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the Case for Fast Boot SSDs (100mS or less) for Petabyte SSD Backup Arrays

by Zsolt Kerekes, editor - February 2, 2010

Can SSDs be Made for AutoMAID?

Massive hard disk archives can reduce running costs by powering down most of the disk - based on useage patterns.

When a hard disk is powered it takes about 10 to 20 seconds (depending on the model) to become usable. Some enterprise - hard drives instead offer an alternative power saving mode in which provides faster start up -closer to 6 seconds - but smaller power savings. This strategy reduces electricity costs but provides slow random access () to infrequently used data. This technique - called AutoMAID - was pioneered by Nexsan Technologies in 2001. Many other vendors now offer similar features in their RAID systems, but they call the technique by differing names.

This hard disk technique is faster than that for typical tape libraries - which have random access times in the range 10S to 60S. But backup SSDs will offer a new dimension of possibilities for backup archive designers.

SSD ad - click for more infothe Need for Fast Startup SSDs

Like HDDS - most 2.5" SSDs designed for the high IOPS enterprise market typically consume 2x to 3x more power when they are reading or writing data than when they are not. And it is unusual for SSDs (except those designed for low power embedded applications) to support power management commands which improve much on this.

If we want to design an affordable massive array of SSDs for an archive systems - we would like the "standby" power consumption to be more like 10x to 50x lower than the standby mode.

Let's look at the feasibility of this. The switch side of that is easy - and already exists. But we need to ask the question - how long does a server class flash SSD take to become operational?

(OK I realize that this is an extremely simple view - because if you really wanted to do this you may be able to optimize the power management - but let's see if it would be worthwhile in this extreme case.)

In the current state of the SSD market - the time taken for an SSD to become operational from the time that power up reset is applied varies considerable depending on the type of internal cache, flash memory (MLC takes longer than SLC) and controller used. Here are some typical figures for 2.5" SATA SSDs. I've assumed the use of SATA SSDs - even though they have longer power up times and consume more power than PATA SSDs because if you are planning to build a cabinet which houses hundreds of SSDs - it's more realistic to assume they are distributed. This is not a notebook.
  • STEC ZEUS (SLC) - 2.5S
  • SMART Xcel-10 (SLC) - 1S.
Today's SSD market does not pay a premium for fast power up operation, But there is no technical reason why the power up time for a skinny flash SSD should take much longer than 100mS.

Below this figure - the requirements for fast start-up begin to conflict with other desirable characteristics such as
  • tight data management which helps the device power down reliably in the event of sudden power loss.
  • the indivisible minimum (or atomic) start-up time cannot be below about 10x a single write-erase cycles - because there may be pending internal housekeeping operations which have to be completed - before it's safe to initiate new R/W activity.
At one extreme end of this characteristics the user would get an enterprise bulk storage SSD array with the following system characteristics
  • power consumption - upto 50x lower than for the same capacity implemented by standard SSDs or low power HDDs.
  • random R/W IOPS and throughput in the powered and ready state - which is similar to that for average notebook SSDs
  • power up to ready time (for any random SSD) on the order of 50 to 100 milli-seconds. When powered up the device controller would keep the device powered for a minimum period of seconds set by the system.
  • high availability fault tolerant design

A new class of high integrity, low standby current, fast power up to ready flash SSDs (50mS to 100mS) would simplify the design of high density petabyte SSD arrays for bulk archiving and backup applications - because most of the SSDs in these arrays could remain unpowered for most of the time. It's possible to design such power management for SSDs using today's technologies. There are some overlaps with the requirements for fast boot notebooks too - but the greater data integrity requirements of SSD backup means these will probably be different products.
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