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 [off-list ref] wrote:

quoted

On Tue, 16 Feb 2016, Scott McGillivray wrote:

quoted

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

quoted

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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