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:

> 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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> >>
> >
> >
> >
> > --
> > Eric Wheeler, President           eWheeler, Inc. dba Global Linux Security
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> >
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