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ProLiant Servers (ML,DL,SL)
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Proliant ML350 Gen8 Backup Write Cache

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Occasional Contributor

Proliant ML350 Gen8 Backup Write Cache

I have never quite understood this. Hopefully someone and clear up my confusione. I plan to buy a new Gen8 Ml350. The server has a smart array 420i zero memory controller.


Is it sufficient to purchase the 1GB P series Flash Back Write cache product code 631679-B21 for activate the RAID5 option or is there a battery backup option also needed. My confusione lies on never understand if you need both or just the one. The wording for the products is so similar its hard to quite know the difference.


The main tech manual only refers to varies other options like:

HP 1GB P-series Smart Array Flash Backed Write Cache 631679-B21
HP 2GB P-series Smart Array Flash Backed Write Cache 631681-B21



So adding a HP 1GB P-series Smart Array Flash Backed Write Cache 631679-B21 to the Zero Mem controller will it give me the same result as this HP Smart Array P420/1GB FBWC 6Gb 2-ports Int SAS Controller 631670-B21?


Thank you




Occasional Contributor

Re: Proliant ML350 Gen8 Backup Write Cache



Can no body even from HP support put me on the right track?



Honored Contributor

Re: Proliant ML350 Gen8 Backup Write Cache

This is a community of HP users, not a communication channel to HP Support.


But I think I can clear your confusion.


Both the older Battery-Backed Write Cache (BBWC) and the new Flash-Backed Write Cache (FBWC) serve the same function. So either of them will allow you to activate the RAID5 option.


Both *BWCs contain an amount of fast RAM for caching. The only difference between BBWC and FBWC is the survival strategy in the event of a power failure:

  • The old BBWC simply used a battery to keep the RAM alive. The battery had a limited lifetime and required periodic replacement (about every 3 years, depending on operating conditions). If the system suffered a power failure and the battery ran out, the data in the cache was lost, potentially causing significant data loss.
  • The new FBWC has a super-capacitor and an amount of Flash memory. If a power failure occurs, the FBWC uses the power in the super-capacitor to copy the contents of the cache RAM into the Flash memory, where it can be preserved for years without any power at all. The Flash memory will only be used in the event of a power failure: for normal operation, the regular RAM is used, exactly like BBWC. The super-capacitor should be maintenance-free for the expected lifetime of the server.

With each FBWC, there should be a super-capacitor module which is similar in size and form factor to the battery modules used in the older BBWC design. However, they are not interchangeable: do not attempt to plug a super-capacitor module to a BBWC or a battery to a FBWC.