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Storage Tiering –
LUN and sub-LUN tiering & cache tiering explanantion

tierIn the perfect world storage systems consists of SSD flash drives with a lot quantity of RAM memory which works as a cache. Then you don’t have to worry about read/write latencies (unless the network part is a bottleneck). It is a perfect world, usually modern storage system consists of mixed SSD  and SAS (serial attached SCSI) disks or even Serial ATA  disks as a proper storage.

Storage tiering is a technique of establishing a hierarchy of different storage types (tiers). This enables storing the right data to the right tier, based on service level requirements, at a minimal cost. Each tier has different levels of protection, performance, and cost. For example, high performance solid-state drives (SSDs) or FC drives can be configured as tier 1 storage to keep frequently accessed data and low cost SATA drives as tier 2 storage to keep the less frequently accessed data. Keeping frequently used data in SSD or FC improves application performance. Moving less-frequently accessed data to SATA can free up storage capacity in high performance drives and reduce the cost of storage. This movement of data happens based on defined tiering policies. The tiering policy might be based on parameters, such as frequency of access. For example, if a policy states “move the data that are not accessed for the last 30 mins to the lower tier,” then all the data matching this condition are moved to the lower tier.
The process of moving the data from one type of tier to another is typically automated. In automated storage tiering, the application workload is proactively monitored; the active data is automatically moved to a higher performance tier and the inactive data is moved to higher capacity, lower performance tier. The data movement between the tiers is performed non- disruptively.
There are three techniques of storage tiering implemented in a block-based storage system: LUN and sub-LUN tiering, cache tiering, and server flash-caching.

LUN and sub-LUN tiering

lun and sub-lun tiering

The process of storage tiering within a storage system is called intra-array storage tiering. It enables the efficient use of SSD, FC, and SATA drives within a system and provides performance and cost optimization. The goal is to keep the SSDs busy by storing the most frequently accessed data on them, while moving out the less frequently accessed data to the SATA drives. Data movements executed between tiers can be performed at the LUN level or at the sub-LUN level. The performance can be further improved by implementing tiered cache.
Traditionally, storage tiering is operated at the LUN level that moves an entire LUN from one tier of storage to another. This movement includes both active and inactive data in that LUN. This method does not give effective cost and performance benefits. Today, storage tiering can be implemented at the sub-LUN level. In sub-LUN level tiering, a LUN is broken down into smaller segments and tiered at that level. Movement of data with much finer granularity, for example 8 MB, greatly enhances the value proposition of automated storage tiering. Tiering at the sub-LUN level effectively moves active data to faster drives and less active data to slower drives.

Cache Tiering

cache tiering

Tiering is also implemented at the cache level. A large cache in a storage system improves performance by retaining large amount of frequently accessed data in a cache; so most reads are served directly from the cache. However, configuring a large cache in the storage system involves more cost. An alternative way to increase the size of the cache is by utilizing the SSDs on the storage system. In cache tiering, SSDs are used to create a large capacity secondary cache and to enable tiering between DRAM (primary cache) and SSDs (secondary cache). Server flash- caching is another tier of cache in which flash-cache card is installed in the server to further enhance the application performance.

server fllash caching

Server flash-caching technology uses intelligent caching software and a PCI Express-based (PCIe) flash card installed on the compute system. This dramatically improves application performance by reducing latency, and accelerates throughput. Server flash-caching technology works in both physical and virtual environments and provides performance acceleration for read-intensive workloads. This technology uses minimal CPU and memory resources from the compute system by offloading flash management onto the PCIe card.
It intelligently determines which data would benefit by sitting in the compute system on PCIe flash and closer to the application. This avoids the latencies associated with I/O access over the network to the storage system. With this, the processing power required for an application’s most frequently referenced data is offloaded from the back-end storage to the PCIe card. Therefore, the storage system can allocate greater processing power to other applications.

Pictures used in this article originate from EMC resources

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