Re: How to use multiple backing devices

[Nikolaus Rath <Nikolaus@rath.org>]

On Feb 09 2016, Kai Krakow <hurikhan77@gmail.com> wrote:
> Am Mon, 08 Feb 2016 20:47:10 -0800
> schrieb Nikolaus Rath <Nikolaus@rath.org>:
>
>> Hello,
>> 
>> If I'm understanding Documentation/bcache.txt correctly, I should be
>> able to use one SSD to cache multiple spinning disks.
>> 
>> However, I'm at a loss of how to set this up in practice. I believe I
>> need to do
>> 
>>   make-bcache -B /dev/sda  # spinning rust
>>   make-bcache -B /dev/sdb  # spinning rust 
>>   make-bcache -C /dev/sdc  # ssd
>> 
>> and then something like
>> 
>>   echo <CSET-UUID> > /sys/block/bcacheX/bcache/attach
>> 
>> 
>> But what do I have to put for X, and what for CSET-UUID? I believe for
>> at least one of those values I will have two options (because there
>> are two backing devices).
>
> Create it in one go:
>
> make-bcache -C /dev/sdc -B /dev/sd{a,b}
>
> It will take away the hassle. Everything should be setup now.

Yeah, but unfortunately I don't have enough space for that. I need to
create one, move the data over from second, create the second, move data
over from the SSD, and then create the SSD is caching device.


> But if you want to do it manually: you have to run "echo" twice - for
> each X (the backing devices). CSET-UUID comes from /sys/fs/bcache where
> you will find a UUID per each bcache caching device, which is just one
> for your case.


Ah, ok. Intuitively attaching the same cache device to two different
backing devices sounds dangerous, but I'll  take your word that the code
is designed for that...

Thanks,
-Nikolaus

(No Cc on replies please, I'm reading the list)
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