Re: can't use ramdisk as cache device. Receive "Can't attach [UUID]: cache set not found" error

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

On Thu, 25 Feb 2016, Eric Wheeler wrote:

> On Thu, 25 Feb 2016, Scott McGillivray wrote:
> 
> > that doesn't work either. It gives the same error from my opening
> > post. Although this time it's also giving a new message in dmesg about
> > block size.
> > 
> > 
> > bcache: bch_cached_dev_attach() Couldn't attach sdf: block size less
> > than set's block size
> > bcache: __cached_dev_store() Can't attach 0bba8852-ad4f-41f4-bfa3-679a886a14ad
> >                : cache set not found
> > 
> > which is strange because both devices block size are the same
> > 
> > $ blockdev --getbsz /dev/sdf
> > 4096
> > 
> > $ blockdev --getbsz /dev/zram0
> > 4096
> 
> 
> Try this:
>   make-bcache -w 4096 -C /dev/zram0

I'm not sure if -w needs to be called on the backing device too---but if 
you do then expect to loose data when reformatting.

If you decide reformat both backing and cache devices to test, then you 
might consider aligning your data to your backing volume's stride width 
with --data-offset NNN while you're at it if you haven't already.

-Eric

> 
> > 
> > 
> > On 25 February 2016 at 14:44, Eric Wheeler <bcache@lists.ewheeler.net> wrote:
> > > On Thu, 25 Feb 2016, Scott McGillivray wrote:
> > >
> > >> No, doesn't' work either i'm afraid. Below is example of failing
> > >> silently with ram disk and succeeded with HDD as cache device.
> > >>
> > >> Tested with /lib/modules/4.5.0-rc3+/kernel/drivers/md/bcache/bcache.ko
> > >>
> > >>
> > >> with zram - doesn't work
> > >> =================
> > >>
> > >> $ ls -al /sys/fs/bcache/
> > >> total 0
> > >> drwxr-xr-x 2 root root    0 Feb 25 13:42 .
> > >> drwxr-xr-x 9 root root    0 Feb 25 13:40 ..
> > >> --w------- 1 root root 4096 Feb 25 13:40 register
> > >> --w------- 1 root root 4096 Feb 25 13:40 register_quiet
> > >>
> > >> $ file /dev/zram0
> > >> /dev/zram0: block special (250/0)
> > >>
> > >> $ fdisk -l /dev/zram0
> > >> Disk /dev/zram0: 1 GiB, 1073741824 bytes, 262144 sectors
> > >>
> > >> $ make-bcache -C /dev/zram0
> > >> UUID: 87e63981-7bd8-4cc3-93db-9ca275367c21
> > >> Set UUID: 0bba8852-ad4f-41f4-bfa3-679a886a14ad
> > >> version: 0
> > >> nbuckets: 2048
> > >> block_size: 8
> > >> bucket_size: 1024
> > >> nr_in_set: 1
> > >> nr_this_dev: 0
> > >> first_bucket: 1
> > >
> > > did you   `echo /dev/zram0 > /sys/fs/bcache/register`
> > > and then  `echo <CSET-UUID> > /sys/block/bcache0/bcache/attach`
> > >
> > > Not sure which UUID is the CSET UUID.  Probably
> > > 0bba8852-ad4f-41f4-bfa3-679a886a14ad.
> > >
> > > See attaching bcache0 to a cache:
> > > https://www.kernel.org/doc/Documentation/bcache.txt
> > >
> > > -Eric
> > >
> > >>
> > >>
> > >> No errors, but nothing in dmesg and contents of /sys/fs/bcache/ remain
> > >> exactly the same... no UUID. Seems to just silently fail.
> > >>
> > >>
> > >> with HDD - works as expected.
> > >> =====================
> > >>
> > >> $ fdisk -l /dev/sdf
> > >> Disk /dev/sdf: 1 GiB, 1073741824 bytes, 2097152 sectors
> > >>
> > >> $ file /dev/sdf
> > >> /dev/sdf: block special (8/80)
> > >>
> > >> $ make-bcache -C /dev/sdf
> > >> UUID: 3234c9e7-8186-4a7c-99a8-ae6a371c6f2c
> > >> Set UUID: 9a71faf7-a408-4f4d-906e-e5d67e43291f
> > >> version: 0
> > >> nbuckets: 2048
> > >> block_size: 1
> > >> bucket_size: 1024
> > >> nr_in_set: 1
> > >> nr_this_dev: 0
> > >> first_bucket: 1
> > >>
