What is UASP?

I was looking for an external USB enclosure to house a spare 256 GB SDD that I have laying here on my desk. While looking around I noticed a housing for about $20 on Amazon that boasts support of “UASP.” Curious, I wanted to find out if this is something I care about.

In short, it is.

USB storage uses something known as USB Bulk Only Transfer (BOT). This is essentially a subset (read: stripped down) of the SCSI command set that was built specifically for USB attached storage. While it is fine for things like thumbdrives and even desktop external disk drives, the BOT scheme has severe performance limitations when it comes to random read/writes and out of order commands as you would get when running VMs from an external USB drive. (Come on now. I am not the only one.)

Screen Shot 2014-10-23 at 11.23.14 PM

This is where the USB attached SCSI protocol (UASP) comes in. UASP addresses a number of the problems with BOT including more robust command set and command queueing to allow out of order commands. It also eliminates much of the CPU overhead. So you get the full SCSI command set over USB with no out of order penalty.

Screen Shot 2014-10-23 at 11.37.00 PM

Keep in mind since most desktop drive use the ATA commands set (aka SATA drive) there is always some level of SCSI to ATA translation happening in the external housing regardless if you are using BOT or UASP.


Network Engineer interested in many areas including switches/routers/firewalls, SAN, and virtualization. I am currently employed by Cisco Systems. While I like to think that everything I write is well reasoned and insightful, the opinions expressed are solely mine and do not represent my employer.

Posted in Storage Area Networking

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Charles Stizza

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October 2014
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