Hi Jens, Please pull: Documentation updates for bcache: git pull https://bitbucket.org/ewheelerinc/linux.git v4.5-rc7-bcache-fixes-2 -- Eric Wheeler On Fri, 11 Mar 2016, Marc MERLIN wrote: > 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@xxxxxxxxxxx> > --- > Documentation/bcache.txt | 160 ++++++++++++++++++++++++++++++++++++++++++++--- > 1 file changed, 152 insertions(+), 8 deletions(-) > > diff --git a/Documentation/bcache.txt b/Documentation/bcache.txt > index 32b6c31..b8302f9 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* > -- > 2.5.3 > > > -- > "A mouse is a device used to point at the xterm you want to type in" - A.S.R. > Microsoft is to operating systems .... > .... what McDonalds is to gourmet cooking > Home page: http://marc.merlins.org/ | PGP 1024R/763BE901 > -- > To unsubscribe from this list: send the line "unsubscribe linux-bcache" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- To unsubscribe from this list: send the line "unsubscribe linux-bcache" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
