C220M3 LSI Megaraid 9266-8i

[rchaseling]

Hi,

I ahve a quick one have the server above and am follwing the TRC. It states

Set Write Policy to one of the following:

- write back with bbu- if using a RAID card with SuperCap  (e.g. RAID-9266CV)

- always write back – if using a RAID card with  legacy Battery Back-Up (BBU) instead of SuperCap (e.g. RAID-9266). This  option helps prevent a UC application performance impact in the event  that the BBU goes into learning mode or the battery dies.

CAn I use the "write back with BBU" for this controller? Its just the 8i vs CV is throwing me ? Not sure if its the same thing or noew

Cheers

[Keny Perez · Accepted solution]

Hi,

The 9266-8i supports the SCPM (SuperCap Power Module) which provides approximately 3 years of backup for the disk write-back cache DRAM in the case of sudden power loss, it does not support the regular BBU, but provides the same functionality, you may use write back with bbu  with no issues.

The difference between write back with bbu & always write back is:

write back with bbu: it enables longer holding of info in the cache and write to disk eventually, but in case of power loss, the BBU/SCPM provides power to write that info to the disks

always write back: If the BBU/SCPM fails or is not present, it will continue saving the info in cache even though there is no backup power.

BBU and SCPM are ALMOST  the same thing, the difference i:

BBU provides approximately 72 hours of battery backup for the disk write-back cache DRAM in the case of sudden power loss. 

SCPM provides approximately 3 years of backup for the  disk write-back cache DRAM in the case of sudden power loss by  offloading the cache to the NAND flash

Again, as a final word.. you may use "write back with BBU" even when you have SCPM.

I hope it helps, if it did, rate it please

-Kenny

[rchaseling]

Excellent that's great info - so as a preference it is always better to use write back with BBU or does it really not matter

[Keny Perez]

Hi,

If I were you and I have a BBU, I rather have write back with BBU, cause that way if the BBU is not present/bad the way it will work will be that the write mode will be changed to Write-through where the performance is a little degraded cause the information is immediately written to the disk, but that will give me the chance to replace/install a new BBU/SCPM with no chances to lose data, since it will be already in the disk.

Just always keep in mind that with no power, data not saved is lost (as always) so the backup batteries are a great source of reliability.

I am at your service, I hope this also helps getting a better idea!

-Kenny