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Storage guide, 3: storage virtualisation and disc reliability

We have often been warned about exceeding 80% of a disc's capacity, yet storage virtualisation promises to allow up to 100% storage utilisation. How do you reconcile these two storage considerations?

There's a lot of focus on storage capacity rather than storage performance. One of the marketing pitches behind storage virtualisation is to increase storage utilisation so that you don't need to buy so many drives. Unfortunately, "how" the storage is being utilised remains a significant consideration. I've heard "rules of thumb" cautioning against running hard drives at more then 50% capacity. Some concerns are myths; some are based on real issues, like application requirements or workload characteristics.

Still, there is an increasing "storage to server I/O performance gap" where the disc drives are offering more capacity, but the performance isn't keeping up. If you're going to drive utilisation up, you also need to consider the performance impact on your service level. You may need faster storage controllers/arrays, caching devices or other technologies to accelerate your effective storage I/O. Ultimately, the top utilisation level is the highest utilisation that you can drive to while still meeting an acceptable response time or service delivery time -- don't focus just on capacity utilisation. Look at storage performance, availability and reliability. Also keep in mind that more disc drives do not always equate to more performance depending on the type of storage system and controller configuration.

Go back to the beginning of the Disc Hardware FAQ Guide.

 

How does vibration really influence disc reliability and performance? How can I minimise disc vibration?

Certainly any drive produces some level of vibration, and that vibration can then be carried to other drives through the physical drive trays or frames in an array. The concern is that vibration can force the afflicted drive(s) to reseek and reposition before a read/write operation -- even cause read/write errors. In extreme cases, excess vibration can adversely affect a drive's reliability. This was particularly notable in low-cost ATA and SATA drives with extremely high storage densities.

 

However, not all ATA/SATA drives are impacted equally by vibration. If I take four or five SATA drives and bolt them together firmly and place them on a hard surface, the resulting vibration can indeed affect performance. The truth is that cheap (non-mission critical) packaging and enclosures may not do a very good job at suppressing vibration. Still, many enclosures today will include some type of vibration damping. High-end, high-density arrays are often designed with careful vibration damping and airflow cooling schemes.

 

Remember that this only relates to "normal" operating conditions. If you put the disc array on an airplane or truck, you'll absolutely need to take added measures to minimise shock and vibration when the drive is in use. In those extreme cases, you might opt for hybrid drives or even full solid-state drives instead of traditional magnetic hard drives.

Go back to the beginning of the Disc Hardware FAQ Guide.


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This was first published in June 2007

 

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