> -----Original Message-----
> From: linux-btrfs-ow...@vger.kernel.org [mailto:linux-btrfs-
> ow...@vger.kernel.org] On Behalf Of Kai Krakow
> Sent: Thursday, 21 September 2017 6:45 AM
> To: linux-btrfs@vger.kernel.org
> Subject: Re: SSD caching an existing btrfs raid1
>
> Am Wed, 20 Sep 2017 17:51:15 +0200
> schrieb Psalle <psalleets...@gmail.com>:
>
> > On 19/09/17 17:47, Austin S. Hemmelgarn wrote:
> > (...)
> > >
> > > A better option if you can afford to remove a single device from
> > > that array temporarily is to use bcache. Bcache has one specific
> > > advantage in this case, multiple backend devices can share the same
> > > cache device. This means you don't have to carve out dedicated cache
> > > space for each disk on the SSD and leave some unused space so that
> > > you can add new devices if needed. The downside is that you can't
> > > convert each device in-place, but because you're using BTRFS, you
> > > can still convert the volume as a whole in-place. The procedure for
> > > doing so looks like this:
> > >
> > > 1. Format the SSD as a bcache cache.
> > > 2. Use `btrfs device delete` to remove a single hard drive from the
> > > array.
> > > 3. Set up the drive you just removed as a bcache backing device
> > > bound to the cache you created in step 1.
> > > 4. Add the new bcache device to the array.
> > > 5. Repeat from step 2 until the whole array is converted.
> > >
> > > A similar procedure can actually be used to do almost any underlying
> > > storage conversion (for example, switching to whole disk encryption,
> > > or adding LVM underneath BTRFS) provided all your data can fit on
> > > one less disk than you have.
> >
> > Thanks Austin, that's just great. For some reason I had discarded
> > bcache thinking that it would force me to rebuild from scratch, but
> > this kind of incremental migration is exactly why I hoped was
> > possible. I have plenty of space to replace the devices one by one.
> >
> > I will report back my experience in a few days, I hope.
>
> I've done it exactly that way in the past and it worked flawlessly (but it
> took
> 24+ hours). But it was easy for me because I was also adding a third disk to
> the pool, so existing stuff could easily move.
Device delete takes freaking ages! I would avoid using it if you can. Device
replace is much faster.
Paul.
N�����r��y����b�X��ǧv�^�){.n�+����{�n�߲)����w* jg��� �����ݢj/���z�ޖ��2�ޙ����&�)ߡ�a���� �G���h��j:+v���w��٥
