How to restart a ‘stopped’ bcache device

bcache

I need to upgrade some drive firmware and I'd like to shut down bcache for the duration.

The docs show how to stop a bcache device:

stop

Write to this file to shut down the bcache device and close the backing device.

For me that will look like this:

echo 1 > /sys/block/bcache0/bcache/stop

and for the cache device:

echo 1 > /sys/fs/bcache/<set-uuid>/stop

But how do I bring the device back (without rebooting the server)?

Best Answer

Simply re-register each bcache device in the cache set (both backing and cache devices) to the kernel:

echo /dev/<path_to_device> > /sys/fs/bcache/register

Or, if the udev rules from bcache-tools are in place, then partprobe will automatically register the devices when they are scanned.

Related Solutions

Ubuntu – BCache and disk encryption

To answer 1., the most sensible thing to do is to put bcache on top of two LUKS virtual devices. LUKS-encrypting a bcache device might work, but there's no guarantee LUKS will consistently put the same virtual sector in the same physical sector every time. You can encrypt both LUKS devices with the same keyfile and unlock both at the same time.

Optimizing bcache

I have same problem. My disk IO still bypass by bcache. After set congested_read_threshold_us and congested_write_threshold_us following bcache documentation. My problem is solved.

- Traffic's still going to the spindle/still getting cache misses

   In the real world, SSDs don't always keep up with disks - particularly with
   slower SSDs, many disks being cached by one SSD, or mostly sequential IO. So
   you want to avoid being bottlenecked by the SSD and having it slow everything
   down.

   To avoid that bcache tracks latency to the cache device, and gradually
   throttles traffic if the latency exceeds a threshold (it does this by
   cranking down the sequential bypass).

   You can disable this if you need to by setting the thresholds to 0:

   # echo 0 > /sys/fs/bcache/<cache set>/congested_read_threshold_us
   # echo 0 > /sys/fs/bcache/<cache set>/congested_write_threshold_us

   The default is 2000 us (2 milliseconds) for reads, and 20000 for writes.

All disk IO are sent to my SSD(sde) now.

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sdb               0.00     0.00    0.00    0.30     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdd               0.00     0.00    0.10    0.30     0.80     0.00     4.00     0.00    3.00   12.00    0.00   3.00   0.12
sdc               0.00     0.00    2.20    0.30    26.00     0.00    20.80     0.00    1.76    2.00    0.00   1.76   0.44
sda               0.00     0.00    0.20    0.20     0.80     0.00     4.00     0.01    8.00   16.00    0.00  13.00   0.52
sde               0.00   293.20   81.70  232.70  1129.20 58220.00   377.54     6.62   21.05   27.69   18.71   3.18 100.00
md1               0.00     0.00    2.50    0.30    27.60     0.00    19.71     0.00    0.00    0.00    0.00   0.00   0.00
md0               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
bcache0           0.00     0.00   83.00  402.40  1156.80 28994.80   124.23    15.36   31.70   27.02   32.67   2.06  99.92

Best Answer

Related Solutions

Ubuntu – BCache and disk encryption

Optimizing bcache

Related Question