Physical drive write cache settings - should I turn it on?

[Philip_75]

Hi,
I have just purchased a shiney new HP ML350 e4210 with 5 x 146GB SAS Drives. The server comes standard with embedded e200i/64. So today I went out and purchased a the 128MB BBWC.

So I have now added the 128 BBWC and have a question using the Array Config utility.

It is my intention to mirror my fisrt two disks raid1 (disk0, disk1) and use Raid5 (hence the purchase of the BBWC) on (Disk3, Disk4, Disk5).

I note in the array config utility I have the option to "ENABLE" the Physical drive write cache. Is this something is should be doing?

Thanks

[Torsten.]

Yes, since you installed the BBWC this option is enabled and can be used.

[Philip_75]

Thanks I appreciate the quick response.

[Hein van den Heuvel]

Torsten, I beg to differ.

Best I know the presence of the BBWC is irrelevant with respect to Physical Drive write back caching being on or off.

If anything its presence is a reason NOT to enable it (any more) as it will increase risk data loss on power failure and will no longer help the application level (as much) as that is now handled by the BBWC.
The BBWC will erroneously be led to believe that the data is safely on the spindle and it can and will walk away from the protected cache lines holding the data as soon as the drive optimistically claims 'I got it'.

Only in extreme load cases should the application be able to benefit from Physical drive write back cache, as it is hidden/shielded by the controller cache.

By enabling the drive write back cache you may reduce valuable (track-readahead) drive read cache effectiveness.

A recent earlier discussion was in:

http://forums11.itrc.hp.com/service/forums/questionanswer.do?threadId=1240003

As per ACU Ref Manual:

"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"

Regards,
Hein van den Heuvel
HvdH Performance Consulting.

[Torsten.]

Doesn't the (battery backed up) controller cache the data?
This is different to enable the cache on the physical drives itself, right?

I agree regarding the cache on the drive directly, but controller caching is something different, right?

[Hein van den Heuvel]

>> Doesn't the (battery backed up) controller cache the data?

In the BBWC yes. But when that write through to the rive, and the drive indicates succes (having transferred teh data only into its cache) then the controller cache only retains the data for future reads. The write is gone and forgotten.

>> This is different to enable the cache on the physical drives itself, right?

Very.

>> I agree regarding the cache on the drive directly, but controller caching is something different, right?

Yes, you want that on.
Raid-5 is unuseable without WB cache.


Philip>>> Raid5 (hence the purchase of the BBWC) on (Disk3, Disk4, Disk5).

Please not that 3-disk raid-5 is a relatively poor choice. All the pain of raid-5, and little gain (loose 1/3 of the storage for redundancy vs 1/2 for raid-10

Depending on your application (space) needs you may be better of with a second raid-1 and keeping the 5th as hot spare.

Too bad that only the old SWXCR's woudl do raid 0+1 on an odd number of spindles. That is ideal IMHO!

Hein.

http://h20219.www2.hp.com/ERC/downloads/4AA0-8805ENW.pdf

[Tom O'Toole]

I've often wondered, do most storage arrays disable the on-disk cache because it's not battery backed up? If so, why do these drives get made with cache? Is it at least used to buffer reads? Seems like a huge waste of money to have all those drives with unused cache memory.