Flush Time

Stephen — Mon, 19 Oct 2009 14:20:02 +0000

Single-parity RAID is under attack. Caching is the hottest trend in storage. The end of the high-performance disk drive is imminent. What happened? Increasing areal bit density has caused disk capacity to grow much faster than disk performance. A presentation at Storage Networking World by Ronald Bianchini of Avere exposed the mathematics of this phenomenon. Of course, hard disk platters are not getting larger – quite the opposite. But the bits are getting smaller, so the effect is the same:

Capacity increases exponentially based on the formula for the area of a disc: π times radius squared
Sequential performance increases algebraically based on the formula for the circumference of a disc: π times diameter

Therefore, sequential performance grows smoothly with disk density, but capacity increases much faster. Double the density of disk media and you can read twice as many bits in the same amount of time, but the disk now contains four times as much data. Iterate this a dozen times, a miracle performed regularly by hard disk drive manufacturers, and you have a serious bottleneck to both performance and reliability.

Disk capacity has outpaced performance over the last decade

Back in 2004, I gave this metric a name: Flush time. It is a simple calculation to answer the question, how long would it take to read the entire content of a hard disk drive? Let’s look at some real-world examples:

In 2000, a 45 GB Western Digital 450AA disk could stream data at 25.4 MB/s, requiring 30 minutes to flush every byte out its UDMA/66 interface. This was a massive and slow drive at the time – enterprise disks were much faster. A 2000 Quantum Atlas 10K II SCSI drive (36 GB and 31 MB/s) could flush in 19 minutes!
A 2004-era Seagate Barracuda 7200.7 boasted 160 GB ad averaged 44.5 MB/s, requiring about an hour for a full flush.
By 2007, high-performance drives like the Hitachi 15K450 had hit 450 GB and about 100 MB/s in sustained throughput, but flush times were well over an hour.
Today’s enterprise drives can push 200 MB/s and average 160 MB/s across the entire 600 GB of capacity. But this is still about an hour for a flush. But large-capacity SATA drives are much more popular for bulk storage. The Samsung Spinpoint F2 EcoGreen drive I use in my Drobo only delivers about 110 MB/s, requiring almost four hours to flush at 1.5 TB of capacity! Think this is unusual? Check out Hitachi’s popular E7K1000, which needs 2.5 hours at 1 TB and 118 MB/s.

What will happen to flush time over the next half decade if density continues to increase? How about 16 TB drives, 400 MB/s, and RAID rebuilds that last more than half a day?

Since (traditional) RAID rebuilds are directly impacted by flush time, today’s massive disk drives are killing RAID. And flush time is only the minimum required time – most RAID rebuilds take much longer! Then there is the issue of media reliability!

Note: Yes, I know there are alternative RAID schemes that get around this problem. Far from ignoring that point, I’ll be promoting these in future posts! Stay tuned for more on these topics…

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© sfoskett for Stephen Foskett, Pack Rat, 2009. | Flush Time
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Flush Time

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