Flash-Based Write Cache(FBWC) & Battery-Backed Write Cache(BBWC) controllers

[chuckk281]

We had a question on HP Smart Array Controller technology - Flash-Based Write Cache(FBWC) & Battery-Backed Write Cache(BBWC) controllers and what are their differences.

Question: 

Any info about the performance of FBWC against BBWC??? Are they the same or is there any performance penalties?

Answer from Yao and Shane:

FBWC is a flash based cache module that does not have the battery limitation of how long it can retain what is written to the module.  You can find FBWC architecture via this whitepaper. HP Smart Array Controller technology - Technology Brief, January 2010. See pages 20 - 23 in:

https://serverfault.com/questions/434692/flash-backed-write-cache-fbwc-without-capacitor-pack

Have you used flash-based Smart Array Controllers? Do they work fine?

[Note: broken link updated by Mod]

[bearme]

The truth is not so simple.

1. BBWC works as everybody knows. There is DRAM memory module which contents are retained as long as the battery has some juice left.

2. FBWC, on the other hand, works differently. It is an ordinary BBWC module with additional FLASH memory. It also needs a power source to operate properly but with much smaller amount of energy thus enabling capacitors instead of batteries to be used. The crash event in case of FBWC use goes as follows:

- no power on the machine,

- get data from DRAM stored onto FLASH using energy from EXTERNAL capacitor, called by HP "Super Capacitor".

- wait for next power-up.

HP does say that Super Capacitor comes in exactly the same form as the Battery Module. They have different part numbers though: 587324-001 for capacitor,  462976-001 for battery pack.

https://support.hpe.com/hpesc/public/home

for yourself please.

Two last things:

1. Because both caches use DRAM memory there is no performance difference there.

2. Super Capacitors wear off eventually but HP does not state how to diagnose them but opening the server and checking leds on the memory module.

Regards,

MS

[Note: broken link updated/removed by Mod]

[Stromberg]

Are there any disadvantages for not using the capacitor pack? I read cache data could be stored about 1 year there. How about without it?

Greetings

Stromberg

[Dennis Handly]

>Are there any disadvantages for not using the capacitor pack? I read cache data could be stored about 1 year there. How about without it?

I'm not sure what you are asking?  You need the capacitor to power the copy from DRAM to the flash.

I assume once it is on the flash, the capacitor isn't used.

If you don't have a capacitor, you don't have a write cache.