bcache: documentation updates and corrections

Bcache documentation updates:
- Added new HOWTO/COOKBOOK section
- fixed a few typos
- /sys/block/bcache0/cache_mode is /sys/block/bcache0/bcache/cache_mode
Signed-off-by: Marc MERLIN <marc@merlins.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/bcache.txt

-Say you've got a big slow raid 6, and an X-25E or three. Wouldn't it be
+Say you've got a big slow raid 6, and an ssd or three. Wouldn't it be
 nice if you could use them as cache... Hence bcache.
 
 Wiki and git repositories are at:
@@ -8,7 +8,7 @@ Wiki and git repositories are at:
 
 It's designed around the performance characteristics of SSDs - it only allocates
 in erase block sized buckets, and it uses a hybrid btree/log to track cached
-extants (which can be anywhere from a single sector to the bucket size). It's
+extents (which can be anywhere from a single sector to the bucket size). It's
 designed to avoid random writes at all costs; it fills up an erase block
 sequentially, then issues a discard before reusing it.
 
@@ -55,7 +55,10 @@ immediately.  Without udev, you can manually register devices like this:
 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.
+If you are thinking about using bcache later, it is recommended to setup all your
+slow devices as bcache backing devices without a cache, and you can choose to add
+a caching device later.
+See 'ATTACHING' section below.
 
 The devices show up as:
 
@@ -72,12 +75,14 @@ To get started:
   mount /dev/bcache0 /mnt
 
 You can control bcache devices through sysfs at /sys/block/bcache<N>/bcache .
+You can also control them through /sys/fs//bcache/<cset-uuid>/ .
 
 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>
 
-ATTACHING:
+ATTACHING
+---------
 
 After your cache device and backing device are registered, the backing device
 must be attached to your cache set to enable caching. Attaching a backing
@@ -105,7 +110,8 @@ but all the cached data will be invalidated. If there was dirty data in the
 cache, don't expect the filesystem to be recoverable - you will have massive
 filesystem corruption, though ext4's fsck does work miracles.
 
-ERROR HANDLING:
+ERROR HANDLING
+--------------
 
 Bcache tries to transparently handle IO errors to/from the cache device without
 affecting normal operation; if it sees too many errors (the threshold is
@@ -127,7 +133,143 @@ the backing devices to passthrough mode.
    writeback mode). It currently doesn't do anything intelligent if it fails to
    read some of the dirty data, though.
 
