Hello, We have a system where the rootfs is a partition on a USB device, and I've noticed upon a few rare cases where the USB controller loses the connection to the USB device after some uptime (days, weeks...), and the USB device reappears a very short time later.
It doesn't really matter why, I guess that USB controllers and storage devices aren't always of the best quality - let's assume that. The issue is that Linux currently makes it problematic to recover the rootfs for several reasons. If /dev/sda1 is mounted as root and the SCSI device goes into offline mode, it is quite impossible to get out of this situation. At first, I had to disable the usermode helper that sends events to udev since it blocks on the dysfunction /dev/sda1. Afterwards, I noticed that the reappearing device is being assigned with /dev/sdb along with a new SCSI host. I guess the the VFS and block layer still keep a reference to the old scsi_disk and as a result sd doesn't free it. So, I gave some thoughts about this, I and came up with two main solutions: 1) Improve the USB storage error handling - bind the already existing SCSI host to the USB port that has the device, e.g., if host2 got created for usb 5-3 then keep it that way for the sake of EH. /dev/sda1 should come to life when the USB device recovers, unless a few seconds have passed or some attributes (such as manufactor id or serial) have changed. 2) Block layer hack - Write a special layering block device driver that acts as a proxy to the currently functioning /dev/sd* which gets auto-detected by this block layer driver. md multipath for the 'poor', you might say.. I chose '2' for the time being. It was much easier than hacking around the USB subsystem... Yes, I know rootfs on USB is an uncommon use-case at the moment but I do like to know if there are some improvements planned in this area. -- Dan Aloni XIV LTD, http://www.xivstorage.com da-x (at) monatomic.org, dan (at) xiv.co.il ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ _______________________________________________ linux-usb-devel@lists.sourceforge.net To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-devel
