** Description changed:
+ [Impact]
+
+ * Current users of bcache devices may encounter unreliable device
+ numbering as the Linux kernel does not guarantee that bcache
+ minor numbers are assigned to the same devices at each boot.
+ Users who may have used /dev/bcacheN in paths to a specific
+ device could possible be pointing to a different dataset
+ altogether. bcache udev rules do provide some mechanism to
+ generate persistent symlinks in /dev/bcache/by-uuid or
+ /dev/bcache/by-label which is based on superblock data on
+ the underlying device. However, the Linux kernel does not
+ always generate an kernel uevent to trigger the udev rules
+ to create the symlink.
+
+ * The fix adds a udev program which will read bcache superblock
+ of slave devices and extract the UUID and LABEL, exporting them
+ to udev for use in the bcache rule files.
+
+ * This is affected in upstream bcache-tools, the owning package
+ of the udev rules. This affects all releases of bcache-tools
+ as the rules rely upon the kernel to trigger these events,
+ though that is not a requirement to resolve the lack of
+ persistent links.
+
+ [Test Case]
+
+ * Launch and Ubuntu Cloud Image with 3 unused disks
+ - apt install bcache-tools tree
+ - make-bcache -C /dev/vdb
+ - make-bcache -B /dev/vdc
+ - make-bcache -B /dev/vdd
+ - echo "vdc" > /sys/class/block/bcache0/bcache/label
+ - echo "vdd" > /sys/class/block/bcache1/bcache/label
+ - reboot
+
+ - Run this test:
+
+ #!/bin/bash
+ FAIL=0
+ [ ! -d /dev/bcache ] && {
+ echo "FAIL: /dev/bcache is not a directory";
+ exit 1
+ }
+ for label in /dev/bcache/by-label/*; do
+ LABEL_TARGET="$(ls -1 /sys/class/block/`basename $label`/holders/)"
+ DEVNAME=`readlink -f $label`;
+ KNAME="${DEVNAME#*/dev/}"
+ if [ "$LABEL_TARGET" != "$KNAME" ]; then
+ echo "FAIL: label points to $LABEL_TARGET but symlink points to
$DEVNAME";
+ FAIL=1
+ fi;
+ done
+ if [ "$FAIL" == "0" ]; then
+ echo "PASS";
+ exit 0
+ fi
+ exit 1
+
+
+ [Regression Potential]
+
+ * As bcache minor numbers and these symlinks have been unreliable in
+ the past there may be code that makes assumptions about
+ /dev/bcache* expanded only to the block devices, versus
+ /dev/bcache which is a directory.
+
+
+ [Original Description]
+
Bcache device names like /dev/bcache0 are unstable. Bcache does not use
any predictable ordering when assembling bcache devices, so on systems
with multiple bcache devices, a symlink to /dev/bcache0 may end up
pointing do a different device.
the bcache dname symlink should point to the /dev/bcache/by-uuid/<UUID>
which matches the backing device UUID that's set at creation time.
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https://bugs.launchpad.net/bugs/1728742
Title:
curtin dname for bcache uses unstable devname instead of UUID
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