Choosing hard drives when purchasing storage is a challenge that all customers face. In a corporate environment, the storage head unit is selected by functionality, and the disk subsystem is selected by performance and volume. To choose the optimal disk subsystem, it is necessary to understand the main differences between the disks used in storage, their features and applications.

Enterprise-class storage currently uses four types of disks:

SAS / NL-SAS 7K 3.5"/2.5"

• Large capacity. The actual operational capacity of 7K drives is now 12TB, including for Class A brands, which test new formats for the longest time before shipping as part of their storage systems.

• Low speed, suitable for a number of undemanding tasks. The 7K drive produces about ~100 IOPS.

• Maximum storage efficiency in terms of price/volume ratio.

As a rule, the format of these disks is 3.5", although there are also 2.5" implementations. In real conditions, the MTBF (average lifetime) is 3-5 years, after which the disks begin to actively "crumble".

This type of disk is recommended to be chosen for archival storage of large amounts of data, for video surveillance systems, for "slow" files that are rarely accessed.

SAS/NL-SAS 7K drives are recommended as part of RAID6 because the tasks for which 7K drives are usually selected do not require high speeds, but require high disk space utilization. At the same time, it is extremely undesirable to use RAID5, because RAID5 rebuld on loaded systems in the event of a disk failure can take up to 2-3 weeks and all this time the remaining disks experience the maximum load and the chance of failure of the second disk is very high, which will entail the loss of all data.

SAS 10K 2.5"

• Average speed. This type of disk demonstrates an increase in performance by one and a half to two times compared to 7K disks due to the higher spindle rotation speed and a smaller physical area of the disk (the disk head makes a shorter path when searching for the requested data). For reference, we can assume that a 10K disk as part of a storage system produces ~150 IOPS.

• Sufficient capacity. Disks up to 2.4TB are available for order in the storage system, which is sufficient for the tasks performed.

These disks are in steady demand for databases (1C, SQL, SAP, etc.)

It is recommended to use RAID1 for small moons or critical data, RAID10 for all fast business tasks.

SAS 15K 2.5"

• High cost. These drives are about twice as expensive as 10K.

• Legacy architecture. Due to low demand and high price, this architecture is recognized as unpromising, its development is suspended.

• Small capacity. At the moment, as part of the storage system from those manufacturers who still support this format, you can order a disk with a maximum capacity of 900GB.

• High speed. You can get approximately ~200 IOPS from a single 15K drive.

This type of disc from many brands of class A is no longer available for order, the rest smoothly withdraw it from the product lines. Due to the increased speed, the reliability of the drives is lower than that of 10K.

Not recommended for use as part of storage.

SSD 2.5"

• The largest volume. As part of the storage systems, 15.3TB disks are available for order, at the end of this year 32TB disks are expected to go on sale, in 2018 - 64TB, in 2019 - 128TB.

• Highest reliability. Surpasses all types of disks. Storage manufacturers extend the warranty on flash-storage up to 7 years without an increasing factor.

• Highest performance. The 15.3TB drives included with the storage system deliver 200,000 IOPS.

• Moderate cost. At the price of SSDs, drives are approaching SAS 15K drives, and the effective cost of storage, taking into account the disposal of disk space when using recommended RAID types, is already approaching the cost of storage on 10K disks.

• Fast-paced technology. The huge demand in the corporate environment for solid-state drives allows the technology to develop rapidly, the available capacity of disks is constantly increasing and the cost of production is decreasing.

• Other benefits, which we describe in detail in this article.

SSD is recommended to be used for any ultra-fast business tasks (billing, database, etc.), as a top level of storage or as a cache for slow disks. Widely popular at the moment hybrid configurations SSD + SAS 7K are cheaper, more productive and more reliable than systems completely on 10K disks.

The optimal RAID type for an SSD is RAID5 with Hot Spare. RAID6 significantly loads the storage system, thereby reducing overall performance; using RAID10 is too wasteful and pointless, because according to the reports of SSD manufacturers, it is a hundred times more reliable than an HDD.

And in conclusion, we recall that hard drives in data storage systems are consumables, any disks fly out with one frequency or another. This fact should be borne in mind from the very beginning, plan the disk space taking into account the types of RAID recommended in the article, and also be sure to budget for the extension of system support for the period from the fourth year of ownership and beyond, until the end of the planned life of the storage system.