[lug] USB3 vs eSATA III question

stimits at comcast.net stimits at comcast.net
Sun Jul 22 14:11:06 MDT 2018

Sorry, I don't have the hardware to test. I think eSATA would be an advantage if you need to boot from the device.
On the other hand, USB3 should be 5000Mbit/s before overhead, so it is actually a lot faster than 3 Gbps (SATA2). Check for "5000M" at the end of the device line of "lsusb -t" to see what speed it is running and what might be competing for bandwidth. Don't bet on USB3 bandwidth being available if the bus is shared with anything else. Each port of any SATA controller should more or less always have its own fully reserved bandwidth without sharing (but then if the controller is on PCIe there may be some indirect sharing since all ports would be using the same PCIe lanes...enough lanes means this would never be a problem).
In theory, if your bootloader has access to your SATA controller (e.g., the BIOS/UEFI has installed a driver), then you could boot from eSATA without the "external" making any difference (neither GRUB nor Linux would care about "external"). For a USB drive to do this as a boot device you'd have to simultaneously have both a driver for USB3 and for whatever the end device is on the USB existing from boot through Linux starting (less likely than just the SATA controller having support...you might be building a custom initrd to do this). The subtle trick comes when switching from bootloader to Linux kernel taking over...unless you have an initrd or some other support for both the USB device and USB controller in the same mode USB was previously running under you would momentarily lose your root device and boot would fail. Re-enumerating USB during a transition from bootloader to Linux will cause failure. eSATA won't be re-enumerating.
eSATA as a boot device should be far more likely to succeed without fuss versus USB3 (fewer complications to boot eSATA). I would expect that if you are not booting from the USB3 device, then it would be possible it is as fast as SATA2, but slower than a 6 Gbs SATA3. I don't know about SSDs, but traditional hard drives cannot sustain this speed without RAID. On a shared bus higher bandwidth than the device can sustain has benefit, but on a dedicated root HUB the bus being faster than the device has less meaning.
----- Original Message -----From: Bear Giles <bgiles at coyotesong.com>To: Boulder (Colorado) Linux Users Group -- General Mailing List <lug at lug.boulder.co.us>Sent: Sun, 22 Jul 2018 18:27:59 -0000 (UTC)Subject: [lug] USB3 vs eSATA III question

This is mostly from curiousity...

USB 3 has a claimed bandwidth of 3 Gbps.

eSATA II has a claimed bandwidth of 3 Gbps, and eSATA III has a claimed benefit of 6 Gbps.

I don't think any USB stick is going to be pushing 3 Gbps (but I'm not 100% certain), but perhaps an external hard drive, esp. an external SSD. But is that actually realistic?

In contrast my experience with eSATA is that it really is akin a traditionally installed disk.

Like I said this is mostly from curiosity. I noticed my recent systems haven't had eSATA ports but it's easy to add a half-height card for one. Ditto external drives - they usually only have USB 3 but if you buy an external drive case for your own drive then it's easy to find them with both USB 3 and eSATA. So I have my eSATA gear but it might not be necessary any longer if the quoted USB 3 numbers are accurate.

Does anyone else have experience with both USB 3 and eSATA?

(Meanwhile I'm wondering what to do with a few old external drives that are USB 2. They're so much slower and smaller than my external USB 3 drives.)

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