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The file /usr/share/doc/linux-doc/usb/persist.txt.gz mentions that the USB-persist capability can be enabled for a given USB device by writing 1 to the file persist in that device's directory in /sys/bus/usb/devices/$device/power. This is said — if I understood correctly — to allow mountings of volumes on the drive to persist across power loss during suspend.

However, I've discovered that the device I'd like to enable this facility for — a USB hard drive — does not have such a file in its corresponding directory, and that attempts to create it are rebuffed.

Is there perhaps a kernel module that needs to be loaded for this to work? Do I need to build a custom kernel? I'm running ubuntu 10.10.

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cross-posted on AskUbuntu – intuited Jan 6 '11 at 7:52
Can you give us the exact path you are trying to use for the file? Where do you echo "1"? Did you try to do that before/after the device was connected? Does this problem still exist? – Patkos Csaba Feb 13 '12 at 20:07
I doubt the exact path will help, it depends on the vagaries of the USB tree of the machine. – vonbrand Jan 25 '13 at 21:56

As noted at the end of answer this is presumably already on. (Also when Q was posted.)

Also note that the cross post was closed due to no answer – thus most appropriate to post here. And I imagine others can have use of the information.

The doc you linked say (emphasize mine):

Note that the "USB-persist" feature will be applied only to those devices for which it is enabled. You can enable the feature by doing (as root):

echo 1 >/sys/bus/usb/devices/.../power/persist

where the "..." should be filled in the with the device's ID. Disable the feature by writing 0 instead of 1. For hubs the feature is automatically and permanently enabled and the power/persist file doesn't even exist, so you only have to worry about setting it for devices where it really matters.

In other words: (it seems like) you are trying to set it for a hub, which both is redundant – as it is always set, and not possible as file does not exist.

To see which paths actually has a persist file do:

$ ls -l /sys/bus/usb/devices/*/power/persist

To check status do:

$ tail /sys/bus/usb/devices/*/power/persist

If you use lsusb to identify device the correct path is

  • <BUS> is from lsusb
  • <DEVPATH> you can match against Device from lsusb by:

    $ tail <BUS>-*/devnum

Or check dmesg, etc. e.g.:

$ dmesg | grep -iA3 "new usb"

You can also verify by e.g.:

$ udevadm info -q all -p /sys/bus/usb/devices/<BUS>-<DEVPATH>

The policy for persist was changed to on as default by kernel 2.6.26-rc5.

Some ref's on the change to enabled by default:

The implementation:

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