[ +cc: linux-doc, kent ] On Tue, 12 Apr 2016, Marc MERLIN wrote: > On Mon, Mar 28, 2016 at 6:09 PM, Eric Wheeler <bcache@xxxxxxxxxxxxxxxxxx> wrote: > > git pull https://bitbucket.org/ewheelerinc/linux v4.6-rc1-bcache-documentation > > Hi Eric, > > Do I have a way to find out if your patch was pulled? I've not seen them in the commit history for 4.6-rc3: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/log/?id=v4.6-rc3&qt=grep&q=bcache: Hello linux-doc list! There is a bcache documentation update available: > > git pull https://bitbucket.org/ewheelerinc/linux v4.6-rc1-bcache-documentation Is this the right list to submit such documentation? -- Eric Wheeler > > Thanks, > Marc > > On Mon, Mar 28, 2016 at 6:09 PM, Eric Wheeler <bcache@xxxxxxxxxxxxxxxxxx> wrote: > > Hi Jens, > > > > I've rebased documentation updates against 4.6-rc1. There is some reflow > > and trailing whitespace cleanup, so its not pretty---but it cleans > > fills in some missing documentation and adds corrections: > > > > git pull https://bitbucket.org/ewheelerinc/linux v4.6-rc1-bcache-documentation > > > > > > -- > > Eric Wheeler > > > > On Sat, 12 Mar 2016, Eric Wheeler wrote: > > > >> 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 > >> > > >> > > > > -- > "A mouse is a device used to point at the xterm you want to type in" - A.S.R. > Microsoft is to operating systems & security .... > .... what McDonalds is to gourmet cooking > Home page: http://marc.merlins.org/ > -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
