Re: adding new devices to degraded raid1

[Zygo Blaxell <ce3g8jdj@umail.furryterror.org>]

On Thu, Aug 27, 2020 at 07:14:18PM +0200, Goffredo Baroncelli wrote:
> On 8/27/20 2:41 PM, Eric Wong wrote:
> > I don't need to do it right away, but is it possible to add new
> > devices to a degraded raid1?
> > 
> > One thing I might do in the future is replace a broken big drive
> > with two small drives.  It may even be used to migrate to SSDs.
> > 
> > Since btrfs-replace only seems to do 1:1 replacements, and I
> > needed to physically remove an existing broken device to make
> > room for the replacements, could I do something like:
> > 
> > 	mount -o degraded /mnt/foo
> > 	btrfs device add small1 small2 /mnt/foo
> > 	btrfs device remove broken /mnt/foo

Note that add/remove is orders of magnitude slower than replace.
Replace might take hours or even a day or two on a huge spinning drive.
Add/remove might take _months_, though if you have 8-year-old disks
then it's probably a few days, weeks at most.

Add/remove does work for raid1* (i.e. raid1, raid10, raid1c3, raid1c4).
At the moment only 'replace' works reliably for raid5/raid6.

> > ?
> > 
> 
> Instead of
> 
>  	btrfs device remove broken /mnt/foo
> 
> You should do
> 
> 	btrfs device remove missing /mnt/foo
> 
> ("missing" has to be write as is, it is a special term, see man page)
> 
> and
> 
> 	btrfs balance start /mnt/foo

If the replacement disks are larger than half the size of the failed disk
then device remove may do sufficient data relocation and you won't need
balance.  Once all the disks have equal amounts of unallocated space in
'btrfs fi usage' you can cancel any balances that are running.

On the other hand, if the replacement disks are close to half the size
of the failed disk, then some careful balance filtering is required in
order to utilize all the available space.  This filtering is more than
what the stock tool offers.  You have to make sure that there are no block
groups with a mirror copy on both of the small disks, as any such block
group removes 1GB of available mirror space for data on the largest disk.

> To redistribute the data to the disks.
> 
> Please before trying it, wait for other suggestions or confirmation from more expert dveloper about that
> 
> BR
> G.Baroncelli
> 
> > Anyways, so far raid1 has been working great for me, but I have
> > some devices nearing 70K Power_On_Hours according to SMART
> > 
> > Thanks.
> > 
> 
> 
> -- 
> gpg @keyserver.linux.it: Goffredo Baroncelli <kreijackATinwind.it>
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