-TROUBLESHOOTING PERFORMANCE:
+
+HOWTO/COOKBOOK
+--------------
+
+A) Your bcache doesn't start.
+   Starting and starting a bcache with a missing caching device
+
+Registering the backing device doesn't help, it's already there, you just need
+to force it to run without the cache:
+host:~# echo /dev/sdb1 > /sys/fs/bcache/register
+[  119.844831] bcache: register_bcache() error opening /dev/sdb1: device already registered
+
+Next, you try to register your caching device if it's present. However if it's
+absent, or registration fails for some reason, you can still start your bcache
+without its cache, like so:
+host:/sys/block/sdb/sdb1/bcache# echo 1 > running
+
+
+B) Bcache not finding its cache and not starting
+
+This does not work:
+host:/sys/block/md5/bcache# echo 0226553a-37cf-41d5-b3ce-8b1e944543a8 > attach
+[ 1933.455082] bcache: bch_cached_dev_attach() Couldn't find uuid for md5 in set
+[ 1933.478179] bcache: __cached_dev_store() Can't attach 0226553a-37cf-41d5-b3ce-8b1e944543a8
+[ 1933.478179] : cache set not found
+
+In this case, the caching device was simply not registered at boot or
+disappeared and came back, and needs to be (re-)registered:
+host:/sys/block/md5/bcache# echo /dev/sdh2 > /sys/fs/bcache/register
+
+
+C) Corrupt bcache caching device crashes the kernel on startup/boot
+
+You'll have to wipe the caching device, start the backing device without the
+cache, and you can re-attach the cleaned up caching device then. This does
+require booting with a kernel/rescue media where bcache is disabled
+since it will otherwise try to access your device and probably crash
+again before you have a chance to wipe it.
+(or if you plan ahead, compile a backup kernel with bcache disabled and keep it
+in your grub config for a rainy day)
+If bcache is not available in the kernel, a filesystem on the backing device is
+still available at an 8KiB offset. So either via a loopdev of the backing device
+created with --offset 8K or by temporarily increasing the start sector of the
+partition by 16 (512byte sectors).
+
+This is how you wipe the caching device:
+host:~# wipefs -a /dev/sdh2
+16 bytes were erased at offset 0x1018 (bcache)
+they were: c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
+
+After you boot back with bcache enabled, you recreate the cache and attach it:
+host:~# make-bcache -C /dev/sdh2
+UUID:                   7be7e175-8f4c-4f99-94b2-9c904d227045
+Set UUID:               5bc072a8-ab17-446d-9744-e247949913c1
+version:                0
+nbuckets:               106874
+block_size:             1
+bucket_size:            1024
+nr_in_set:              1
+nr_this_dev:            0
+first_bucket:           1
+[  650.511912] bcache: run_cache_set() invalidating existing data
+[  650.549228] bcache: register_cache() registered cache device sdh2
+
+start backing device with missing cache:
+host:/sys/block/md5/bcache# echo 1 > running
+
+attach new cache:
+host:/sys/block/md5/bcache# echo 5bc072a8-ab17-446d-9744-e247949913c1 > attach
+[  865.276616] bcache: bch_cached_dev_attach() Caching md5 as bcache0 on set 5bc072a8-ab17-446d-9744-e247949913c1
+
+
+D) Remove or replace a caching device
+
+host:/sys/block/sda/sda7/bcache# echo 1 > detach
+[  695.872542] bcache: cached_dev_detach_finish() Caching disabled for sda7
+
+host:~# wipefs -a /dev/nvme0n1p4
+wipefs: error: /dev/nvme0n1p4: probing initialization failed: Device or resource busy
+Ooops, it's disabled, but not unregistered, so it's still protected
+
+We need to go and unregister it:
+host:/sys/fs/bcache/b7ba27a1-2398-4649-8ae3-0959f57ba128# ls -l cache0
+lrwxrwxrwx 1 root root 0 Feb 25 18:33 cache0 -> ../../../devices/pci0000:00/0000:00:1d.0/0000:70:00.0/nvme/nvme0/nvme0n1/nvme0n1p4/bcache/
+host:/sys/fs/bcache/b7ba27a1-2398-4649-8ae3-0959f57ba128# echo 1 > stop
+kernel: [  917.041908] bcache: cache_set_free() Cache set b7ba27a1-2398-4649-8ae3-0959f57ba128 unregistered
+
+Now we can wipe it:
+host:~# wipefs -a /dev/nvme0n1p4
+/dev/nvme0n1p4: 16 bytes were erased at offset 0x00001018 (bcache): c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
+
+
+E) dmcrypt and bcache
+
+First setup bcache unencrypted and then install dmcrypt on top of /dev/bcache<N>
+This will work faster than if you dmcrypt both the backing and caching
+devices and then install bcache on top.
+
+
+F) Stop/free a registered bcache to wipe and/or recreate it
+(or maybe you need to free up all bcache references so that you can have fdisk
+run and re-register a changed partition table, which won't work if there are any
+active backing or caching devices left on it)
+
+1) Is it present in /dev/bcache* ? (there are times where it won't be)
+If so, it's easy:
+host:/sys/block/bcache0/bcache# echo 1 > stop
+
+2) But if your backing device is gone, this won't work:
+host:/sys/block/bcache0# cd bcache
+bash: cd: bcache: No such file or directory
+
+In this case, you may have to unregister the dmcrypt block device that
+references this bcache to free it up:
+host:~# dmsetup remove oldds1
+bcache: bcache_device_free() bcache0 stopped
+bcache: cache_set_free() Cache set 5bc072a8-ab17-446d-9744-e247949913c1 unregistered
+
+This causes the backing bcache to be removed from /sys/fs/bcache and then it can
+be reused
+
+3) In other cases, you can also look in /sys/fs/bcache/:
+host:/sys/fs/bcache# ls -l */{cache?,bdev?}
+lrwxrwxrwx 1 root root 0 Mar  5 09:39 0226553a-37cf-41d5-b3ce-8b1e944543a8/bdev1 -> ../../../devices/virtual/block/dm-1/bcache/
+lrwxrwxrwx 1 root root 0 Mar  5 09:39 0226553a-37cf-41d5-b3ce-8b1e944543a8/cache0 -> ../../../devices/virtual/block/dm-4/bcache/
+lrwxrwxrwx 1 root root 0 Mar  5 09:39 5bc072a8-ab17-446d-9744-e247949913c1/cache0 -> ../../../devices/pci0000:00/0000:00:01.0/0000:01:00.0/ata10/host9/target9:0:0/9:0:0:0/block/sdl/sdl2/bcache/
+
+The device names will show which UUID is relevant, cd in that directory
+and stop the cache:
+host:/sys/fs/bcache/5bc072a8-ab17-446d-9744-e247949913c1# echo 1 > stop
+this will free up bcache references and let you reuse the partition for other
+purposes.
+
+
+
+TROUBLESHOOTING PERFORMANCE
+---------------------------
 
 Bcache has a bunch of config options and tunables. The defaults are intended to
 be reasonable for typical desktop and server workloads, but they're not what you
@@ -140,7 +282,7 @@ want for getting the best possible numbers when benchmarking.
    maturity, but simply because in writeback mode you'll lose data if something
    happens to your SSD)
 
-   # echo writeback > /sys/block/bcache0/cache_mode
+   # echo writeback > /sys/block/bcache0/bcache/cache_mode
 
  - Bad performance, or traffic not going to the SSD that you'd expect
 
@@ -193,7 +335,9 @@ want for getting the best possible numbers when benchmarking.
    Solution: warm the cache by doing writes, or use the testing branch (there's
    a fix for the issue there).
 
-SYSFS - BACKING DEVICE:
+
+SYSFS - BACKING DEVICE
+----------------------
 
 Available at /sys/block/<bdev>/bcache, /sys/block/bcache*/bcache and
 (if attached) /sys/fs/bcache/<cset-uuid>/bdev*