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USB Attached SCSI (UAS) and UAS Protocol (UASP) vs. BOT

USB Attached SCSI (UAS or UASP) is a communication protocol designed to improve the performance and efficiency of USB storage devices, particularly SSDs.

Legacy USB Storage vs. UAS

The Old Way (BOT): BOT (Bulk-Only Transport) was the traditional protocol for USB mass storage. It was designed with simple devices like flash drives and older hard drives in mind. BOT handles data transfers in sequential blocks but that would very commonly lead to bottlenecks with faster storage devices.

The UAS Way: UAS is a more intelligent protocol. It allows for command queuing and out-of-order data transfers. Think of it like switching from a single-lane road to a multi-lane highway for data going to and from your device.

Benefits of UAS

Speed Boost (Especially for SSDs): UAS really shines with SSDs. You’ll see significantly faster file transfers both in reading and writing reaching closer to the actual capabilities of your drive.

Reduced CPU Overhead: UAS is more efficient in how it communicates with your computer by freeing up CPU resources for other tasks. This will slightly improve overall system responsiveness.

Command Queuing: UAS can handle multiple commands simultaneously. This is particularly helpful with workloads involving lots of small file transfers making the process far less laggy.

Compatibility Checkpoints

To realize the benefits of UAS, you need the following:

UAS-capable Device: Most modern SSDs and some higher-end flash drives support UAS.

UAS-compatible Operating System: Windows 8 onwards, macOS, and most modern Linux distributions have built-in UAS support.

UAS-capable USB Host Controller: Check if your computer’s motherboard or USB card has chipsets that support UAS. This information will be found in your computer’s specifications or in the Control Panel in Windows.

How to Tell if UAS is Active

Operating System Tools:

Windows: Device Manager will list “USB Attached SCSI (UAS) Mass Storage Device” if it’s active.

macOS: System Information tool sometimes shows if UAS is negotiated with a specific device.

Third-Party Utilities: Some drive benchmark tools do indicate whether UAS is active or not.

Important Things

UAS vs. SATA/NVMe: Even with UAS, USB storage will still have a slight performance disadvantage compared to internal SATA or NVMe SSDs because of the limits of the USB interface itself.

Small File Transfer Gains: With very small files even with UAS you will not see dramatic speed differences compared to BOT. The benefits are more pronounced with larger files or multiple and simultaneous mixed reads/writes.

Illustrative Examples

Scenario 1: Copying Large File (e.g., Video) to SSD

BOT: The transfer process will be jerky and fluctuate. The hard drive makes noticeable clicking sounds because of the frequent repositioning for sequential data access. Task completion will be significantly longer.

UAS: Smoother transfer at higher sustained speeds. Drive activity will be quieter thanks to the optimized data access.

Scenario 2: Editing Photos on an External SSD

BOT: You will experience slight delays when opening RAW photo files or applying extensive editing. Background tasks on your computer will feel sluggish.

UAS: Faster loading and saving of files. The editing experience feels more responsive especially with high-resolution images. Overall system responsiveness should be slightly better during these tasks.

Technicals for Nerds

TRIM Command Support: UAS enables the TRIM command to pass through to SSDs even over USB connections. TRIM helps keep SSDs perform at their peak by managing deleted data correctly.

Not All UAS Implementations are Equal: The quality of a device’s firmware and your computer’s USB controller will still influence real-world performance even with UAS active.

UAS and HDDs: While UAS technically works with traditional hard drives (HDDs) the benefits are far less visible. BOT’s bottleneck in this case is more likely the drive’s mechanical speed not the protocol itself.

Troubleshooting and Further Testing

Check Cabling: A poor quality or long USB cable will negatively impact performance even with UAS.

Benchmarking: Tools like CrystalDiskMark can perform sequential and random file transfer tests on a drive with and without UAS drivers enabled for a direct speed comparison.

Resource Monitoring: While performing file transfers look at your CPU usage to see if UAS helped reduce the load.

What if I don’t have UAS?

If your device and system aren’t UAS compatible there’s not much you can do short of upgrading hardware components.

For basic storage needs with older devices the Bulk-Only Transport (BOT) protocol still gets the job done.

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