Re: incomplete conversion to RAID1?

[Duncan <1i5t5.duncan@cox.net>]

Nicholas D Steeves posted on Wed, 02 Mar 2016 20:25:46 -0500 as excerpted:

> btrfs fi show
> Label: none  uuid: 2757c0b7-daf1-41a5-860b-9e4bc36417d3
>         Total devices 2 FS bytes used 882.28GiB
>         devid    1 size  926.66GiB used 886.03GiB path /dev/sdb1
>         devid    2 size  926.66GiB used 887.03GiB path /dev/sdc1
> 
> But this is what's troubling:
> 
> btrfs fi df /.btrfs-admin/
> Data, RAID1: total=882.00GiB, used=880.87GiB
> Data, single: total=1.00GiB, used=0.00B
> System, RAID1: total=32.00MiB, used=160.00KiB
> Metadata, RAID1: total=4.00GiB, used=1.41GiB
> GlobalReserve, single: total=496.00MiB, used=0.00B
> 
> Do I still have 1.00GiB that isn't in RAID1?

You have a 1 GiB empty data chunk still in single mode, explaining both 
the extra line in btrfs fi df, and the 1 GiB discrepancy between the two 
device usage values in btrfs fi show.

It's empty, so it contains no data or metadata, and is thus more a 
"cosmetic oddity" than a real problem, but wanting to be rid of it is 
entirely understandable, and I'd want it gone as well. =:^)

Happily, it should be easy enough to get rid of using balance filters.  
There are at least a two such filters that should do it, so take your 
pick. =:^)

btrfs balance start -dusage=0

This is the one I normally use.  -d is of course for data chunks. usage=N 
says only balance chunks with less than or equal to N% usage, this 
normally being used as a quick way to combine several partially used 
chunks into fewer chunks, releasing the space from the reclaimed chunks 
back to unallocated.  Of course usage=0 means only deal with fully empty 
chunks, so they don't have to be rewritten at all and can be directly 
reclaimed.

This used to be needed somewhat often, as until /relatively/ recent 
kernels (tho a couple years ago now, 3.17 IIRC), btrfs wouldn't 
automatically reclaim those chunks as it usually does now, and a manual 
balance had to be done to reclaim them.  Btrfs normally reclaims those on 
its own now, but probably missed that one somewhere in your conversion 
process.  But that shouldn't be a problem as you can do it manually. =:^)

Meanwhile, a hint.  While btrfs normally reclaims usage=0 chunks on its 
own now, it still doesn't automatically reclaim chunks that actually 
still have some usage, and over time, it'll likely still end up with a 
bunch of mostly empty chunks, just not /completely/ empty.  These can 
still take all your unallocated space, creating problems when the other 
type of chunk needs a new allocation (normally it's data chunks that take 
the space, and metadata chunks that need a new allocation and can't get 
it because the data chunks are hogging it all, but I've seen at least one 
report of it going the other way, metadata hogging space and data being 
unable to allocate, as well).

To avoid that, you'll want to keep an eye on the /unallocated/ space, and 
when it drops below say 10 GiB, do a balance with -dusage=20, or as you 
get closer to full, perhaps -dusage=50 or -dusage=70 (above that will 
take a long time and not get you much), or perhaps -musage instead of
-dusage, if metadata used plus globalreserve total gets too far from 
metadata total. (global-reserve total comes from metadata and should be 
added to metadata used, tho if it ever says global-reserve used above 0, 
you know your filesystem is /very/ tight in regard to space usage, since 
it won't use the reserve until it really /really/ has to.)

btrfs balance start -dprofiles=single

This one again uses -d for data chunks only, with the profiles=single 
filter saying only balance single-profile chunks.  Since you have only 
the one and it's empty, again, it should simply delete it, returning the 
space it took to unallocated.


Of course either way assumes you don't run into some bug that will 
prevent removal of that chunk, perhaps exactly the same one that kept it 
from being removed during the normal raid1 conversion.  If that happens, 
the devs may well be interested in tracking it down, as I'm not aware of 
anything similar being posted to the list.  But it does say zero usage, 
so by logic, either of the above balance commands should just remove it, 
actually pretty fast, as there's only a bit of accounting to do to remove 
it.  And if they don't, then it /is/ a bug, but I'm guessing they will. 
=:^)

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman