Difference between BBWC and FBWC

[Francesco Stupazzoni]

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

[KML]

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]

[ICTstudent]

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

[Zygmunt Krawczyk]

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.

[ptlinva12]

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

If so, which one is faster?

Thanks!
Paul L.

[Matti_Kurkela]

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