<s>
Btrfs	B-Operating_System
(	O
pronounced	O
as	O
"	O
better	O
F	O
S	O
"	O
,	O
"	O
butter	O
F	O
S	O
"	O
,	O
"	O
b-tree	B-Architecture
F	O
S	O
"	O
,	O
or	O
simply	O
by	O
spelling	O
it	O
out	O
)	O
is	O
a	O
computer	O
storage	O
format	O
that	O
combines	O
a	O
file	B-Application
system	I-Application
based	O
on	O
the	O
copy-on-write	O
(	O
COW	O
)	O
principle	O
with	O
a	O
logical	B-Application
volume	I-Application
manager	I-Application
(	O
not	O
to	O
be	O
confused	O
with	O
Linux	B-Application
's	O
LVM	B-Application
)	O
,	O
developed	O
together	O
.	O
</s>
<s>
It	O
was	O
initially	O
designed	O
at	O
Oracle	B-Application
Corporation	I-Application
in	O
2007	O
for	O
use	O
in	O
Linux	B-Application
,	O
and	O
since	O
November	O
2013	O
,	O
the	O
file	B-Application
system	I-Application
's	O
on-disk	O
format	O
has	O
been	O
declared	O
stable	O
in	O
the	O
Linux	B-Application
kernel	O
.	O
</s>
<s>
According	O
to	O
Oracle	B-Application
,	O
Btrfs	B-Operating_System
"	O
is	O
not	O
a	O
true	O
acronym	O
"	O
.	O
</s>
<s>
Btrfs	B-Operating_System
is	O
intended	O
to	O
address	O
the	O
lack	O
of	O
pooling	B-General_Concept
,	O
snapshots	B-Application
,	O
checksums	B-Algorithm
,	O
and	O
integral	O
multi-device	O
spanning	O
in	O
Linux	B-Application
file	B-Application
systems	I-Application
.	O
</s>
<s>
Chris	O
Mason	O
,	O
the	O
principal	O
Btrfs	B-Operating_System
author	O
,	O
stated	O
that	O
its	O
goal	O
was	O
"	O
to	O
let	O
 [ Linux ] 	O
scale	O
for	O
the	O
storage	O
that	O
will	O
be	O
available	O
.	O
</s>
<s>
The	O
core	O
data	O
structure	O
of	O
Btrfsthe	O
copy-on-write	O
B-treewas	O
originally	O
proposed	O
by	O
IBM	O
researcher	O
Ohad	O
Rodeh	O
at	O
a	O
presentation	O
at	O
USENIX	B-Operating_System
2007	O
.	O
</s>
<s>
Chris	O
Mason	O
,	O
an	O
engineer	O
working	O
on	O
ReiserFS	B-Application
for	O
SUSE	O
at	O
the	O
time	O
,	O
joined	O
Oracle	B-Application
later	O
that	O
year	O
and	O
began	O
work	O
on	O
a	O
new	O
file	B-Application
system	I-Application
based	O
on	O
these	O
B-trees	B-Architecture
.	O
</s>
<s>
In	O
2008	O
,	O
the	O
principal	O
developer	O
of	O
the	O
ext3	B-Application
and	O
ext4	B-Application
file	B-Application
systems	I-Application
,	O
Theodore	O
Ts'o	O
,	O
stated	O
that	O
although	O
ext4	B-Application
has	O
improved	O
features	O
,	O
it	O
is	O
not	O
a	O
major	O
advance	O
;	O
it	O
uses	O
old	O
technology	O
and	O
is	O
a	O
stop-gap	O
.	O
</s>
<s>
Ts'o	O
said	O
that	O
Btrfs	B-Operating_System
is	O
the	O
better	O
direction	O
because	O
"	O
it	O
offers	O
improvements	O
in	O
scalability	O
,	O
reliability	O
,	O
and	O
ease	O
of	O
management	O
"	O
.	O
</s>
<s>
Btrfs	B-Operating_System
also	O
has	O
"	O
a	O
number	O
of	O
the	O
same	O
design	O
ideas	O
that	O
reiser3/4	O
had	O
"	O
.	O
</s>
<s>
Btrfs	B-Operating_System
1.0	O
,	O
with	O
finalized	O
on-disk	O
format	O
,	O
was	O
originally	O
slated	O
for	O
a	O
late-2008	O
release	O
,	O
and	O
was	O
finally	O
accepted	O
into	O
the	O
Linux	B-Application
kernel	O
mainline	O
in	O
