HPE Smart Array (SR) controllers with cache use flash-backed write cache (FBWC): writes are posted to fast cache memory and completed to the host at once, then written to the drives later. An energy pack protects that cache during a power loss. If the energy pack is missing or failed, write caching is affected, so it is worth knowing where to check it. This guide explains the documented behaviour, how to read cache and energy pack status, and which cache settings you can change.
Before you start
- Know your controller model. Cache sizes and supported options differ by model; some settings are not available on every controller.
- The energy pack must only be installed, removed or replaced while the server is powered off and the AC power cords are removed.
- Have SSA, ssacli or iLO access, or console access for UEFI System Utilities.
- For background on write-back and write-through, see RAID controller cache.
How FBWC and the energy pack work
With write-back caching, the controller posts write data to cache and immediately returns completion to the OS, so a write finishes in microseconds instead of milliseconds. The controller writes the data to disk later, coalescing small writes and reordering commands. FBWC keeps cache contents in flash during a power loss. The energy pack only has to power the controller while DRAM contents are copied to flash, so there is no 48-hour limit as with older battery-backed cache; the data is written to the drives at the next power-up.
HPE documents two energy pack types:
- HPE Smart Storage Battery: an optional lithium-ion centralized backup source. The guides give a recharge time of two hours for the 96 W battery and one hour for the 12 W battery.
- HPE Smart Storage Hybrid Capacitor: battery-free; recharges in less than one minute. The SR Gen10 guide says it supports up to three Gen10 controllers; the SR Gen11 guide says up to two controllers.
The energy pack sits at the front of the server and plugs into a 14-pin connector on the system board; power reaches the controller through a 3-pin backup power cable. Type-a modular controllers (and the SR416ie-m Gen11 mezzanine controller) do not use that cable; their backup power is delivered through the PCI connector.
Step 1 — Check energy pack and cache status
- iLO: open Power & Thermal > Power and check the Smart Storage Energy Pack panel.
- SSA GUI: select the controller, click Configure, then under Controller Configuration Summary click View more details and scroll to Controller Status, which includes the cache and the energy pack.
- UEFI System Utilities: System Configuration > (controller) > Configure Controller Settings, then the Backup Power Source screen. Status is Failed, Not present, Charging or Charged.
- ssacli: the controller
showoutput includes Cache Status, Accelerator Ratio, No-Battery Write Cache, Cache Backup Power Source and Battery/Capacitor Status.
ssacli ctrl slot=0 showiLO Redfish also exposes the status (CacheSummary under the StorageController resource, and SmartStorageBattery under the Chassis resource). For alerting, the controller guides list Redfish alerts such as BatteryFailure, BatteryMissing, BatteryCharging, WriteCacheDegraded and WriteCacheDataLoss; send them out with iLO alerts.
Step 2 — Set the cache ratio
The cache ratio splits controller memory between read and write caching. HPE's default is 10% read and 90% write, which the guide calls the best ratio for most workloads; highly sequential reads, or reads of recently written data, may benefit from more read cache. The SR controller guides say you can change the ratio when the controller cache has backup power (battery or hybrid capacitor) and logical drives are configured; the CLI guide words this condition differently, so if your tool does not offer the change, check the guide for your controller.
- SSA: select the controller and open Modify Controller Settings, which includes the array accelerator cache ratio on supported controllers.
- ssacli: query the allowed values, then set read/write percentages.
ssacli ctrl slot=0 modify cr=?
ssacli ctrl slot=0 modify cr=25/75While HPE SR SmartCache is enabled, SSA does not offer Change Cache Ratio.
Step 3 — Turn controller caching on or off per logical drive
Disabling caching on one logical drive reserves the cache for the others, for example to favour a database volume.
- SSA: Tools > Cache Manager > Controller Cache > Modify Caching Settings, select the logical drives to cache, verify and click OK.
- UEFI (SR Gen11 guide): Manage Arrays > (array) > List Logical Drives > (logical drive) > Edit Logical Drive, Acceleration method: Controller cache or None.
- ssacli:
ssacli ctrl slot=0 ld 1 modify aa=enableoraa=disable.
Step 4 — Understand SSD Smart Path
SSD Smart Path lets I/O bypass the controller firmware and go straight to the SSDs. It speeds up reads for all RAID levels and writes for RAID 0. SSA enables it by default on volumes built from SSDs, and controller cache is not configured for those volumes. To use controller cache on an SSD volume, disable Smart Path first: in SSA go to Configure > Logical Devices, select the array, click Disable SSD Smart Path and Save, or run:
ssacli ctrl slot=0 array A modify ssdsmartpath=disableStep 5 — Drive write cache, bypass threshold and no-battery write cache
- Physical drive write cache is the cache inside each drive. In SSA it is under Modify Controller Settings > Physical Drive Write Cache State or Manage Drive Write Cache Policy, depending on the controller; in ssacli it is
modify dwc=enable|disable. HPE warns that enabling it can increase write performance but risks losing the data in the drive cache on a sudden power loss. - Write cache bypass threshold: writes larger than this value skip the write cache and go straight to disk. It applies only to non-parity RAID volumes and is set in multiples of 16 KiB in Cache Manager > Controller Cache > Modify Caching Settings.
- No-battery write cache (NBWC): lets the controller use write cache when no battery is present or the battery has failed. The default is disabled, and some controllers do not support it.
Warning: Enabling physical drive write cache or no-battery write cache means some acknowledged writes are not protected by the energy pack, so a power cut can lose or corrupt data. Use them only with a tested UPS and current backups. Power the server off and remove the AC cords before replacing an energy pack.
Frequently asked questions
Why does my new SSD logical drive show no controller cache?
Because SSA enables SSD Smart Path by default on SSD volumes and does not configure controller cache for them. Disable Smart Path on that array if you want controller caching.
Does this apply to HPE MR controllers?
The energy pack types are the same, but MR controllers set Write Through, Write Back or Always Write Back per logical drive and use StorCLI or MR Storage Administrator. See RAID on HPE MR Gen11 controllers.
Checklist
- Energy pack shows Charged/OK in iLO, SSA, UEFI or ssacli.
- Cache Status OK and accelerator ratio as intended (default 10% read / 90% write).
- Caching enabled on the logical drives that need it.
- SSD Smart Path decision made for SSD volumes.
- Drive write cache and NBWC left disabled unless the risk is accepted.
- Battery and write-cache alerts forwarded from iLO.
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