Hi Coly, On Wed, Feb 1, 2023 at 4:03 AM mingzhe <mingzhe.zou@xxxxxxxxxxxx> wrote: > > > > 在 2023/1/31 23:51, Coly Li 写道: > > > > > >> 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/ . > >> 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. I didn't test it extensively but it looks like that the patch to achieve this is just a one-line patch, it could be very beneficial. (I just realized that mingzhe sent a relevant patch on this, thank for your work) Our use case will be to be able to take advantage of different blocking devices that differ in performance and cost. Some of these blocking devices are ephemeral and not suitable for wb cache mode, but stacking them with non-ephemeral ones would be a very nice and neat solution. Cheers, Andrea > > Hi, Coly > > Maybe we have a case like this. We are considering make-bcache a hdd as > a cache device and create a flash device in it, and then using the flash > device as a backing. So， completely converts writeback to sequential > writes. > > However, we found that there may be many unknown problems in the flash > device, such as the created size, etc. > > For now, we've put it due to time，but we think it might be a good thing > to do. We also have some patches, I will post them. > > mingzhe > > > 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. > > > > Thanks. > > > > Coly Li
