What Is Paired Storage?

Datacenter servers commonly store data on shared networked storage arrays, but the laptops and desktops used by individuals continue to rely on internal hard disk drives. Enterprise arrays employ a number of performance enhancements, including large RAM caches and fast flash storage, but end users are stuck with single disks. This is changing, however, as users increasingly deploy flash SSD for performance while retaining a hard disk as well for capacity. This is “paired storage”, and it’s a major trend.

Tiering and Pairing Storage

Tiered storage is commonly used in the enterprise server space. Storage pros deploy a few different categories of storage and assign them to servers as needed: Slower RAID-5 for capacity and faster 15k rpm mirrors or flash SSD for performance. Some systems now have the ability to actively move data on the fly between according to demand, often on a block-by-block basis.

Tiered storage makes sense in the enterprise because storage arrays are shared by multiple servers. But it was impractical to deploy multiple storage types in a desktop, and often impossible in a laptop. The typical client machine still has a single hard disk drive, often chosen for capacity rather than performance.

Avid gamers challenged the status quo, however, by deploying faster hard drives, multiple drive types, and even RAID storage. Drives like the 15k rpm Western Digital VelociRaptor and OCZ Vertex SSDs took this enthusiast market by storm, and the excitement about the massive performance offered by these devices is spilling over into other markets.

Today, high-end laptop and desktop buyers are rapidly adopting dual-drive strategies, pairing an SSD for booting and applications with a hard disk drive for capacity. PC makers are responding, offering built-to-order paired storage configurations in high-end machines like the Apple iMac.

Leave The Optical Drive At Home

A common aftermarket paired storage configuration replaces the optical drive with a hard disk drive (HDD) using a specialized bracket. This is especially common in the Apple world, with MacBook Pro users snapping up the MCE OptiBay and OWC DataDoubler and moving their DVD SuperDrives to external enclosures.

One reason for this shift is the lagging capacity of optical media. Today’s hard disk drives have up to 100 times the capacity of a DVD, and “ripped” media files are even more compact. A 1 TB hard disk drive can hold over a thousand hours of high-definition video and can serve double duty storing virtual machine disks, music, and other space hogs as well.

Another reason to skip the DVD drive is a shift in the distribution of software from optical disc to online download. Apple has rapidly moved to Internet-based distribution with their Mac App Store, and independent publishers commonly rely on digital downloads rather than box-and-disc distribution. Many users simply no longer need an optical drive.

Fitting an SSD in place of a DVD or Blu-Ray drive is something of a challenge, however. The Slim SATA connectors used by optical drives is mechanically incompatible with laptop hard disks (microSATA), as is the drive bay mounting screw locations.

Data Placement Difficulties

But mainstream operating systems like Microsoft Windows 7 and Apple Mac OS X are not inherently suited for paired storage. No desktop operating is able to make optimal use of an SSD and hard drive by dynamically placing data according to performance demands or frequency of access. They will not even combine the SSD and HDD into a single logical drive.

Instead, users must manually configure their storage, often placing most of their data on the SSD and moving data to the hard disk by hand. This wastes valuable flash capacity and limits the effectiveness of a paired storage configuration. But users are willing to sacrifice some price efficiency for the performance they get from the SSD.

Stephen’s Stance

Paired storage is a growing trend in the laptop computer market, with many high-end machines sporting both a SSD and hard disk drive. But it remains a game for the rich, adding many hundreds of dollars to the cost of a computer, and manually placing data is inefficient. It will be interesting to see if future operating systems bring better support for paired storage, and if it will reach into the server world.