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SmartCache Feature Guide

OceanStor V5 Series V500R007

This document is applicable to OceanStor 5110 V5, 5300 V5, 5500 V5, 5600 V5, 5800 V5, 6800 V5, 18500 V5, and 18800 V5 storage systems. It describes the implementation principles and application scenarios of the SmartCache feature. Also, it explains how to configure and manage SmartCache.
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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
Working Principle

Working Principle

This section describes the working principle, data read process, and SmartCache partition management of the SmartCache feature.

Related Concepts

  • SmartCache pool

    A SmartCache pool consists of SSDs and can act as a supplement to a RAM cache. A SmartCache pool is used to store hotspot data.

  • SmartCache partition

    SmartCache divides a SmartCache pool into multiple partitions. Different services can share the same partition. In addition, they are allowed to use different partitions without a mutual impact, thereby providing more cache resources for mission-critical applications to ensure their performance.

Principle Description

SmartCache is a feature that uses SSDs to provide read cache resources. Working with the RAM cache, SmartCache accelerates the read of hotspot data to improve the performance of the entire storage system.

SmartCache manages SSD resources based on SmartCache pools and SmartCache partitions.

  • SmartCache pool
    • A SmartCache pool manages all SSDs in the local controller to ensure that resources allocated to each SmartCache partition come from different SSDs, thereby preventing load imbalance among SSDs.
    • By default, a storage system generates a SmartCache pool on each controller.
  • SmartCache partition
    • SmartCache partitions provide fine-grained SSD cache resources for services. (The granularity ranges from 4 KB, 8 KB, 16 KB, 32 KB, 64 KB, and 128 KB, adaptive to front-end I/Os. To be specific, SSD cache resources of different granularities are requested based on the sizes of I/Os delivered by hosts.)
    • A storage system creates a default SmartCache partition for dual-controller. In addition, dual-controller supports a maximum of eight user-defined partitions.
    • A SmartCache partition must reside in the same engine as the LUN or file system that needs to be accelerated.

Data Read Process

Before SmartCache is enabled, hotspot data is mixed with cold data on hard disk drives (HDDs). HDDs require seek time to read data, which adversely affects the data read performance.

After SmartCache is enabled, the storage system migrates hotspot data to the SmartCache pool of SSDs. Compared with HDDs, SSDs are relieved of seek time; therefore, the read performance of hotspot data is remarkably improved.

The SmartCache data read process can be either of the following circumstances:

  • Read cache hit
  • Read cache miss

Read Cache Hit

A read cache hit means that an application server reads the desired data from the SmartCache pool, as shown in Figure 1-1.

Figure 1-1 Read cache hit process
  1. A read I/O request from the application server is first delivered to the RAM cache before arriving at LUNs or file systems.
  2. If a read cache miss occurs, the RAM cache transfers the read I/O request to the SmartCache pool.
  3. If a cache hit occurs in the SmartCache pool, an SSD read request is initiated and data read from SSDs is returned to the RAM cache.
  4. The RAM cache returns the data to the application server.

Read Cache Miss

A read cache miss means that an application server reads the desired data from HDDs, as shown in Figure 1-2.

Figure 1-2 Read cache miss process
  1. A read I/O request from the application server is first delivered to the RAM cache before arriving at LUNs or file systems.
  2. If a read cache miss occurs, the RAM cache transfers the read I/O request to the SmartCache pool.
  3. If a read cache miss occurs again in the SmartCache pool, this cache read result is returned to the RAM cache.
  4. The RAM cache transfers the I/O read request to the back-end HDDs.
  5. HDDs return data to the RAM cache.
    NOTE:

    Data returned from HDDs is stored in the RAM cache.

  6. The RAM cache returns the data to the application server. In addition, the RAM cache synchronizes the data to the SmartCache pool. If the capacity of the SmartCache pool is insufficient, the SmartCache pool evicts old data in chronological order to release memory resources.

SmartCache Partition Management

SmartCache partition management allows each service to use its own SmartCache partitions, preventing a mutual impact between applications of different types and ensuring the overall quality of service (QoS).

You can configure partition sizes for different services to strike an optimal balance between services and performance. For example, you can restrict cache resources for non-critical applications and grant more cache resources to critical applications to ensure their performance. Figure 1-3 shows the cache partition management process.

Figure 1-3 SmartCache partition management process

  • Services 1 and 2 use SmartCache partitions 1 and 2 respectively. The other services share the default SmartCache partition.
  • SmartCache partitions provide fine-grained SSD cache resources. Different services can share one SmartCache partition or use different SmartCache partitions. The SmartCache partitions are isolated from each other.
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Updated: 2019-07-11

Document ID: EDOC1000181482

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