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​Since FLARE R4.30 of CLARiiON, FAST Cache was introduced to boost the performance of the entire storage system. Considering there is no sufficient DRAM cache in middle range storage system, the industry developed a concept of Cache Tier, which leverages SSD as a complement of DRAM Cache to improve system performance.​

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​Q1: Do we need a license for FAST Cache?​

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​A1: Yes, a license is needed to enable FAST Cache. FAST Cache is included in FAST Suit of VNX storage system. Please contact pre-sales to get more information.​

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​Q2: Would the installation of FAST Cache affect the storage system operation?​

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​A2: Yes, performance would be degraded during the installation of FAST Cache. The system disables read/write cache to re-allocate memory space because FAST Cache bitmap needs memory. We would recommend installing FAST Cache under a maintenance Window whenever possible. [​​emc272105​​]​

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​Q3: As FAST Cache checks every incoming I/O against memory map, would it have performance penalty?​

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​A3: The cost of memory map checking is really low. As compared to the performance boost bringing to the whole system, we can ignore the penalty from memory map checking.​

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​Q4: FAST Cache is a “Cache”, would we suffer data loss incurred by power failure?​

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​A4: The data of FAST Cache is stored in the SSD persistent storage which is different than volatile physical RAM.​

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​Q5: Do we need capacity planning before FAST Cache deployment?​

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​A5: Yes, we do. Please engage EMC professional service to better understand the hotspot of your production workloads. We must make sure there is always sufficient storage space for FAST Cache data promotion.​

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​Q6: Is FAST Cache suitable to all kinds of application workloads?​

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​A6: No. FAST Cache prefers data with high locality of reference. For more detailed information, please refer to ​​emc278482​​and ​​emc251589​​.​

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​Q7: Is FAST Cache actually RAID1 mirror?​

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​A7: Yes, FAST Cache drives are all RAID 1 for high availability. Please refer to ​​emc251589​​ for configuration best practices for FAST Cache drive positioning.​

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​Q8: I did not see any FAST Cache I/O promotion through UnisphereAnalyzer, why?​

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​A8: The system always services I/O through DRAM Cache as possible as she can. If you were in a test environment, disabling DRAM Cache could show the FAST Cache promotion immediately.​

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​Q9: Does hot spare support FAST Cache?​

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​A9: Yes, you must have a SSD spare and proactive hot spare is also supported.​

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​Q10: What is the difference between FAST Cache and FAST VP?​

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​A10: FAST Cache is designed for performance boosting while FAST VP aims to reduce TCO (total cost of ownership). Although both solution involve SSD for better performance, but different working mechanism make them have different effect on performance gains. [​​emc296541​​]​

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​Author: Steve Zhou​

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