Tiered storage is one of those terms which people use freely and assume that everyone understands. The basic concept is that you can reduce the cost of storage by assigning your data to different cost tiers of storage depending on the requirements of the data.

However, there are different technologies to address tiered storage. The technology you choose can make a great deal of difference in the value or benefts that can be derived. In fact some implementations of tiered storage may end up causing more complexity and cost. Here are a number of considerations which may be helpful.

Often I hear people talk about assigning data to tiers of storage based upon the “value” of the data, and they go through a very complicated study to classify the data by “value”. Some companies have spent several years on the classifcation of data and never finish.

First, I would say that all data is valuable or you shouldn’t be keeping it. Secondly I would separate out primary data from replicated data. Replicas are growing faster than primary data since we cannot afford to stop applications today to do backups, development/test, data transformation, data mining, data distribution, and disaster recovery, etc. Rather than disrupt the application server to make these copies, it is simpler to have the storage systems make those copies, especially if we want a consistent copy across a group of related volumes.

These copies do not have to be on the same tier of storage as the primary data. With storage virtualisation you can snap them off to lower cost tiers of storage in the same storage system or to lower cost, externally attached, storage systems. There are also technologies to reduce the time and capacity required for making copies such as copy on write and dynamic (thin) provisioning.

Another way to classify a tier is by performance. This makes sense if there is a signifcant difference in price/performance between the storage tiers. As you can imagine, the differences in performance and cost per GB between different types of storage media can be very signifcant. There are performance differences in rotation speed and RAID mapping which may make a difference for some types of workloads that are assigned to static tiers, but these differences may not justify the work to dynamically move data up and down tiers of storage on a frequent basis.

Today, movement of data between tiers of storage is done by volumes or fles, and moving large volumes and files is a very heavy workload that you might not want to do on a frequent basis. You can start by allocating a volume to a mid tier of storage initially and if it turns out that it needs need higher performance you can promote it to a higher performance tier with storage virtualisation. Storage virtualisation forgives you if you happen to make a bad choice with your initial allocation.

Data centres that implement disaster recovery classify applications on the basis of RPO/RTO and assign critical application data to storage systems which have the capability to do distance replication for business continuity. Typically if an application must recover in hours, it uses enterprise storage to do synchronous and/or asynchronous replication. Here again we can use dynamic (thin) provisioning to reduce the time and capacity needed for replication.

With the price and performance differences that we see today between fash drives, FC/SAS disks, and large capacity SATA disks, the benefts of tiered storage have become very compelling. Storage virtualisation makes it easy to copy, move, and replicate data between internal and external tiers of storage without disrupting the applications. Additional services like copy on write, and dynamic (thin) provisioning can decrease the workload needed to do the tiering.

Hubert Yoshida is VP & CTO Hitachi Data Systems, and is responsible for defining direction for Hitachi Data Systems.