bcache: Refresh usage docs
Mention udev autoregistration, symlinks. Write down some sysfs paths.
Signed-off-by: Gabriel de Perthuis <g2p.code@gmail.com>
Signed-off-by: Kent Overstreet <koverstreet@google.com>
diff --git a/Documentation/bcache.txt b/Documentation/bcache.txt
index c3365f2..32b6c31 100644
--- a/Documentation/bcache.txt
+++ b/Documentation/bcache.txt
@@ -46,29 +46,33 @@
have to manually attach:
make-bcache -B /dev/sda /dev/sdb -C /dev/sdc
-To make bcache devices known to the kernel, echo them to /sys/fs/bcache/register:
+bcache-tools now ships udev rules, and bcache devices are known to the kernel
+immediately. Without udev, you can manually register devices like this:
echo /dev/sdb > /sys/fs/bcache/register
echo /dev/sdc > /sys/fs/bcache/register
-To register your bcache devices automatically, you could add something like
-this to an init script:
+Registering the backing device makes the bcache device show up in /dev; you can
+now format it and use it as normal. But the first time using a new bcache
+device, it'll be running in passthrough mode until you attach it to a cache.
+See the section on attaching.
- echo /dev/sd* > /sys/fs/bcache/register_quiet
+The devices show up as:
-It'll look for bcache superblocks and ignore everything that doesn't have one.
+ /dev/bcache<N>
-Registering the backing device makes the bcache show up in /dev; you can now
-format it and use it as normal. But the first time using a new bcache device,
-it'll be running in passthrough mode until you attach it to a cache. See the
-section on attaching.
+As well as (with udev):
-The devices show up at /dev/bcacheN, and can be controlled via sysfs from
-/sys/block/bcacheN/bcache:
+ /dev/bcache/by-uuid/<uuid>
+ /dev/bcache/by-label/<label>
+
+To get started:
mkfs.ext4 /dev/bcache0
mount /dev/bcache0 /mnt
+You can control bcache devices through sysfs at /sys/block/bcache<N>/bcache .
+
Cache devices are managed as sets; multiple caches per set isn't supported yet
but will allow for mirroring of metadata and dirty data in the future. Your new
cache set shows up as /sys/fs/bcache/<UUID>
@@ -80,11 +84,11 @@
device to a cache set is done thusly, with the UUID of the cache set in
/sys/fs/bcache:
- echo <UUID> > /sys/block/bcache0/bcache/attach
+ echo <CSET-UUID> > /sys/block/bcache0/bcache/attach
This only has to be done once. The next time you reboot, just reregister all
your bcache devices. If a backing device has data in a cache somewhere, the
-/dev/bcache# device won't be created until the cache shows up - particularly
+/dev/bcache<N> device won't be created until the cache shows up - particularly
important if you have writeback caching turned on.
If you're booting up and your cache device is gone and never coming back, you
@@ -191,6 +195,9 @@
SYSFS - BACKING DEVICE:
+Available at /sys/block/<bdev>/bcache, /sys/block/bcache*/bcache and
+(if attached) /sys/fs/bcache/<cset-uuid>/bdev*
+
attach
Echo the UUID of a cache set to this file to enable caching.
@@ -300,6 +307,8 @@
SYSFS - CACHE SET:
+Available at /sys/fs/bcache/<cset-uuid>
+
average_key_size
Average data per key in the btree.
@@ -390,6 +399,8 @@
SYSFS - CACHE DEVICE:
+Available at /sys/block/<cdev>/bcache
+
block_size
Minimum granularity of writes - should match hardware sector size.