2009	O
.	O
</s>
<s>
Several	O
Linux	B-Application
distributions	I-Application
began	O
offering	O
Btrfs	B-Operating_System
as	O
an	O
experimental	O
choice	O
of	O
root	B-Application
file	I-Application
system	I-Application
during	O
installation	O
.	O
</s>
<s>
In	O
July	O
2011	O
,	O
Btrfs	B-Operating_System
automatic	O
defragmentation	B-Application
and	O
scrubbing	B-Error_Name
features	O
were	O
merged	O
into	O
version	O
3.0	O
of	O
the	O
Linux	B-Application
kernel	O
mainline	O
.	O
</s>
<s>
Besides	O
Mason	O
at	O
Oracle	B-Application
,	O
Miao	O
Xie	O
at	O
Fujitsu	O
contributed	O
performance	O
improvements	O
.	O
</s>
<s>
In	O
June	O
2012	O
,	O
Chris	O
Mason	O
left	O
Oracle	B-Application
for	O
Fusion-io	O
,	O
which	O
he	O
left	O
a	O
year	O
later	O
with	O
Josef	O
Bacik	O
to	O
join	O
Facebook	B-Application
.	I-Application
</s>
<s>
While	O
at	O
both	O
companies	O
,	O
Mason	O
continued	O
his	O
work	O
on	O
Btrfs	B-Operating_System
.	O
</s>
<s>
In	O
2012	O
,	O
two	O
Linux	B-Application
distributions	I-Application
moved	O
Btrfs	B-Operating_System
from	O
experimental	O
to	O
production	O
or	O
supported	O
status	O
:	O
Oracle	B-Application
Linux	B-Application
in	O
March	O
,	O
followed	O
by	O
SUSE	O
Linux	B-Application
Enterprise	O
in	O
August	O
.	O
</s>
<s>
In	O
2015	O
,	O
Btrfs	B-Operating_System
was	O
adopted	O
as	O
the	O
default	O
filesystem	B-Application
for	O
SUSE	O
Linux	B-Application
Enterprise	O
Server	O
(	O
SLE	O
)	O
12	O
.	O
</s>
<s>
In	O
August	O
2017	O
,	O
Red	O
Hat	O
announced	O
in	O
the	O
release	O
notes	O
for	O
Red	O
Hat	O
Enterprise	O
Linux	B-Application
(	O
RHEL	O
)	O
7.4	O
that	O
it	O
no	O
longer	O
planned	O
to	O
move	O
Btrfs	B-Operating_System
to	O
a	O
fully	O
supported	O
feature	O
(	O
it	O
's	O
been	O
included	O
as	O
a	O
"	O
technology	O
preview	O
"	O
since	O
RHEL	O
6	O
beta	O
)	O
noting	O
that	O
it	O
would	O
remain	O
available	O
in	O
the	O
RHEL	O
7	O
release	O
series	O
.	O
</s>
<s>
Btrfs	B-Operating_System
was	O
removed	O
from	O
RHEL	O
8	O
in	O
May	O
2019	O
.	O
</s>
<s>
RHEL	O
moved	O
from	O
ext4	B-Application
in	O
RHEL	O
6	O
to	O
XFS	B-Application
in	O
RHEL	O
7	O
.	O
</s>
<s>
In	O
2020	O
,	O
Btrfs	B-Operating_System
was	O
selected	O
as	O
the	O
default	O
file	B-Application
system	I-Application
for	O
Fedora	O
33	O
for	O
desktop	O
variants	O
.	O
</s>
<s>
As	O
of	O
version	O
5.0	O
of	O
the	O
Linux	B-Application
kernel	O
,	O
Btrfs	B-Operating_System
implements	O
the	O
following	O
features	O
:	O
</s>
<s>
Checksums	B-Algorithm
on	O
data	O
and	O
metadata	O
(	O
CRC-32C	O
)	O
.	O
</s>
<s>
New	O
hash	O
functions	O
are	O
implemented	O
since	O
5.5	O
:	O
xxHash	O
,	O
SHA256	B-Algorithm
,	O
BLAKE2B	B-Algorithm
.	O
</s>
<s>
In-place	O
conversion	O
from	O
ext3/4	O
to	O
Btrfs	B-Operating_System
(	O
with	O
rollback	O
)	O
.	O
</s>
<s>
This	O
feature	O
regressed	O
around	O
btrfs-progs	O
version	O
4.0	O
,	O
rewritten	O
from	O
scratch	O
in	O
4.6	O
.	O
</s>
<s>
Persistent	O
read	O
and	O
write	O
cache	O
(	O
