random writes a lot quicker, so why such a difference?

Well if you look at NAND based SSDs, firstly there is usally an element of DRAM that acts as a buffer to the NAND media which is usually around 1GB of DRAM for every TB of media, so any incoming writes hit the DRAM buffer first, this can be a positive boost in short, low block size write bursts, but cannot be sustained over a longer period of time, in an Optane™ SSD there is no such DRAM buffer so the data is being written to directly to the media. The VxRAIL team at Dell EMC have done some extensive testing around this and clearly demonstrated that a NAND based SSD cannot sustain the same level of write performance in a continuous fashion whereas the Optane™ SSD maintains the same level of write performance consistently, below is the results of their performance testing:


The way NAND based SSDs and Optane™ SSDs perfrom write operations is fundamentally different, in everybody’s NAND, media has to be read and written in pages, but everything has to be erased in blocks. Page updates are typically written to a new unused block, as new data is written, old pages become stale, and on an SSD these stale pages can build up fairly quickly which means at some point there a significant chunks of blocks that are obsolete, this then has to be garbage collected. This will then clear the block and allow that block to receive data, and the process starts all over again.

Optane™ SSDs are transistor-less which essentially means that each cell state can be changed from a 0 or 1 independently of other cells on the device. This means that Optane™ SSDs are completely bit addressable as opposed to having to write in pages, there is also no garbage collection required, and this obviously has a positive impact on performance as well as endurance which is why Optane™ SSDs have very high endurance capabilities.

So what does all this mean from an application perspective?
Well the VxRAIL Guys at Dell EMC also did some performance testing using Hammer DB and shown some significant performance gains when using Intel Optane™ SSDs versus traditional NAND as Cache as much as a 61% gain in performance in a complex OLTP workload

As we all know latency is critical in any type of workloads, what I have seen in performance testing is that Intel Optane™ SSDs consistently provide lower latency as well as a much more tightly controlled standard deviation on latency versus the P4610, even though in some smaller block size tests the performance of both devices was similar, in larger block size tests the Optane™ SSD again delivered lower latency and tightly controlled standard deviation in latency but also provided a much higher performance in comparison to the P4610. You also have to remember that the P4610 device was only using 37% Span due to vSAN Currently having a limit of 600GB write buffer per disk group, whereas the Optane™ SSD was using 100% Span, so the P4610 had a bit of an unfair advantage here.

What is clear from a vSAN perspective, endurance plays a critical role in the vSAN cache tier, in the very early days of vSAN there was no other choice but SAS or SATA based NAND devices with a ranging DWPD of between 10 and 25 based on an 800GB Drive, but as the technology evolution pushes the boundaries of performance and endurance, technology like Intel Optane™ SSDs clearly have an edge offering up to 60 DWPD on a smaller capacity of 375GB.

Smaller cache device…are you serious?

In the testing I have done on full NVMe systems where Intel Optane™ SSDs are being used in the vSAN Cache Tier, and standard more read-intensive NVMe drives like the Intel P4510 are being used in the capacity tier, a 375GB Optane™ SSD is more than sufficient, in most workloads a 750GB Optane™ SSD did not improve performance, even with 375GB I was only able to saturate the write buffer by 60% (based on vSAN 6.7 Update 3).

So whilst NAND based devices are fully supported as a vSAN cache device, they may not be the right choice when it comes down to consistent performance and endurance required for a modern infrastructure.