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Promise Pegasus J4 — Initializing Volumes

Last updated 2013-07-03 - Send Feedback
Related: backup, RAID, RAID-0, SSD, storage, Thunderbolt

Update: see issues.

The 4-bay Promise Pegasus J4 is available as 0GB solution (you supply the drives) or a 2TB (4 X 500GB) solution.

The Pegasus J4 acts as a HBA (host bus adapter), so up to four drives can be installed into it, and each hard drive or SSD appears separately in Disk Utility.

That flexibility can be very useful, but it does not include high performance and fault tolerant hardware solutions such as RAID-5 (striping with parity). However, RAID-1 mirroring can be used, or with some low-level configuration effort at the command line, RAID 0+1 (a mirror of stripes).

Example drive install

Shown below, four OWC Mercury Elite Pro 6G SSDs are installed in the Pegasus J4 (cover is removed for the photo). Since each of these drives is by itself a very high performance device, a setup like this is a very high performance arrangement.

As shown, these are 240GB SSDs, but they could just as well be 120GB or 480GB units, capacity being the question.

Possible ways to configure them include:

  • 4 X 240GB RAID-0 stripe producing ultra high performance 960GB volume.
  • Two pairs of 2 X 240GB RAID-0 striped drives (480GB each volume).
  • Two pairs of 2 X 240GB RAID-1 mirror drives (240GB each volume).
  • Four separate 240GB volumes.
  • 3 X 240GB = 720GB RAID-0 stripe + one 240GB volume.
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How about a 3-drive stripe + 4th drive for clone backup?

That last 3-drive stripe configuration is interesting: a 3-drive stripe provides essentially the same performance as a 4-drive stripe due to the Thunderbolt bus speed limit.

One could instead make that 4th drive a high capacity 2.5-inch 9mm-thick hard drive which serves as a backup clone of the 3-drive stripe (or Time Machine backup). This is not the same as RAID-5 striping+parity, but it achieves a similar reliability goal.

Promise Pegasus J4 with 4 X OWC Mercury Extreme Pro 6G SSD

Initialized as single drives

Here the four OWC 6G SSDs I installed were initialized as separate volumes d1/d2/d3/d4, as shown.

There are reasons to do this, namely to provide separate chunks of space (volumes) for different projects that do not need ultra high performance and are better off not taking the increased failure risks of RAID striping.

Promise Pegasus J4 with 4 X OWC Mercury Extreme Pro 6G SSD initialized singly

Initialized as a single volume — 4-way RAID-0 stripe

Here the four OWC 6G SSDs I installed were initialized as a 4 X 240GB RAID-0 stripe, producing a 960GB single ultra high performance volume.

Promise Pegasus J4 with 4 X OWC Mercury Extreme Pro 6G SSD initialized as 4-way striped RAID-0
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