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Diffstat (limited to 'Documentation/bcache.txt')
| -rw-r--r-- | Documentation/bcache.txt | 197 |
1 files changed, 187 insertions, 10 deletions
diff --git a/Documentation/bcache.txt b/Documentation/bcache.txt index 32b6c31..a9259b5 100644 --- a/Documentation/bcache.txt +++ b/Documentation/bcache.txt @@ -1,4 +1,4 @@ -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 @@ -127,12 +133,181 @@ 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) 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: + 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 + +Note that this may cause data loss if you were running in writeback mode. + + +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] 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 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 + +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 + + +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:~# 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 + + +G) dm-crypt 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?] + + +H) Stop/free a registered bcache to wipe and/or recreate 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 + +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. 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/ +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 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 @@ -140,7 +315,7 @@ 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 +368,9 @@ 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* @@ -238,7 +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: @@ -325,7 +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 |
