Changing allocation mode

Changing allocation mode

[Fredrik Tolf]

Dear list,

I'm still in the process of transferring all the data I have to the btrfs 
filesystem I have had your help in debugging in a previous thread, and I 
have a slight question, if you will humour me.

I have the data I want to transfer on an old ReiserFS partition, 
consisting of 2 mdraid mirrors, one of which consists of two 1.5 TB disks, 
and the other of two 3 TB disks. The btrfs I'm copying the data to 
consists of two 3 TB disks only that I have put in RAID-1 mode, and the 
data on the old filesystem is only slightly larger than 3 TB. I am now at 
the point where I have transferred just under 3 TB.

If I were transferring the data to a new filesystem on mdraid, the 
procedure I would use for that last portion of the data would be to remove 
one disk only from either of the old mdraid mirror arrays (putting that 
array in degraded mode), and then create a new mirror in degraded mode 
with only that disk, add that mirror to the new filesystem, expand it, 
copy the last data, and then delete the old mirrors, moving the rest of 
the disks to the new filesystem.

Is there a way to mirror this procedure in btrfs? I'm not yet quite so 
familiar with all btrfs concepts that I know quite what I'm talking about, 
but I'm guessing that what I want to do is to merely temporarily set the 
allocator to allocate new btrfs on a single disk only, and then add a 
single disk to the filesystem. And then copy the rest of the data, abandon 
the old filesystem and add another disk and rebalance those 
singly-allocated extents to RAID-1 mode.

Have I described a conceptionable idea in saying so? And if so, how does 
one actually do that? I don't know if I'm just blind, but I haven't found 
any btrfs command to change the allocation algorithm without having to 
rebalance the existing data, which seems a bit unnecessary in this case.

Thanks for any help you can offer!

--

Fredrik Tolf

Re: Changing allocation mode

[Liu Bo]

On Sat, Feb 23, 2013 at 01:46:03AM +0100, Fredrik Tolf wrote:
> Dear list,
> 
> I'm still in the process of transferring all the data I have to the
> btrfs filesystem I have had your help in debugging in a previous
> thread, and I have a slight question, if you will humour me.
> 
> I have the data I want to transfer on an old ReiserFS partition,
> consisting of 2 mdraid mirrors, one of which consists of two 1.5 TB
> disks, and the other of two 3 TB disks. The btrfs I'm copying the
> data to consists of two 3 TB disks only that I have put in RAID-1
> mode, and the data on the old filesystem is only slightly larger
> than 3 TB. I am now at the point where I have transferred just under
> 3 TB.
> 
> If I were transferring the data to a new filesystem on mdraid, the
> procedure I would use for that last portion of the data would be to
> remove one disk only from either of the old mdraid mirror arrays
> (putting that array in degraded mode), and then create a new mirror
> in degraded mode with only that disk, add that mirror to the new
> filesystem, expand it, copy the last data, and then delete the old
> mirrors, moving the rest of the disks to the new filesystem.

That sounds like using seed device, although seed disk is designed for another
different purpose.

thanks,
liubo

> 
> Is there a way to mirror this procedure in btrfs? I'm not yet quite
> so familiar with all btrfs concepts that I know quite what I'm
> talking about, but I'm guessing that what I want to do is to merely
> temporarily set the allocator to allocate new btrfs on a single disk
> only, and then add a single disk to the filesystem. And then copy
> the rest of the data, abandon the old filesystem and add another
> disk and rebalance those singly-allocated extents to RAID-1 mode.
> 
> Have I described a conceptionable idea in saying so? And if so, how
> does one actually do that? I don't know if I'm just blind, but I
> haven't found any btrfs command to change the allocation algorithm
> without having to rebalance the existing data, which seems a bit
> unnecessary in this case.
> 
> Thanks for any help you can offer!
> 
> --
> 
> Fredrik Tolf
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: Changing allocation mode

[Fredrik Tolf]

On Wed, 27 Feb 2013, Liu Bo wrote:
> On Sat, Feb 23, 2013 at 01:46:03AM +0100, Fredrik Tolf wrote:
>> If I were transferring the data to a new filesystem on mdraid, the
>> procedure I would use for that last portion of the data would be to
>> remove one disk only from either of the old mdraid mirror arrays
>> (putting that array in degraded mode), and then create a new mirror
>> in degraded mode with only that disk, add that mirror to the new
>> filesystem, expand it, copy the last data, and then delete the old
>> mirrors, moving the rest of the disks to the new filesystem.
>
> That sounds like using seed device, although seed disk is designed for another
> different purpose.

It does? I must admit I don't see quite how that would be applicable.

--

Fredrik Tolf