L2ARC	B-Application
+	O
ZIL	O
,	O
lvmcache	B-Application
,	O
etc	O
.	O
)	O
</s>
<s>
In	O
2009	O
,	O
Btrfs	B-Operating_System
was	O
expected	O
to	O
offer	O
a	O
feature	O
set	O
comparable	O
to	O
ZFS	B-Application
,	O
developed	O
by	O
Sun	O
Microsystems	O
.	O
</s>
<s>
After	O
Oracle	B-Application
's	O
acquisition	O
of	O
Sun	O
in	O
2009	O
,	O
Mason	O
and	O
Oracle	B-Application
decided	O
to	O
continue	O
with	O
Btrfs	B-Operating_System
development	O
.	O
</s>
<s>
Btrfs	B-Operating_System
provides	O
a	O
clone	O
operation	O
that	O
atomically	B-General_Concept
creates	O
a	O
copy-on-write	O
snapshot	B-Application
of	O
a	O
file	B-Operating_System
.	O
</s>
<s>
Such	O
cloned	O
files	O
are	O
sometimes	O
referred	O
to	O
as	O
reflinks	O
,	O
in	O
light	O
of	O
the	O
proposed	O
associated	O
Linux	B-Application
kernel	O
system	B-Operating_System
call	I-Operating_System
.	O
</s>
<s>
By	O
cloning	O
,	O
the	O
file	B-Application
system	I-Application
does	O
not	O
create	O
a	O
new	O
link	O
pointing	O
to	O
an	O
existing	O
inode	B-Application
;	O
instead	O
,	O
it	O
creates	O
a	O
new	O
inode	B-Application
that	O
initially	O
shares	O
the	O
same	O
disk	O
blocks	O
with	O
the	O
original	O
file	B-Operating_System
.	O
</s>
<s>
As	O
a	O
result	O
,	O
cloning	O
works	O
only	O
within	O
the	O
boundaries	O
of	O
the	O
same	O
Btrfs	B-Operating_System
file	B-Application
system	I-Application
,	O
but	O
since	O
version	O
3.6	O
of	O
the	O
Linux	B-Application
kernel	O
it	O
may	O
cross	O
the	O
boundaries	O
of	O
subvolumes	O
under	O
certain	O
circumstances	O
.	O
</s>
<s>
The	O
actual	O
data	O
blocks	O
are	O
not	O
duplicated	O
;	O
at	O
the	O
same	O
time	O
,	O
due	O
to	O
the	O
copy-on-write	O
(	O
CoW	O
)	O
nature	O
of	O
Btrfs	B-Operating_System
,	O
modifications	O
to	O
any	O
of	O
the	O
cloned	O
files	O
are	O
not	O
visible	O
in	O
the	O
original	O
file	B-Operating_System
and	O
vice	O
versa	O
.	O
</s>
<s>
Cloning	O
should	O
not	O
be	O
confused	O
with	O
hard	B-Application
links	I-Application
,	O
which	O
are	O
directory	O
entries	O
that	O
associate	O
multiple	O
file	B-Operating_System
names	O
with	O
a	O
single	O
file	B-Operating_System
.	O
</s>
<s>
While	O
hard	B-Application
links	I-Application
can	O
be	O
taken	O
as	O
different	O
names	O
for	O
the	O
same	O
file	B-Operating_System
,	O
cloning	O
in	O
Btrfs	B-Operating_System
provides	O
independent	O
files	O
that	O
initially	O
share	O
all	O
their	O
disk	O
blocks	O
.	O
</s>
<s>
Support	O
for	O
this	O
Btrfs	B-Operating_System
feature	O
was	O
added	O
in	O
version	O
7.5	O
of	O
the	O
GNU	B-Application
coreutils	I-Application
,	O
via	O
the	O
--reflink	O
option	O
to	O
the	O
cp	B-Application
command	O
.	O
</s>
<s>
In	O
addition	O
to	O
data	O
cloning	O
(	O
)	O
,	O
Btrfs	B-Operating_System
also	O
supports	O
out-of-band	O
deduplication	O
via	O
.	O
</s>
<s>
A	O
Btrfs	B-Operating_System
subvolume	O
can	O
be	O
thought	O
of	O
as	O
a	O
separate	O
POSIX	O
file	B-Operating_System
namespace	O
,	O
mountable	B-Application
separately	O