> > >> $ ls -al /sys/fs/bcache/
> > >> total 0
> > >> drwxr-xr-x 3 root root    0 Feb 25 13:48 .
> > >> drwxr-xr-x 9 root root    0 Feb 25 13:40 ..
> > >> drwxr-xr-x 7 root root    0 Feb 25 13:48 9a71faf7-a408-4f4d-906e-e5d67e43291f
> > >> --w------- 1 root root 4096 Feb 25 13:40 register
> > >> --w------- 1 root root 4096 Feb 25 13:40 register_quiet
> > >>
> > >> $ dmesg
> > >> [  714.168488] bcache: register_cache() registered cache device sdf
> > >>
> > >>
> > >> Thank you.
> > >>
> > >> On 24 February 2016 at 15:37, Eric Wheeler <lists@ewheeler.net> wrote:
> > >> >
> > >> > On Tue, 16 Feb 2016, Scott McGillivray wrote:
> > >> >
> > >> >> Hello
> > >> >>
> > >> >> I am using Ubuntu 15.10 with latest nightly 4.5 kernel. Ubuntu creates
> > >> >> 16 RAM drives as /dev/ram0 - ram16 by default and i am trying to test
> > >> >> bcache and use one of these as a cache device.
> > >> >>
> > >> >> wipefs /dev/sdd
> > >> >> wipefs /dev/sdd -a
> > >> >> wipefs /dev/ram0
> > >> >> wipefs /dev/ram0 -a
> > >> >> make-bcache -B /dev/sdd -C /dev/ram0
> > >> >
> > >> > Does it work with /dev/zram0?   You would prep it something like this:
> > >> >
> > >> >         modprobe zram
> > >> >         echo $((1024*1024*1024)) > /sys/block/zram0/disksize # 1GB
> > >> >         make-bcache  -C /dev/zram0
> > >> >
> > >> > -Eric
> > >> >
> > >> >
> > >> >
> > >> >> At this point there were no errors output but there is also no UUID
> > >> >> entry in /sys/fs/bcache/. Only register and register_quiet.
> > >> >>
> > >> >> bache-super-show correctly prints info about both block and cache
> > >> >> devices when queried but when i try to manually attach the cache
> > >> >> device with echo cset.uuid > /sys/block/bcache0/bcache/attach i get no
> > >> >> error but in dmesg i see
> > >> >>
> > >> >> bcache: __cached_dev_store() Can't attach
> > >> >> 3f2521a6-2103-4e2e-a47d-cc56558e8f7d: cache set not found
> > >> >>
> > >> >> If i repeat this example but use another sd block device as cache then
> > >> >> it works OK.
> > >> >>
> > >> >> Can anyone confirm if it should be possible to use the ramdisk based
> > >> >> block device as a caching device with bcache ?
> > >> >>
> > >> >> I also tried to use a loop device as cache and that didn't work
> > >> >> either.. i got "kernel bug" in dmesg for
> > >> >> drivers/md/bcache/super.c:1812. RIP []
> > >> >> cache_alloc.isra.21+0x646/0x670[bcache]
> > >> >>
> > >> >> Does bcache need the cache device to be a certain block type i.e sd 8
> > >> >> ? Should i be able to use any block device from /proc/devices as a
> > >> >> cache device ?
> > >> >>
> > >> >> The kernel options used for ramdisk are:
> > >> >>
> > >> >> CONFIG_BLK_DEV_RAM=y
> > >> >> CONFIG_BLK_DEV_RAM_COUNT=16
> > >> >> CONFIG_BLK_DEV_RAM_SIZE=65536
> > >> >> CONFIG_BLK_DEV_RAM_DAX=y
> > >> >>
> > >> >> Thank you.
> > >> >> Scott
> > >> >> --
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> > >> >> the body of a message to majordomo@vger.kernel.org
> > >> >> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > >> >>
> > >> >
> > >> >
> > >> >
> > >> > --
> > >> > Eric Wheeler, President           eWheeler, Inc. dba Global Linux Security
> > >> > 888-LINUX26 (888-546-8926)        Fax: 503-716-3878           PO Box 25107
> > >> > www.GlobalLinuxSecurity.pro       Linux since 1996!     Portland, OR 97298
> > >> >
> > >> --
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> > >>
> > 
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