Easy Tier is even better than we thought!

IBM storage architects and IBM Business Partners are encouraged to use Disk Magic to model performance when recommending disk systems to meet a customer requirement. Recently v9.1 of Disk magic was released and it listed nine changes from v9. This little gem was one of them:

“The Easy Tier predefined Skew Levels have been updated based on recent measurements.”

Knowing that sometimes low-key mentions like this can actually be quite significant, I thought I’d check it out.

It turns out that v9 had three settings

  • low skew (2)
  • medium skew (3.5)
  • heavy skew (7)

While v9.1 has

  • very low (2)
  • low (3.5)
  • intermediate (7)
  • high (14)
  • very high (24)

If I take a model that I did recently for Storwize V7000 customer:

  • 40 x 450GB 10K 2.5″ drives RAID5
  • 5 x 200GB SSDs RAID5
  • plus hot spares
  • 16KB I/O size
  • 70/30 read/write ratio

The v9 predictions were:

  • 12,000 IOPS at light skew (2)
  • 13,000 IOPS at medium skew (3.5)
  • 17,000 IOPS at heavy skew (7)

I have generally used medium skew (3.5) when doing general sizing, but the help section in Disk Magic now says “In order to get a realistic prediction, we recommend using the high skew (14) option for most typical environments.  Use the intermediate skew level (7) for a more conservative sizing.”

The v9.1 predictions are now:

  • 12,000 IOPS at very low (2)
  • 13,000 IOPS at low (3.5)
  • 17,000 IOPS at intermediate (7)
  • 28,000 IOPS at high (14)
  • 52,000 IOPS at very high (24)

So what we can see from this is that the performance hasn’t changed for a given skew, but what was previously considered heavy skew is now classed as intermediate. It seems that field feedback is that I/Os are more heavily skewed towards a fairly small working set as a percentage of the total data. Easy Tier is therefore generally more effective than we had bargained on. So apparently I have been under-estimating Easy Tier by a considerable margin (the difference between 13,000 IOPS and 28,000 IOPS in this particular customer example).

The Disk Magic help also provides this graph to show how the skew relates to real life. “In this chart the intermediate skew curve (the middle one) indicates that for a fast tier capacity of 20%, Easy Tier would move 79% of the Workload (I/Os) to the fast tier.”

For more reading on Easy Tier see the following:

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