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Riedsio
Asked: 2010-12-23 22:58:05 +0800 CST

How can I tell if my hard drive(s) have Battery Backed Write Cache?

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How can I tell if my hard drives have a battery backed write cache (BBWC)?

How can I tell if it is enabled and/or configured correctly?

I don't have physical access to my server. It's a GNU/Linux box.

I can provide supplemental incremental information/details as requested. My frame of reference is that of a DBA -- I have access and privileges, but (usually) only tread where I know am supposed to. :)

hard-drive linux battery disaster-recovery storage

3 Answers

  1. I've never actually seen battery backed write cache in harddisks - only in RAID controllers. How to query the controller is rather specific to your hardware. It will almost certainly require root privileges on the box.

    For example, using Dell PERC controllers, and having dell-omsa-repository installed, I can do:

    # omreport storage battery
List of Batteries in the System

Controller SAS 6/iR Integrated (Slot Embedded)
No Batteries found

    On another server:

    # omreport storage battery
List of Batteries in the System

Controller PERC 4e/Di (Slot Embedded)
ID                        : 0
Status                    : Ok
Name                      : Battery 0
State                     : Ready
Recharge Count            : 0
Max Recharge Count        : 1100
Predicted Capacity Status : Not Applicable
Learn State               : Not Applicable
Next Learn Time           : Not Applicable
Maximum Learn Delay       : Not Applicable

    I haven't been able to find right now how to ask it what the current write cache setting is, but I think I've seen it in there before. However, if you're not using Dell RAID controllers, you will need different software.

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  2. This way you can see if the battery backup is speeding up your server, on this server it is working:

    root@server:~# omreport storage vdisk
List of Virtual Disks in the System

Controller PERC H700 Adapter (Slot 1)
ID                        : 0
Status                    : Ok
Name                      : VD1
State                     : Ready
Hot Spare Policy violated : Not Assigned
Encrypted                 : Not Applicable
Layout                    : RAID-5
Size                      : 1,862.00 GB (1999307276288 bytes)
Device Name               : /dev/sda
Bus Protocol              : SATA
Media                     : HDD
Read Policy               : Adaptive Read Ahead
Write Policy              : Write Back
Cache Policy              : Not Applicable
Stripe Element Size       : 64 KB
Disk Cache Policy         : Enabled

    The important line here is the

    Write Policy              : Write Back

    Make sure you have a battery before enabling this, using:

    root@server:~# omreport storage battery

    Then you can enable using:

    root@server:~# omconfig storage vdisk controller=0 vdisk=0 action=changepolicy writepolicy=wb
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  3. It is called KNOWING. A dark secret by experienced system admins. They remember what hardware they order and how they configure it. Very arcane stuff. Part of the dark art of administration and requires you to read the ancient scrolls containing the purchase orders.

    In the end, if you are the DBA, ask the people responsible for the system.

    • Discs normally dont have a BBWC. Discs arep owered by the computer. so that would be a redundant power supply witha USV. I know of not a single disc having the availablility top plug in a battery.
    • BBWC are a raid controller feature. Sadly ,there is no common API. Where exactly you see stuff like controlelr configuration etc. depends on the hardware. On modern hardware you may even be protrected WITHOUT battery - Adaptec recently released a controller without battery. It has a condensator keeping thing alive long enough... to write the whoe cache onto basically an on board flash non volatile memory. Same result, no maintenance (batteries needs regular replacement, they wear out within 2-3 years).

    Depending - if you are the DBA - on how large the company is you may not have discs, a USV or even a raid controller available at all, but run against a SAN (which has all this in a much larger configuration).

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