Re: [PULL v4.6-rc1] bcache: documentation updates

[Eric Wheeler <bcache@lists.ewheeler.net>]

[ +cc: linux-doc, kent ]

On Tue, 12 Apr 2016, Marc MERLIN wrote:
> On Mon, Mar 28, 2016 at 6:09 PM, Eric Wheeler <bcache@lists.ewheeler.net> 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@lists.ewheeler.net> 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@merlins.org>
> >> > ---
> >> >  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@vger.kernel.org
> >> > 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/
>