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| -rw-r--r-- | Documentation/bcache.txt | 160 |
1 files changed, 152 insertions, 8 deletions
diff --git a/Documentation/bcache.txt b/Documentation/bcache.txt index 32b6c3189d98..0aba405d3368 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 @@ -127,7 +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* |