ISS Technology Update, Volume 9, Number 3

Risks of enabling physical drive write cache

Unlike controller-level cache, individual drive write caches are not protected. If the server loses power or if a hot plug drive is

physically removed from the array during operations, unwritten data in the physical drives’ write caches will be lost.

Although this window of vulnerability is fairly small, it still exists. Therefore, HP drives have drive write cache disabled by

default.

When to enable physical drive write cache

If your environment has write intensive applications that tend to write in bursts, enabling drive write cache can improve your

storage performance. Applications that continuously write large amounts of data to the array will eventually fill up write

cache, causing throughput to slow down to the basic rate supported by the drives. In general, physical drive write cache

should not be enabled when using a Smart Array controller in any application environment where data loss would be

completely unrecoverable. However, enabling drive write cache can prove beneficial in certain write intensive application

environments, such as high-end video editing, that make extensive use of temporary files in the editing process.

Earlier generations of Smart Array controllers may require a firmware upgrade to enable physical drive write cache

(Table 1). This feature is supported for all firmware versions of the current genereation of Smart Array controllers.

Table 1. Smart Array controller firmware requirements for enabling physical drive write cache

Controller family

Required firmware

E200

1.72 and above

P400/P400i/E500/P800

4.06 and above

P410/P411/P212/P812

All versions

Additional resources

For additional information on the topics discussed in this article, visit:

Resource

URL

Microsoft Support article on use drive cache

with SQL Server

http://support.microsoft.com/kb/234656

Does HP Enclosure Dynamic Power Capping affect individual server blade

performance?

HP has designed Enclosure Dynamic Power Capping technology specifically for BladeSystem enclosures. It allows your

administrator to set a power cap on an enclosure by using either HP Insight Control power management (ICpm) version 2.0

or later, or Onboard Administrator (OA) firmware version 2.30 or later

. The OA monitors and maintains the enclosure’s

power cap by adjusting cap levels on individual server blades. Special hardware on each server blade lowers the processor

performance states (P-states), throttles the processor clock, or both, to keep the server’s energy use below the cap.

The total power for an enclosure includes the power used by the managed server blades as well as the power used by

unmanaged devices such as I/O peripherals (switches) and cooling fans. The OA cannot control the power use of

unmanaged devices, so it calculates a blade server power budget that represents the maximum amount of power the servers

can consume. With the blade power budget as its limit, the OA software uses a sophisticated algorithm to increase the

See the Blade Servers Support Matrix at http://h18004.www1.hp.com/products/servers/management/dynamic-power-capping/support.html.