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bcache: documentation formatting, edited for clarity, stripe alignment notes

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Signed-off-by: Eric Wheeler <bcache@linux.ewheeler.net>
Cc: Marc MERLIN <marc@merlins.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
tirimbino
Eric Wheeler 9 years ago committed by Jonathan Corbet
parent c9b2ffc022
commit c0b8c9a344
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  1. 161
      Documentation/bcache.txt

161
Documentation/bcache.txt
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@ -137,48 +137,60 @@ the backing devices to passthrough mode.
HOWTO/COOKBOOK
--------------
A) Your bcache doesn't start.
Starting and starting a bcache with a missing caching device
A) Starting a bcache with a missing caching device
Registering the backing device doesn't help, it's already there, you just need
If 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
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
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
Note that this may cause data loss if you were running in writeback mode.
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
B) Bcache does not find its cache
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
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).
C) Corrupt bcache crashes the kernel at device registration time:
This should never happen. If it does happen, then you have found a bug!
Please report it to the bcache development list: linux-bcache@vger.kernel.org
Be sure to provide as much information that you can including kernel dmesg
output if available so that we may assist.
D) Recovering data without bcache:
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 any value defined by
--data-offset when you originally formatted bcache with `make-bcache`.
For example:
losetup -o 8192 /dev/loop0 /dev/your_bcache_backing_dev
This should present your unmodified backing device data in /dev/loop0
If your cache is in writethrough mode, then you can safely discard the
cache device without loosing data.
E) Wiping a cache device
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
@ -205,56 +217,60 @@ 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
F) 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:/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
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
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
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
G) dm-crypt and bcache
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. [benchmarks?]
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.
H) Stop/free a registered bcache to wipe and/or recreate it
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)
Suppose that you need to free up all bcache references so that you can
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
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
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
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
This causes the backing bcache to be removed from /sys/fs/bcache and
then it can be reused. This would be true of any block device stacking
where bcache is a lower device.
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/
@ -262,9 +278,10 @@ lrwxrwxrwx 1 root root 0 Mar 5 09:39 5bc072a8-ab17-446d-9744-e247949913c1/cache
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.
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.
@ -275,6 +292,22 @@ 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
want for getting the best possible numbers when benchmarking.
- Backing device alignment
The default metadata size in bcache is 8k. If your backing device is
RAID based, then be sure to align this by a multiple of your stride
width using `make-bcache --data-offset`. If you intend to expand your
disk array in the future, then multiply a series of primes by your
raid stripe size to get the disk multiples that you would like.
For example: If you have a 64k stripe size, then the following offset
would provide alignment for many common RAID5 data spindle counts:
64k * 2*2*2*3*3*5*7 bytes = 161280k
That space is wasted, but for only 157.5MB you can grow your RAID 5
volume to the following data-spindle counts without re-aligning:
3,4,5,6,7,8,9,10,12,14,15,18,20,21 ...
- Bad write performance
If write performance is not what you expected, you probably wanted to be
@ -382,7 +415,7 @@ sequential_merge
against all new requests to determine which new requests are sequential
continuations of previous requests for the purpose of determining sequential
cutoff. This is necessary if the sequential cutoff value is greater than the
maximum acceptable sequential size for any single request.
maximum acceptable sequential size for any single request.
state
The backing device can be in one of four different states:
@ -469,7 +502,7 @@ bucket_size
Size of buckets
cache<0..n>
Symlink to each of the cache devices comprising this cache set.
Symlink to each of the cache devices comprising this cache set.
cache_available_percent
Percentage of cache device which doesn't contain dirty data, and could

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