Hi All,
Summary: the kernel can't use an existing /dev/bcache0 as backing device
for /dev/bcache1, and throws a lot of errors in /var/log/syslog.
In detail:
I would like to use a small slow ssd as a write cache for a hard drive,
and a large, fast SSD as read cache. Therefore I'm trying to stack two
bcaches on top of each other like this: (Note, I have done all tests on
one HDD with a lot of partitions, so all devices start with /dev/sde)
+---------------------------------+
| btrfs file system | |
+---------------------------------+
| /dev/Bcache1 |
+---------------------------------+
| Read cache on fast ssd |
| /dev/sde3 (5GB) |
+---------------------------------+
| /dev/Bcache0 |
+---------------------------------+
| Write Cache on slow SSD |
| /dev/sde2 (2GB) |
+---------------------------------+
| Data on big spinning hard drive |
| /dev/sde1 (40GB) |
+---------------------------------+
I create the stack from bottom to top:
1) Format the big spinning hard drive as backing device
# make-bcache -B /dev/sde1
Name /dev/sde1
Label
Type data
UUID: a2b424ce-c7dd-4d16-8600-fa4b47306865
Set UUID: f9698940-513b-4649-9d0e-58216587bd6f
version: 1
block_size_in_sectors: 1
data_offset_in_sectors: 16
# dmesg
[52742.372299] bcache: register_bdev() registered backing device sde1
2) Format the slow SSD as cache device
# make-bcache -C /dev/sde2
Name /dev/sde2
Label
Type cache
UUID: 58092ee5-2412-4f25-af21-ed8d30fb3f1c
Set UUID: a5bfbb35-dd42-4593-af63-9af53a5cc3a0
version: 0
nbuckets: 3814
block_size_in_sectors: 1
bucket_size_in_sectors: 1024
nr_in_set: 1
nr_this_dev: 0
first_bucket: 1
# dmesg
[52965.412606] bcache: run_cache_set() invalidating existing data
[52965.438891] bcache: register_cache() registered cache device sde2
3) Attach the slow ssd cache (/dev/sde2) to the big spinning hard drive
(/dev/sde1)
# bcache-super-show /dev/sde2 | grep cset
cset.uuid a5bfbb35-dd42-4593-af63-9af53a5cc3a0
echo a5bfbb35-dd42-4593-af63-9af53a5cc3a0 > /sys/block/bcache0/bcache/attach
# dmesg
Mar 2 17:45:17 cedric kernel: bcache: bch_cached_dev_run() cached dev
sde1 is running already
Mar 2 17:45:17 cedric kernel: bcache: bch_cached_dev_attach() Caching
sde1 as bcache0 on set a5bfbb35-dd42-4593-af63-9af53a5cc3a0
4) Format /dev/bcache0 as backing device
# make-bcache -B /dev/bcache0
Name /dev/bcache0
Label
Type data
UUID: 21cc37f3-9aa7-4eba-b652-8cd6b1f812ae
Set UUID: 2d45c3c2-1efc-43bb-80e0-33859649aba2
version: 1
block_size_in_sectors: 1
data_offset_in_sectors: 16
# tail -F /var/log/everything.log
Mar 2 17:48:47 cedric kernel: sysfs: cannot create duplicate filename
'/devices/virtual/block/bcache0/bcache'
Mar 2 17:48:47 cedric kernel: CPU: 1 PID: 37431 Comm: kworker/1:4
Tainted: G OE 5.16.11-arch1-1 #1
ded1ca8dd1dd660648f829b67fad213afe36c9c9
Mar 2 17:48:47 cedric kernel: Hardware name: Gigabyte Technology Co.,
Ltd. B550 AORUS PRO AC/B550 AORUS PRO AC, BIOS F12 01/18/2021
Mar 2 17:48:47 cedric kernel: Workqueue: events register_bdev_worker
[bcache]
Mar 2 17:48:47 cedric kernel: Call Trace:
Mar 2 17:48:47 cedric kernel: <TASK>
Mar 2 17:48:47 cedric kernel: dump_stack_lvl+0x48/0x5e
Mar 2 17:48:47 cedric kernel: sysfs_warn_dup.cold+0x17/0x24
Mar 2 17:48:47 cedric kernel: sysfs_create_dir_ns+0xc6/0xe0
Mar 2 17:48:47 cedric kernel: kobject_add_internal+0xbd/0x2c0
Mar 2 17:48:47 cedric kernel: kobject_add+0x98/0xd0
Mar 2 17:48:47 cedric kernel: ? bcache_device_init+0x242/0x2a0 [bcache
eb8586620a25cfc9a2e260d15875c9beb6c2953d]
Mar 2 17:48:47 cedric kernel: register_bdev_worker+0x30d/0x3b0 [bcache
eb8586620a25cfc9a2e260d15875c9beb6c2953d]
Mar 2 17:48:47 cedric kernel: process_one_work+0x1e8/0x3c0
Mar 2 17:48:47 cedric kernel: worker_thread+0x50/0x3b0
Mar 2 17:48:47 cedric kernel: ? rescuer_thread+0x3a0/0x3a0
Mar 2 17:48:47 cedric kernel: kthread+0x15c/0x180
Mar 2 17:48:47 cedric kernel: ? set_kthread_struct+0x40/0x40
Mar 2 17:48:47 cedric kernel: ret_from_fork+0x22/0x30
Mar 2 17:48:47 cedric kernel: </TASK>
Mar 2 17:48:47 cedric kernel: kobject_add_internal failed for bcache
with -EEXIST, don't try to register things with the same name in the
same directory.
Mar 2 17:48:47 cedric kernel: bcache: register_bdev() error bcache0:
error creating kobject
Mar 2 17:48:47 cedric kernel: bcache: register_bdev_worker() error
/dev/bcache0: fail to register backing device
Mar 2 17:48:47 cedric kernel: bcache: bcache_device_free() bcache1 stopped
# ls -l /dev/bca*
brw-rw---- 1 root disk 254, 0 Mar 2 17:48 /dev/bcache0
Expected result:
I expected step 4 to create the new bcache device /dev/bcache1.
>That would have enabled me to take these steps:
5) Format the fast SSD as cache device:
# make-bcache -C /dev/sde3
6) Attach the fast ssd cache /dev/sde3 to /dev/bcache1 created in step 4
# bcache-super-show /dev/sde3 | grep cset
cset.uuid 957377e0-ae6f-45fb-9ad8-9ff7ca83a861
# echo 957377e0-ae6f-45fb-9ad8-9ff7ca83a861 >
/sys/block/bcache1/bcache/attach
7) Finally use /dev/bcache1 as device for the btrfs filesystem:
# mkfs.btrfs /dev/bcache1
My Kernel:
# uname -a
Linux cedric 5.16.11-arch1-1 #1 SMP PREEMPT Thu, 24 Feb 2022 02:18:20
+0000 x86_64 GNU/Linux
How can I help to debug this?
Is linux-bcache the best place to report this problem?
Is there a workaround for this problem?
Kind regards,
Cedric
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