topic Difference between BBWC and FBWC in ProLiant Servers (ML,DL,SL)

Difference between BBWC and FBWC

Francesco Stupazzoni — Thu, 29 Jul 2010 12:38:12 GMT

Hi,

on the DL370G7 Quickspec there are references to both 512 MB BBWC (P/N: 462967-B21) and 512 MB FBWC (P/N: 534916-B21).

Which are the difference between the two, i.e. which are the difference between a Battery Backed Write Cache and a Flash Backed Write Cache?

Thanks

Ciao

Re: Difference between BBWC and FBWC

KML — Thu, 29 Oct 2020 07:23:36 GMT

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://support.hpe.com/hpesc/public/docDisplay?docId=emr_na-c02231971
Hope this clarifies..

Moderator Note: [Updated with new links]

Re: Difference between BBWC and FBWC

ICTstudent — Wed, 05 Jan 2011 12:51:51 GMT

But what do they both need to operate ?
Does de 512 MB cache module require a battery ? if it does, which one ?
please help and Explain !!

Thanks,

ICT student

Re: Difference between BBWC and FBWC

Zygmunt Krawczyk — Wed, 05 Jan 2011 13:07:05 GMT

ICT student,
for your ML350 G6 you need one of the two:

534916-B21 HP 512MB Flash Backed Write Cache
534562-B21 HP 1G Flash Backed Cache

These modules do not require a battery. They have super capacitors to provide power to flash write cache memory.

Re: Difference between BBWC and FBWC

ptlinva12 — Sat, 30 Apr 2011 20:11:25 GMT

Is there a performance difference between the two (BBWC/FBWC)?

If so, which one is faster?

Thanks!
Paul L.

Re: Difference between BBWC and FBWC

Matti_Kurkela — Thu, 29 Oct 2020 07:28:21 GMT

According to page 4 of this document...

~~http://h20000.www2.hp.com/bc/docs/support/SupportManual/c02249094/c02249094.pdf~~

[Admin Note: Above link is broken-try below link instead]

Alternate link: https://support.hpe.com/hpesc/public/docDisplay?docId=emr_na-c02231971

...the flash in the FBWC is used only when the power is lost: the cache RAM is copied to flash using the power reserve of the supercapacitors.

The actual cache RAM is probably identical in both BBWC and FBWC, because its primary requirement must be speed in both read and write operations. So I'd expect their performance to be identical if both modules have the same size.

As far as I know, writing to flash memory is usually significantly slower than reading from it. The flash memory cells also have a limited number of write cycles, although the limit can be pretty high nowadays. But these two factors make raw flash a bad choice for cache. On the other hand, copying the content of the cache RAM to a backup flash in the event of power loss makes a lot of sense.

MK