Storage Subsystems Disk Types

This is the first Blog I am doing in a series on Storage and Disks. It is important to understand the “Black Box” so that the proper Architecture can be designed to maximize the value of the technology investment.

Disks Types

The type of hard drive utilized in the host server or the storage array will have the most significant impact on the overall storage architecture performance. The critical performance factors for hard disks are the interface architecture (for example, SCSI, SATA, SAS, SSD and FC), the rotational speed of the drive (7200, 10k, 15k RPM), and the average latency in milliseconds. Additional factors, such as the cache on the drive, and support for advanced features, such as Native Command Queuing (NCQ), can improve performance. As with the storage connectivity, high IOPS and low latency are more critical than maximum sustained throughput when it comes to performance capacity. When selecting drives, this translates into selecting those with the highest rotational speed and lowest latency possible. Utilizing 15k RPM drives over 10k RPM drives can result in up to 35% more IOPS per drive.

Storage Tiers

Storage administrators are increasingly revisiting tiered storage strategies to lower asset and operational costs, and to improve application performance. This is accomplished by establishing and assigning tiers according to drive cost and performance. In its simplest form, this means assigning non-critical data to higher-capacity and lower-cost drives.

Recent technology improvements aiding the success of tiered storage include new solid state disks (SSDs), improved Serial Attached SCSI (SAS) and more efficient data movement, classification and quality of service (QoS) software. These developments provide more tiers of storage to choose from, and facilitate moving data to the optimal available storage tier.


Small Computer System Interface (SCSI) is a parallel interface used for attaching peripherals such as hard disks. SCSI drives are rapidly being replaced by SATA, SAS, and Fibre Channel drives. SCSI drives are not recommended for storage configurations.


Serial Advanced Technology Attachment (SATA) drives are a low cost and relatively high performance option for storage. SATA drives are available primarily in the 1.5 GB/s and 3.0 GB/s standards (SATA I and SATA II) with a rotational speed of 7200 RPM and average latency of around 4 ms. There are a few SATA I drives that operate at 10k RPM and average latency of 2 ms that can provide alow cost storage solution.


SAS drives are typically much more expensive than SATA drives but can provide significantly higher performance in both throughput, and more importantly, low latency. SAS drives typically have a rotational speed of 10k or 15k RPM with an average latency of 2 to 3 ms.


Solid State Disk (SSD) is just another name for Enterprise Flash Drives (EFD) or Flash drives. Flash drives deliver vastly increased performance to database applications when compared to traditional Hard Disk Drives (HDD) both in transaction rates per minute as well as transaction response time. Since there are no moving mechanical parts, Flash drives consume up to 98% less energy per I/O than traditional HDD drives. Flash drives are especially well suited for low-latency applications that require consistently low read/write response times. Flash drives exhibit as much as 30 times improvement in IOPS over traditional HDD drive technologies.

EMC and Microsoft SQL Server Customer Advisory Team (SQLCAT) collaborated on a white paper that examines deployments of SQL Server 2008 using Flash drives. The white paper is for database administrators and storage architects who want to understand the implementations of enterprise Flash drives in Microsoft SQL Server environments to improve the performance of business applications and assist in reducing overall TCO.


  • Please review the EMC and Microsoft white paper on enterprise Flash drives technology compared to traditional Hard Disk Drives.

EMC Symmetrix DMX-4 Enterprise Flash Drives with Microsoft SQL Server Databases.


Fibre Channel (FC) drives are usually the most expensive and typically have similar performance characteristics to SAS drives but use a different interface. The choice of Fibre Channel or SAS drives is usually determined by the choice of storage array. As with SAS, they are typically offered in 10k and 15k RPM variants with similar average latencies.