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.
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
