On Tue, 31 Jan 2023, Coly Li wrote: > > 2023年1月26日 19:30，Andrea Tomassetti <andrea.tomassetti-opensource@xxxxxxxx> 写道： > > > > Hi, > > I know that bcache doesn't natively support multi-level caching but I > > was playing with it and found this interesting "workaround": > > make-bcache -B /dev/vdb -C /dev/vdc > > the above command will generate a /dev/bcache0 device that we can now > > use as backing (or cached) device: > > make-bcache -B /dev/bcache0 -C /dev/vdd > > This will make the kernel panic because the driver is trying to create > > a duplicated "bcache" folder under /sys/block/bcache0/ . panic? Ouch. > > So, simply patching the code inside register_bdev to create a folder > > "bcache2" if "bcache" already exists does the trick. > > Now I have: > > vdb 252:16 0 5G 0 disk > > └─bcache0 251:0 0 5G 0 disk > > └─bcache1 251:128 0 5G 0 disk /mnt/bcache1 > > vdc 252:32 0 10G 0 disk > > └─bcache0 251:0 0 5G 0 disk > > └─bcache1 251:128 0 5G 0 disk /mnt/bcache1 > > vdd 252:48 0 5G 0 disk > > └─bcache1 251:128 0 5G 0 disk /mnt/bcache1 > > > > Is anyone using this functionality? I assume not, because by default > > it doesn't work. > > Is there any good reason why this doesn't work by default? > > > > I tried to understand how data will be read out of /dev/bcache1: will > > the /dev/vdd cache, secondly created cache device, be interrogated > > first and then will it be the turn of /dev/vdc ? > > Meaning: can we consider that now the layer structure is > > > > vdd > > └─vdc > > └─bcache0 > > └─bcache1 > > ? > > IIRC, there was a patch tried to achieve similar purpose. I was not > supportive for this idea because I didn’t see really useful use case. In > general, extra layer cache means extra latency in the I/O path. What I > see in practical deployments are, people try very hard to minimize the > cache layer and place it close to application. > > Introduce stackable bcache for itself may work, but I don’t see real > usage yet, and no motivation to maintain such usage still. Hi Coly, If Andrea's patch is simple and prevents a panic, would you consider accepting it? Users should not be able to crash the kernel no matter what they do. Perhaps this should be considered a bug. If so, then Andrea, can you post your suggested patch for review? -Eric > > Thanks. > > Coly Li
