Why USB doesn't get these mount option?

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1

mount output:

/dev/sda1 on /mnt/usb1 type btrfs (rw,relatime,seclabel,space_cache=v2,subvolid=5,subvol=/,x-gvfs-show)
/dev/mmcblk1p3 on /home type btrfs (rw,relatime,seclabel,compress=zstd:1,ssd,discard=async,space_cache=v2,subvolid=256,subvol=/home)

fstab:

UUID=**** /home           btrfs   subvol=home,compress=zstd:1 0 0
UUID=**** /mnt/usb1       btrfs   nofail,x-gvfs-show 0 0

/dev/mmcblk1p3 is an SD card with system install, /dev/sda1 is a USB stick.
Why the USB doesn’t get ssd,discard=async? And where do the extra options not in fstab come from?
man btrfs says ssd is auto detected on “status of a device with respect to rotational or non-rotational type”. Pretty sure USB stick is non-rotational…
discard default is “async when devices support it”, does this mean the USB stick doesn’t it? Is there a way to check?

2

You can use for i in /sys/block/sd?/queue/rotational; do echo "$i: $(<$i)"; done to check if the block devices in the scsi subsystem are being detected as rotational (1) or not (0).

As for discard support not being auto detected, I think that can be an artifact of the USB device being “behind” a USB bridge. You can use the smartctl command to see if discard/trim support is being detected.

# smartctl -x /dev/sda | grep -i trim
TRIM Command:     Available, deterministic, zeroed

# smartctl -x /dev/sdc
...
/dev/sdc: Unknown USB bridge [0x13fe:0x6500 (0x110)]
Please specify device type with the -d option.
...

I don’t know a good way to determine if a device actually supports the discard command. I’ve seen the blkdiscard command directly report that a device does not support discard commands when used against a block device that lacks such support. But that is a very dangerous command that will instantly erase your entire block device if it works (though maybe if you have a spare that is the same model, you could test it against the spare).

2 Likes
3

It is indeed detected as rotational (1)… Is that normal?
Doesn’t seem like it supports TRIM, but mounting with discard=async gives no error unlike here.

$ sudo fstrim -v /mnt/usb1
fstrim: /mnt/usb1: the discard operation is not supported
$ sudo mount /mnt/usb1 -o discard=async,ssd
dev/sda1 on /mnt/usb1 type btrfs (rw,relatime,seclabel,ssd,discard=async,space_cache=v2,subvolid=5,subvol=/,x-gvfs-show)

EDIT: Seems yes to rotational, though it might be better to change it.