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, 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

> 
> 
> 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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