+============================
+A block layer cache (bcache)
+============================
+
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:
- http://bcache.evilpiepirate.org
- http://evilpiepirate.org/git/linux-bcache.git
- http://evilpiepirate.org/git/bcache-tools.git
+
+ - http://bcache.evilpiepirate.org
+ - http://evilpiepirate.org/git/linux-bcache.git
+ - http://evilpiepirate.org/git/bcache-tools.git
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
Getting started:
You'll need make-bcache from the bcache-tools repository. Both the cache device
-and backing device must be formatted before use.
+and backing device must be formatted before use::
+
make-bcache -B /dev/sdb
make-bcache -C /dev/sdc
make-bcache has the ability to format multiple devices at the same time - if
you format your backing devices and cache device at the same time, you won't
-have to manually attach:
+have to manually attach::
+
make-bcache -B /dev/sda /dev/sdb -C /dev/sdc
bcache-tools now ships udev rules, and bcache devices are known to the kernel
-immediately. Without udev, you can manually register devices like this:
+immediately. Without udev, you can manually register devices like this::
echo /dev/sdb > /sys/fs/bcache/register
echo /dev/sdc > /sys/fs/bcache/register
a caching device later.
See 'ATTACHING' section below.
-The devices show up as:
+The devices show up as::
/dev/bcache<N>
-As well as (with udev):
+As well as (with udev)::
/dev/bcache/by-uuid/<uuid>
/dev/bcache/by-label/<label>
-To get started:
+To get started::
mkfs.ext4 /dev/bcache0
mount /dev/bcache0 /mnt
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
device to a cache set is done thusly, with the UUID of the cache set in
-/sys/fs/bcache:
+/sys/fs/bcache::
echo <CSET-UUID> > /sys/block/bcache0/bcache/attach
important if you have writeback caching turned on.
If you're booting up and your cache device is gone and never coming back, you
-can force run the backing device:
+can force run the backing device::
echo 1 > /sys/block/sdb/bcache/running
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
read some of the dirty data, though.
-HOWTO/COOKBOOK
+Howto/cookbook
--------------
A) Starting a bcache with a missing caching device
If registering the backing device doesn't help, it's already there, you just need
-to force it to run without the cache:
+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:
+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 does not find its cache
+B) Bcache does not find its cache::
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] : 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:
+or disappeared and came back, and needs to be (re-)registered::
+
host:/sys/block/md5/bcache# echo /dev/sdh2 > /sys/fs/bcache/register
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:
+For example::
+
losetup -o 8192 /dev/loop0 /dev/your_bcache_backing_dev
This should present your unmodified backing device data in /dev/loop0
E) Wiping a cache 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
+::
+
+ 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::
-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
+ 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
+start backing device with missing cache::
-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
+ host:/sys/block/md5/bcache# echo 1 > running
+attach new cache::
-F) Remove or replace a caching device
+ 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
+
+
+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
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:
+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:
+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
1) Is it present in /dev/bcache* ? (there are times where it won't be)
-If so, it's easy:
+ 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:
+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:
+ 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. This would be true of any block device stacking
-where bcache is a lower device.
+ 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/::
-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/
-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::
-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.
+ This will free up bcache references and let you reuse the partition for
+ other purposes.
-TROUBLESHOOTING PERFORMANCE
+Troubleshooting performance
---------------------------
Bcache has a bunch of config options and tunables. The defaults are intended to
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:
+ 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:
+ 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
running in writeback mode, which isn't the default (not due to a lack of
maturity, but simply because in writeback mode you'll lose data if something
- happens to your SSD)
+ happens to your SSD)::
- # echo writeback > /sys/block/bcache0/bcache/cache_mode
+ # echo writeback > /sys/block/bcache0/bcache/cache_mode
- Bad performance, or traffic not going to the SSD that you'd expect
accessed data out of your cache.
But if you want to benchmark reads from cache, and you start out with fio
- writing an 8 gigabyte test file - so you want to disable that.
+ writing an 8 gigabyte test file - so you want to disable that::
- # echo 0 > /sys/block/bcache0/bcache/sequential_cutoff
+ # echo 0 > /sys/block/bcache0/bcache/sequential_cutoff
- To set it back to the default (4 mb), do
+ To set it back to the default (4 mb), do::
- # echo 4M > /sys/block/bcache0/bcache/sequential_cutoff
+ # echo 4M > /sys/block/bcache0/bcache/sequential_cutoff
- Traffic's still going to the spindle/still getting cache misses
throttles traffic if the latency exceeds a threshold (it does this by
cranking down the sequential bypass).
- You can disable this if you need to by setting the thresholds to 0:
+ You can disable this if you need to by setting the thresholds to 0::
- # echo 0 > /sys/fs/bcache/<cache set>/congested_read_threshold_us
- # echo 0 > /sys/fs/bcache/<cache set>/congested_write_threshold_us
+ # echo 0 > /sys/fs/bcache/<cache set>/congested_read_threshold_us
+ # echo 0 > /sys/fs/bcache/<cache set>/congested_write_threshold_us
The default is 2000 us (2 milliseconds) for reads, and 20000 for writes.
a fix for the issue there).
-SYSFS - BACKING DEVICE
+Sysfs - backing device
----------------------
Available at /sys/block/<bdev>/bcache, /sys/block/bcache*/bcache and
still be added to the cache until it is mostly full; only meant for
benchmarking. Defaults to on.
-SYSFS - BACKING DEVICE STATS:
+Sysfs - backing device stats
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
There are directories with these numbers for a running total, as well as
versions that decay over the past day, hour and 5 minutes; they're also
bypassed
Amount of IO (both reads and writes) that has bypassed the cache
-cache_hits
-cache_misses
-cache_hit_ratio
+cache_hits, cache_misses, cache_hit_ratio
Hits and misses are counted per individual IO as bcache sees them; a
partial hit is counted as a miss.
-cache_bypass_hits
-cache_bypass_misses
+cache_bypass_hits, cache_bypass_misses
Hits and misses for IO that is intended to skip the cache are still counted,
but broken out here.
cache_readaheads
Count of times readahead occurred.
-SYSFS - CACHE SET:
+Sysfs - cache set
+~~~~~~~~~~~~~~~~~
Available at /sys/fs/bcache/<cset-uuid>
Echoing a size to this file (in human readable units, k/M/G) creates a thinly
provisioned volume backed by the cache set.
-io_error_halflife
-io_error_limit
+io_error_halflife, io_error_limit
These determines how many errors we accept before disabling the cache.
Each error is decayed by the half life (in # ios). If the decaying count
reaches io_error_limit dirty data is written out and the cache is disabled.
Detaches all backing devices and closes the cache devices; if dirty data is
present it will disable writeback caching and wait for it to be flushed.
-SYSFS - CACHE SET INTERNAL:
+Sysfs - cache set internal
+~~~~~~~~~~~~~~~~~~~~~~~~~~
This directory also exposes timings for a number of internal operations, with
separate files for average duration, average frequency, last occurrence and max
trigger_gc
Writing to this file forces garbage collection to run.
-SYSFS - CACHE DEVICE:
+Sysfs - Cache device
+~~~~~~~~~~~~~~~~~~~~
Available at /sys/block/<cdev>/bcache
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