why is usb boot bad? longevity or connection?

[mappo]

I'm about to install Truenas Scale on a new build (old parts). I've understood that installation to usb drive is adviced against - but why?
Is it due to the short lifespan of usb sticks, or is there something inherent to usb?
My plan is to install to a mSata SSD in a mSata->usb adapter. That should take care of the lifespan, but it's still usb.
Thoughts?

 

[ChrisRJ]

The advice is not against USB as an interface, but your typical USB stick. Unless the latter is enterprise-grade, which would be prohibitively expensive, it is not durable enough. What works in general is a USB-SATA adapter (as you plan), although there is a very recent thread about someone having issues with that for TrueNAS Scale (Core is ok).

 

[rvassar]

I ran mirrored USB thumb drives for a while, and I found they simply have poor wear leveling and wear out quickly. At one point I think I lost three sticks inside of 6 weeks. Newer devices seem worse than 5+ year old sticks I have lying around with my collection of Raspberry Pi's. I eventually picked up some discount 60Gb SATA SSD's and mirrored the first one in using a USB to SATA enclosure. I then moved it to a real SATA port, and mirrored it to a second unit in the USB enclosure, leaving me with 5 SATA ports for storage drives. But FreeNAS / TrueNAS is for tinkerers, and I eventually picked up a SAS HBA and ditched USB entirely. There's really only two problems with USB, it's sorta slow (v2.0, 3.x is better), and it's focus is on hot plug devices mostly outside the case. Even with a pair of sticks plugged in the back, you have to deal with little hands grabbing the blinking lego brick, etc... It did work OK for me with the SATA enclosures. If you plan to use it long term, make sure you use a quality chipset that allows SMART pass thru, so you can monitor your drive health.

 

[thinkjarvis]

I am running a HP Proliant Gen 8 and use the internal USB with adaptor to SSD. Works very well.

Reasons for doing this for my setup:

1. On the Gen 8 Proliant - If you do not use an SD card or USB for TrueNAS then you have to use one of the 4 storage drives. This means you cannot run all 4 drives as storage.
2. This particular server wont boot from the OD Sata port internally either so the only way to add a bootable SSD is to sacrifice one of the 4 main drives for the OS, use a USB, or install a grub on a USB or the SSD to launch from the OD Sata port.
3. The easier option here is to just boot from SSD connected via USB.
4. Emergency situations - It is easy to replace the SSD with a new one or with a USB if something terrible goes wrong.

As you correctly said - using an SSD over USB resolves the poor lifespan of cheap USB sticks. It also moves any speed issues connected to a slow usb stick further down the process. The bottleneck becomes the max speed of the USB adaptor and then the USB interface itself.

To stop any of the 4 drives being used for the OS or for silly data transfers. I use an SSD connected via adaptor to USB internal for the OS and then use the OD Port Sata for a second small SSD for quick non-important network data transfers. This frees up all 4 drives to be used as NAS as intended while adding a further 2 drives to an otherwise 4 drive server (Totalling to 6).

 

[mappo]

Wow! Three great answers almost immediately.
I'm very interested in the reply from @thinkjarvis since my planned setup is a HPE microserver as well (although an older one with a soldered-on Turion)