2 drives btrfs raid1 with one ssd

2 drives btrfs raid1 with one ssd

[Pavol Cupka]

Hi all,

I would like to use a bcache for btrfs filesystem residing on two identical disk as raid1 using btrfs' raid (without soft/hard raid).

Is it possible? I am migrating from a different mirror setup and have to create btrfs in "degraded" mode and then add the second drive/bcache.

I would like to use the same ssd/ssd partition for caching both drives/mirror. Is this doable? 

Did someone encounter problems with bcache build as module? I can not stop the current caching device.

Thank you 
pavol

Re: 2 drives btrfs raid1 with one ssd

[Matthew Patton]

On Sun, 15 Dec 2013 20:01:57 -0500, Pavol Cupka <pavol.cupka@gmail.com>  
wrote:

> I would like to use a bcache for btrfs filesystem residing on two  
> identical disk as raid1 using btrfs' raid (without soft/hard raid).
>
> Is it possible?

if I understood you correctly, no.

> I would like to use the same ssd/ssd partition for caching both  
> drives/mirror. Is this doable?

You will have to build the btrfs 'raid' on top of two bcache devices. If  
the caching device for both devices in the same SSD (partitioned) what's  
the point? Assuming Btrfs has a similar feature set to MD's Raid1  
write-mostly, you could accelerate the one side, and run the other half of  
the mirror without acceleration. Otherwise I would just use MD's Raid1 and  
be done with it. Put btrfs/ext4/xfs on top as you like.

Does your intended workload really benefit from putting an SSD into the  
mix?

Re: 2 drives btrfs raid1 with one ssd

[Pavol Cupka]

I was thinking of simulating L2ARC like design from ZFS for btrfs.
that's why I wanted one ssd partition taking care of both mirrored
drives, mostly for read cache.


thanks for your reply.
pavol

On Mon, Dec 16, 2013 at 2:11 AM, Matthew Patton <pattonme@yahoo.com> wrote:
> On Sun, 15 Dec 2013 20:01:57 -0500, Pavol Cupka <pavol.cupka@gmail.com>
> wrote:
>
>> I would like to use a bcache for btrfs filesystem residing on two
>> identical disk as raid1 using btrfs' raid (without soft/hard raid).
>>
>> Is it possible?
>
>
> if I understood you correctly, no.
>
>
>> I would like to use the same ssd/ssd partition for caching both
>> drives/mirror. Is this doable?
>
>
> You will have to build the btrfs 'raid' on top of two bcache devices. If the
> caching device for both devices in the same SSD (partitioned) what's the
> point? Assuming Btrfs has a similar feature set to MD's Raid1 write-mostly,
> you could accelerate the one side, and run the other half of the mirror
> without acceleration. Otherwise I would just use MD's Raid1 and be done with
> it. Put btrfs/ext4/xfs on top as you like.
>
> Does your intended workload really benefit from putting an SSD into the mix?

Re: 2 drives btrfs raid1 with one ssd

[Pavol Cupka]

could you do it without mdraid? directly using the btrfs raid1 settings?

thanks

On Thu, Dec 19, 2013 at 3:30 AM, matthew patton <pattonme@yahoo.com> wrote:
>> I was thinking of simulating L2ARC like design from ZFS for btrfs.
>
>
> construct one leg of the MD Raid1 from a bCache device. Construct the other leg from regular disk and put it into write-mostly. Put BTRfs on top. Now you have L2ARC.