best way to get the bdev of a bcache block device

best way to get the bdev of a bcache block device

[Stefan Priebe - Profihost AG]

Hello,

what's the best way to get the correct bcache backing device (bdev) form
a known dev (f.e. /dev/bcache2)?

I haven't found an easy reliable way through /sys/block.

Thanks!

Greets,
Stefan

Re: best way to get the bdev of a bcache block device

[Michael Lyle]

On 01/22/2018 01:59 PM, Stefan Priebe - Profihost AG wrote:
> Hello,
> 
> what's the best way to get the correct bcache backing device (bdev) form
> a known dev (f.e. /dev/bcache2)?
> 
> I haven't found an easy reliable way through /sys/block.
> 
> Thanks!
> 
> Greets,
> Stefan
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/sys/block/bcache0/slaves

/sys/block/bcache0/bcache/cache/bdev0

Re[2]: best way to get the bdev of a bcache block device

[Pavel Goran]

Hello Michael,

Tuesday, January 23, 2018, 1:20:28 AM, you wrote:

> On 01/22/2018 01:59 PM, Stefan Priebe - Profihost AG wrote:
>> Hello,
>> 
>> what's the best way to get the correct bcache backing device (bdev) form
>> a known dev (f.e. /dev/bcache2)?
>> 
>> I haven't found an easy reliable way through /sys/block.
>> 
>> Thanks!
>> 
>> Greets,
>> Stefan

> /sys/block/bcache0/slaves

This lists both the backing device and the cache device(s), without an
(apparent) way of identifying which one is backing.

> /sys/block/bcache0/bcache/cache/bdev0

/sys/block/bcache0/bcache/cache links to the cache set (/sys/fs/bcache/[UUID])
which can serve several bcache devices, so bdev0 would link to the first
backing device registered in the set, not the backing device that corresponds
to the bcache device we started with.

However, it's actually possible to go further: iterate all bdevX devices and
identify the correct one by looking at the "dev" symlink:

/sys/block/bcache0/bcache/cache/bdev0/dev
/sys/block/bcache0/bcache/cache/bdev1/dev
etc.

The device whose "dev" symlink points to bcache0 should be the backing device
for bcache0.

Pavel Goran