Egg Timers, a Million Pounds and Read Intensive Flash
bwhyte 310000B8UF Comments (7) Visits (7793)
Yesterday I spent an hour or so looking for something I knew was out in the blogosphere so I could pull down the reference to it, its frustrating that even when you know something is out there, and no amount of google search phrases can find it for you. But what I did find was a couple of my old posts from back in about 2008/9 discussing a couple of interesting things.
Firstly I found a post where I was knocking Hu Yoshida over at HDS for being a marketing blog…
It made me think back to the 1990’s band The KLF, who started producing ‘sampled’ music back in 1987, by taking other peoples music, sampling, chopping, mashing it up and themselves creating other tunes as a way of rebelling against the state of the pop music industry, with manufactured artists and vocal tuning regularly being used. In the early 90’s they had started to produce their own chart topping songs and in 1994 they infa
I realise too that my mocking of Hu had come full circle and most of my posts of late have been marketing posts essentially, but providing a different perspective hopefully from the mainstream marketing around our product updates and releases - more of an insiders view.
So a few people have been asking me about the new class of Flash drives we released for the Spectrum Virtualize based products back in June. I don’t really like the current term ‘Read Intensive’ Flash. My understanding is we will be renaming them in the future, and I’ve been referring to them as ‘General Purpose’ Flash in my client briefings.
MLC vs MLC
First off, lets look at the facts of MLC. Essentially they are the same technology, MLC (Multi level cell) used in all of the current Enterprise Flash drives, and the General Purpose Flash drives. They are usually 2-bits per cell, some maybe 3-bits per cell. The Flash itself ranges from 5-10,000 writes per cell before wear out. I don’t think anyone is still shipping SLC (Single level cell) with single bits per cell as Enterprise drives these days mainly due to cost.
The main difference between the Enterprise and General Purpose is the level of over-provisioning that the capacity uses. For example, some of the early MLC Enterprise drives had as much as 2x the capacity in raw Flash terms as the reported capacity.
The over provisioning generally dictates the amount of write data that can be sustained by the drives, and this is reported as a metric of ‘Drive Writes per Day’ - As of 7.7.1 code we are now reporting the DWPD as a percentage of used drive writes per day. This is really a percentage of endurance used of the drive, and so you can easily see if you are over using a drive.
Drive Writes per Day
What does this really mean? Well in terms of writing, it means whatever the capacity of the drive you can write X times that capacity per day to the drive and it will last the 5 year warranty. Best served by the example below, but you need to know the metrics.
Current Enterprise Flash drives in Storwize (400,800,1600 GB) can support up to 10 DWPD.
Assume we have a 3.84TB drive, and we are writing at 10MB/s on average, 24x7x365.
In reality, you can think of this as a conservative 10MB/s per TB per day.
Some investigation of our existing Enterprise install base suggested that over 90% of all existing installs would be absolutely fine swapping their Enterprise for General Purpose.
In getting your head around this, it basically says that typical workloads that benefit from Flash latency and performance are smaller block transactional style workloads, which tend to be low in MB/s The kind of workloads that Easy Tier today has moved onto Flash will be ideal for this, as it avoids high bandwidth streaming workloads today.
Unless you have something that is doing very high continuous write streaming workloads, the new General Purpose is going to be ideal.
When you upgrade to the latest code, you will be able to view the ‘write endurance used’ percentage on existing Flash drives also.
If you have other questions about this new class of drive feel free to ask.
Oh and the second thing I found while googling, the Storage Egg timer in which my predictions have almost come true (except for the 15K drive bit… and nobody is using SLC these days either!)