On 20/09/17 22:45, Kai Krakow wrote:
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.
I suggest to initialize bcache to writearound mode while converting, so
your maybe terabytes of disk don't go through the SSD.
If you later decide to remove bcache or not sure about future bcache
usage, you can wrap any partition into a bcache container - just don't
connect it to a cache and it will work like a normal partition.
Those are good advices. I've finished now and it seems to have gone
without a hitch. Thanks!
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