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Faster disk drive rebuilds: Hot Spots

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RAID might not be the best choice for high-capacity drives. It's time to rethink your rebuild strategies.
 

There's a lot of talk about shortening rebuild times for large capacity disk drives in today's storage environments. Fast rebuild technology is widely deployed nowadays, but plenty of users still don't think in terms of hardware RAID and individual drive rebuild times. And here's a new angle on the discussion: perhaps the best way to shorten rebuild times is to not have to rebuild in the first place.

Roughly 50% of failed SATA drives returned to vendors result in a diagnosis of "no trouble found" and are returned to service as replacement drives that typically function like new. That's because SATA drives were originally designed for lightly loaded desktops and laptops, rather than high-performance enterprise arrays, and they occasionally experience slowdowns in performance that result in a disk being diagnosed as non-responsive. As a result, several vendors have introduced technology to diagnose these issues and determine if the disk is actually failing or if it's just an intermittent slow down. This technology is important to understand because it reduces the risk of data loss due to the potential of a second drive failing during rebuild.

Before deciding on which approach or vendor best addresses your RAID rebuild challenges, let's look at how we got here. The term RAID, or redundant array of independent

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(or inexpensive) disks, was introduced in the late 1980s to describe a method of protecting disk drives in an array. Despite the standardization efforts of the now defunct RAID Advisory Board, most vendors developed protection schemes that met basic RAID definitions but varied widely in their implementations. No matter the strategy, RAID rebuild times across the board get longer as disk drive size increases. That's because there's more data to copy or rebuild from parity. In the event of a single disk drive failure in most RAID modes, data is left unprotected until the RAID rebuild is finished, and RAID rebuilds suck up significant processing power.

However, there are ways to keep data protected in the event of a single disk failure. Users can implement dual-parity RAID 6 -- which keeps data available in the event of a dual drive failure in a single RAID group -- or go as far as implementing remote mirroring technology to protect themselves not just from a drive failure but to keep data available in the event of a full site failure. But there are costs associated with each layer of protection added, and these need to be balanced against the value of the data to be protected; the overhead required to allocate capacity for data protection is, in some cases, three to four times the amount of data stored.

With the advent of high capacity TB-size Serial ATA (SATA) disk drives, the problem is compounded. SATA drives spin at less than half the speed of Fibre Channel (FC) drives, but hold up to 1 TB (twice the capacity of FC drives). The density of the drive doesn't make up for the slower rotation speed, however; average latency for a 7,200 rpm disk drive is more than two times the latency of a 15,000 rpm drive. With TB SATA drives, rebuilds could extend for multiple days, depending on how busy the system is, and become onerous to the point that they have an unacceptable impact on the business. There are significant cost advantages to storing data on large capacity drives: the price per MB is much less than high-performance FC drives and, thanks to their price advantage, SATA drives have been widely deployed in archive systems and scale-out storage architectures while higher performance FC drives have continued to hold court at the top storage tiers.

This was first published in January 2009

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