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Promise Pegasus J4 — SSD Speed (Incompressible)

Tested on the 2.7 GHz MacBook Pro with Retina display, which has a Thunderbolt connector.

This test is for incompressible data.

Update: see issues.

What is incompressible data ?

Some types of data cannot be compressed: ZIP files, videos, image files (JPEG, PNG, GIF, compressed TIF, some PSD and PSB files), compressed audio files (MP3, etc), some (but not all) digital camera raw format files—such file types are already compressed internally and cannot be compressed any smaller.

Read and write speed with incompressible data

The Sandforce controller in many SSDs including the OWC Mercury Elite Pro 6G SSD drops in write performance with incompressible data.

Read performance on incompressible data is generally unaffected, and since most scenarios see reads dominate, the write speed is often not terribly important so long as it is still reasonably fast.

Other factors

While one can wish that this write speed were unaffected by incompressible data, it is an adaptive behavior designed to extend the life of the SSD. Also, there are other behaviors that can cause slowdowns, and some brands start having trouble with increased capacity usage. So one should not ignore the context of real world usage.

Sustained transfer speed — incompressible data

This test is a worst-case: a pure random stream of data without repetition. Real-world data is usually not so unforgiving.

While the random data generation in the test runs in its own computing thread, there can be a small performance 'hit' as speeds increases, adding 10% to the 3 and 4 drive cases is probably realistic for real-world where there is no compute load.

Detailed conclusions follow below. In brief:

  • For most every real-world case, two SSDs provides outstanding performance. Using 3 or 4 SSDs is unlikely to help for anything but rare situations.
  • Critical performance environments where performance variation must be held to a tight level should stripe four SSDs. Otherwise, no particular benefit other than capacity.
  • Either two or three SSDs provide read performance on par with four SSDs.
  • A single SSD is still very fast! But the sweet spot for users looking for high performance at reasonable cost is a dual-SSD RAID-0 stripe.

Tested with diglloydTools DiskTester using the fill-volume test.

RAID-0 stripes of 2/3/4 SSDs + single SSD
(Data from diglloydTools DiskTester data graphed from fill-volume test)


  • With two or three or four SSDs striped, the read speed is essentially identical. Hence two SSDs are enough to deliver maximum speed for reads.
  • Two SSDs striped: write speed for incompressible data is about the same as for writing highly compressible data with a single SSD.
  • Three SSDs striped: write speed variation is at or above the average speed for two SSDs striped.
  • Four SSDs striped: nice “tight” read/write performance at ~800MB/sec.
  • Four SSDs striped: the Thunderbolt bus is still the limiting factor (or very close to it); performance has no practical difference from the compressible-data case.
Read speed for compressible vs incompressible data
Write speed for compressible vs incompressible data
RAID-0 stripes of 2/3/4 SSDs + single SSD for incompressible data
(diglloydTools DiskTester data graphed from fill-volume test)
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