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