tech: Mac Direct Attached Storage (DAS) Disk Array

silver hard drive interals

tech: Mac Direct Attached Storage (DAS) Disk Array

OK, getting a Direct Attached Storage (DAS) is useful if you run out of storage and your Network Attached Storage (NAS). The main reason for this is bandwidth, if you use a Thunderbolt 4 DAS for instance, then you are running at 40Gbps vs Gigabit Ethernet at 1Gbps or if you are direct connected, then you could be running at 2.5Gbps.

This won't matter too much with a traditional mechanical disk drive which is limited to 400MBps (4Gbps) but it does make a big difference if using an NVMe drive (2.7GBps or 30Gbps)

Interfaces in a Disk Array

There is a relatively large and confusing collection of interfaces, so the first thing is to figure out what you need inside the system, but for SSDs, you can use either:

  1. U-2 (aka SFF-8639) costs a fortune but works up to 32Gbps. And the main thing to understand is that it is essentially 4x PCI-E 3.0 channels and supports the NVMe standard protocol. As an aside, the input to the disk array is mini-SAS (SFF-8643) and there is a new U.2 connector
  2. M.2 (aka NGFF). This is a cheaper standard than U.2 and is used for SSD, this is B or B+M running at 16Gbps, and the M notch only is 32Gbps.

Then for hard disks:

  1. SAS. This uses SFF8482 connectors and those drives are 10x more reliable, so if you can get SAS and it is not too much more expensive, that's a win. There is SAS-2, 3 and 4 (6Gbps, 12Gbps and 22.5Gbps). It is not for SSDs and doesn't support NVMe.
  2. SATA Express. This is an improvement to SATA and works up to 16Gbps. it is the cheapest interface for SSDs as an example

Then there are standards that

  1. SATA. This is the relatively slower or cheaper interface with SATA 3.0 (6Gbps) being the fastest and it doesn't support NVMe, so not useful for SSDs. But 6Gbps is plenty for traditional hard disks. This is related to the disk-spinning speed so a 7,200 RPM drive ru8ns at 1Gbps with a maximum sustained for 204MBps (2Gbps) fits well into SATA 3.0. mSATA is the version for offboard drives (up to
  2. PCI Express. This is used if you are putting SSDs directly onto the computer bus, it can be divided into how many lanes (each lane in PCI-e 3.0 is 8Gbps), so X16 is 16 lanes, X8 is 8 lanes. The most common for SSDs on PCI Express is X4 or 32Gbps. These typically implement the NVMe protocol

Disk Array Choices

There are not that many Disk Arrays that support the Mac, but Apple Insider (and iMore) does a good overview, but most have been converted into OWC offerings, the Akiteo is now an OWC as an example.

The other choices are the CalDigit T4 RAID that is also a combination dock and disk array. As a dock it has a DisplayPort and comes with a CalDigit RAID installer, however you can't this as a bare enclosure, the cheaper thing is an 32TB version (8TB drives) for $2.3K so not that much density. The same is true with the Western Digital G-Speed Shuttle which comes with drives.

USB C Dual Dock: Low Cost

If you don't need the Thunderbolt 3 performance at 40Gbps, then you can get the OWC Drive Dock USB-C Dual Drive for $99. This is perfectly good for backup and ordinary purposes, but won't have the extreme speed of Thunderbolt.

Picking a Good Drive Array: OWC $900 for 8-bay or $499 for 4-bay no-RAID

I have a cheap dual-disk Thunderbolt dock which works well with Apple's built-in RAID-1 and RAID-0 support. But now that I need a bigger DAS that can hold more. A 4-disk array for instance can support 4x20TB as a RAID10 array but then you need the $100 SoftRaid system from OWC. The cost for this from OWC is $499 for a 4-bay and $799 for an ThunderBay 8 8-bay so there is quite a discount for getting an 8-bay even with the $100 SoftRAID XT, you it is $900.

The low cost option is to go with a OWC ThunderBay 4 without the RAID and just create two sets of RAID1 drives so a pair of 20TB storage, although having a single volume that is 40TB is more flexible (assuming you are using Seagate X20 drives).

ThunderBay Flex 8 and Flex 1U4

As an aside, if you are doing some major production work and the cost is no object, the new OWC ThunderBay Flex 8 is pretty cool if expensive at $1,400. It is a combination Thunderbolt Dock with USB 3.1 and USB-C ports, SD, and CFexpree cards, has two PCI-e slots (not that useful with Apple Silicon for GPUs, but it does have a hardware SATA controller in it for free and you can put in a DeckLink SDI PCI-e card) and downstream, there is a 15W Thunderbolt port for down. So it is like getting a $300 Thunderbolt dock which is pretty cool.

Then it has U.2 SSD slots in the top 4 drive bays. These are much faster than SATA in typical 2.5" SSDs running at 32Gbps vs 16Gbps, so you can put these into RAID-0 for scratch storage for huge video files. Note that at least for me, ordinary mortals are going to get along fine with say a single SSD running 16Gbps which will let the SSDs run at 2GBps (the U.2 can run up to an extraordinary 5GBps or 32Gbps).

And if you need a rack mounted system to go ith your Mac Studio, the OWC Flex 1U4 has four drives and full U.2 support. It is also a dock with USB-C and USB-A 10Gbps ports. And it has a PCIe x16 and x4 slots.

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