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Enable disk write cache, P400

Jonas Forsberg
Occasional Advisor

Enable disk write cache, P400

Hi.
Is there any way to enable write cache on the SA P400 without adding battery? (and yes, I know the potential risk of data loss)

Anyhow, we try to enable the disk write cache on the physical disks attached to a P400 without a battery. And it fails.
The ACU tells us that write cache is disabled on the disk but also tells us that the cache is changeable.

we try to enable it using acu cli, but gets this result:
"Error: This operation is not supported with the current configuration. Use the "show" command on devices to show additional details about the configuration."

is the battery a mandatory detail to enable write cache on the disk, if so: why?

Firmware on P400 : 2.10
Disk Model: HP DG146ABAB4
Disk Firmware Revision: HPD5

Thanks in advance
13 REPLIES
Patrick Terlisten
Honored Contributor

Re: Enable disk write cache, P400

Hello Jonas,

AFAIK there is no way to enable write cache without a BBWC Unit.

For which typ of disks did you try to enable the cache? A logical drive or a physical disk?

Kind regards,
Patrick
Best regards,
Patrick
Jonas Forsberg
Occasional Advisor

Re: Enable disk write cache, P400

Hi Patrick.
Thanks for a quick respons.

Ok, så I can't enable the write cache on the P400 without the battery.

But we try to enable the write cache on the physical disks with acu cli with the following command:
=> controller slot=1 modify drivewritecache=enable

and we get a warning about potential dataloss and a question about going ahead any way.
But then we get the following response:
"Error: This operation is not supported with the current configuration. Use the "show" command on devices to show additional details about the configuration."

You see the disk model in my original post, and we do have the latest firmwares.

cheers
Patrick Terlisten
Honored Contributor

Re: Enable disk write cache, P400

Hello,

I can't verify it here, but IMHO you need also a BBWC unit to activate it.

Maybe someone else can verify it with a ProLiant and a P400 controller.

Kind regards,
Patrick
Best regards,
Patrick
Jonas Forsberg
Occasional Advisor

Re: Enable disk write cache, P400

Ok. thanks for all help Patrick.

But I can't see why the disk write cache needs a BBWC on the P400. The Disk write cache will not be backedup with that anyway..

I hope someone else can give me a hint on how to proceed.
Rob Leadbeater
Honored Contributor

Re: Enable disk write cache, P400

Hi,

As per the previous posts, you need the BBWC to enable the write cache.

Note that the battery backs up the controller cache, not the cache on the disks.

Cheers,

Rob
Jonas Forsberg
Occasional Advisor

Re: Enable disk write cache, P400

Hi Rob.

So. I do need a BBWC to enable the write cache on the disk even though the BBWC doesn't back it up?

Seems strange to me, but if thats the case I guess I have to live with that.

Thanks for the replyes.
Rob Leadbeater
Honored Contributor

Re: Enable disk write cache, P400

Hi,

If you check the QuickSpecs of the P400, it seems quite clear.

http://h18000.www1.hp.com/products/quickspecs/12400_div/12400_div.HTML

I suspect the confusion might arise because of the terminology being used by the ACU CLI. I could be wrong, but I suspect the command "drivewritecache=enable" is actually referring to the logical drives on the controller, rather than the physical drives.

Cheers,

Rob
Jonas Forsberg
Occasional Advisor

Re: Enable disk write cache, P400

Hi Rob.
I do appreciate your help and the fact that you take time trying to help me.

Bu according to the ACU manual, and I do quote:
---------------------
Enabling or disabling the drive cache
On controllers and drives that support physical drive write cache, you can use this command to enable or disable the write cache for all drives on the controller.

CAUTION: Because physical drive write cache is not battery-backed, you could lose data if a power failure occurs during a write process. To minimize this possibility, use a backup power supply.

Syntax:
modify drivewritecache=enable|disable|? [forced]
---------------------

So it seems to me that this is a command to manipulate the physical and not the logical drives.

It still could be that it requires the BBWC, but that seems strange to me.
And as I said in my first post ACU tells me that the physical drive write cache is disabled but changeable.
Rob Leadbeater
Honored Contributor

Re: Enable disk write cache, P400

Interesting... I did say I could be wrong !

The manual does appear to be slightly contradictory though...

The line below the one that you quoted says

"where is a controller that supports drive write cache."

That sort of implies you do need the battery...

Hmmm. Have you tried using the forced option ?

=> controller slot=1 modify drivewritecache=enable forced

that *might* work, although there's no mention in the manual of what the forced option is for...

Cheers,

Rob
Jonas Forsberg
Occasional Advisor

Re: Enable disk write cache, P400

Hi Rob.

Thanks for all the help.

The forced flag seams to suppress the warning text about loosing data, nothing else.
We now tried to enable the disk write cache with a BBWC attached to the P400 but we get the same result.

I think itâ s time to make a support case of this.

Cheers
Patrick Terlisten
Honored Contributor

Re: Enable disk write cache, P400

Hello,

what about some points. ;)

Best regards,
Patrick
Best regards,
Patrick
nomadsole
Advisor

Re: Enable disk write cache, P400

I had the same problem with P600 / BBWC. Besides I couldn't enable the write cache of the physical drives, I also found that the write back cache feature was disabled for logical drives in Windows. And I coundn't enable it as well. I think this is attributed to the fact that the write caches on the physical drives were disabled. I am going to log a support case next Monday.
nomadsole
Advisor

Re: Enable disk write cache, P400

HP advised me SA-P600 does not support write cache on SATA drives. If you want write cache, you must use SAS. I suspect they technically disable the feature for sales reason. There is no point for a battery backed controller with battery backed write cache which has write cache disabled on the drives. The write performance really sucks compared with a SAS system.