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November 20th, 2009 09:00
CX4 Power Failure Behavior
On a CX500, one SPS failure will disable write cache.
On the CX4 write cache will stay on with one SPS faulted - this is the new GWCA (Greater Write Cache Availability).
Single SP failure
Single vault drive failure
Power supply failures and fan failures
Single SPS failure
glen
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kelleg
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November 20th, 2009 10:00
No problem, your more complete explanation is much appreciated.
glen
jps00
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November 20th, 2009 10:00
Because the CX4 Power Failure behavior, and the CX4 Persistent Write Cache is so different from previous CLARiiONs, I thought it might be of some interest to post how it works here.
CX4 AC power failure behavior
The different generations of CLARiiON also behave differently to power failures. The CLARiiON CX4 series with its persistent cache continues operation though a wider range of failures than legacy CLARiiONs. (See the Vault and write cache availability section.) The CLARiiON CX-4 series behavior in a AC power failure is dependent on the failure cause, and the number SPSs installed. The AC power failure scenarios are power lost to:
• Both SPSs
• One SPS in a dual SPS system
• Only SPS in a single SPS system
• The non-SPS-protected-SP in a single SPS system
AC power loss to both SPSs: This is the most serious condition. An example of this is a complete data center AC mains failure. When AC power is lost, all SPSs will report "Battery Online", to the FLARE operating environment. FLARE will regularly poll the SPS status for a short period to determine if the failure is an intermittent power loss or a complete outage. If all SPSs continue to report "Battery Online" through the polling period, the dump of the write cache to the CLARiiON vault is started. After the write cache is written to the vault, FLARE continues to check the SPS state. if "Battery Online" state persists, the SPs and Bus 0 Enclosure 0 are powered off. When AC power returns, the SPs and Bus 0 Enclosure 0 are automatically power up and operation is restored.
AC power loss one SPS in a dual SPS system: This condition may occur if one of the two data center AC mains providing power to the CLARiiON fails. As in the full AC power loss scenario, the SPS reports "Battery Online" to the connected SP and the peer SP, which will still be running on AC power from its SPS. In this case, the SPS with no AC power input is shutdown by FLARE to prevent unnecessary discharge. Both SPs continue operation powered from the power supplies connected to the SPS that still has AC power. CLARiiON, write cache is still enabled.
AC power loss to the only SPS in a single SPS system: The SPS reports "Battery Online to its connected SP and the peer Storage Processor, which will still be running on AC power. Note, the peer has no connected SPS. If the SPS reports "Battery Online" through the polling period, the write cache dump to the vault is started. After the write cache dump completes, the storage system continues to operate without write cache at a reduced level of performance until AC power to the SPS is restored and the SPS battery is fully recharged.
AC power loss to the non-SPS-protected-SP in a single SPS system: The SP reports AC power fail to FLARE, but the storage system continues to operate on the power from the peer SP’s power supplies. Write cache remains enabled.
The above will be found in the forthcoming 'EMC CLARiiON Storage System Fundamentals for Performance and Availability, Rev. 1.1'. My apologies to Glen for Thread-jacking.