by	O
passing	O
subvol	O
or	O
subvolid	O
options	O
to	O
the	O
utility	O
.	O
</s>
<s>
It	O
can	O
also	O
be	O
accessed	O
by	O
mounting	B-Application
the	O
top-level	O
subvolume	O
,	O
in	O
which	O
case	O
subvolumes	O
are	O
visible	O
and	O
accessible	O
as	O
its	O
subdirectories	O
.	O
</s>
<s>
Subvolumes	O
can	O
be	O
created	O
at	O
any	O
place	O
within	O
the	O
file	B-Application
system	I-Application
hierarchy	O
,	O
and	O
they	O
can	O
also	O
be	O
nested	O
.	O
</s>
<s>
Any	O
Btrfs	B-Operating_System
file	B-Application
system	I-Application
always	O
has	O
a	O
default	O
subvolume	O
,	O
which	O
is	O
initially	O
set	O
to	O
be	O
the	O
top-level	O
subvolume	O
,	O
and	O
is	O
mounted	O
by	O
default	O
if	O
no	O
subvolume	O
selection	O
option	O
is	O
passed	O
to	O
mount	O
.	O
</s>
<s>
A	O
Btrfs	B-Operating_System
snapshot	B-Application
is	O
a	O
subvolume	O
that	O
shares	O
its	O
data	O
(	O
and	O
metadata	O
)	O
with	O
some	O
other	O
subvolume	O
,	O
using	O
Btrfs	B-Operating_System
 '	O
copy-on-write	O
capabilities	O
,	O
and	O
modifications	O
to	O
a	O
snapshot	B-Application
are	O
not	O
visible	O
in	O
the	O
original	O
subvolume	O
.	O
</s>
<s>
Once	O
a	O
writable	O
snapshot	B-Application
is	O
made	O
,	O
it	O
can	O
be	O
treated	O
as	O
an	O
alternate	O
version	O
of	O
the	O
original	O
file	B-Application
system	I-Application
.	O
</s>
<s>
For	O
example	O
,	O
to	O
roll	O
back	O
to	O
a	O
snapshot	B-Application
,	O
a	O
modified	O
original	O
subvolume	O
needs	O
to	O
be	O
unmounted	O
and	O
the	O
snapshot	B-Application
needs	O
to	O
be	O
mounted	O
in	O
its	O
place	O
.	O
</s>
<s>
The	O
copy-on-write	O
(	O
CoW	O
)	O
nature	O
of	O
Btrfs	B-Operating_System
means	O
that	O
snapshots	B-Application
are	O
quickly	O
created	O
,	O
while	O
initially	O
consuming	O
very	O
little	O
disk	O
space	O
.	O
</s>
<s>
Since	O
a	O
snapshot	B-Application
is	O
a	O
subvolume	O
,	O
creating	O
nested	O
snapshots	B-Application
is	O
also	O
possible	O
.	O
</s>
<s>
Taking	O
snapshots	B-Application
of	O
a	O
subvolume	O
is	O
not	O
a	O
recursive	O
process	O
;	O
thus	O
,	O
if	O
a	O
snapshot	B-Application
of	O
a	O
subvolume	O
is	O
created	O
,	O
every	O
subvolume	O
or	O
snapshot	B-Application
that	O
the	O
subvolume	O
already	O
contains	O
is	O
mapped	O
to	O
an	O
empty	O
directory	O
of	O
the	O
same	O
name	O
inside	O
the	O
snapshot	B-Application
.	O
</s>
<s>
Taking	O
snapshots	B-Application
of	O
a	O
directory	O
is	O
not	O
possible	O
,	O
as	O
only	O
subvolumes	O
can	O
have	O
snapshots	B-Application
.	O
</s>
<s>
Having	O
that	O
available	O
,	O
a	O
snapshot	B-Application
of	O
this	O
new	O
volume	O
can	O
be	O
created	O
.	O
</s>
<s>
A	O
subvolume	O
in	O
Btrfs	B-Operating_System
is	O
quite	O
different	O
from	O
a	O
traditional	O
Logical	B-Application
Volume	I-Application
Manager	I-Application
(	O
LVM	B-Application
)	O
logical	B-Application
volume	I-Application
.	O
</s>
<s>
With	O
LVM	B-Application
,	O
a	O
logical	B-Application
volume	I-Application
is	O
a	O
separate	O
block	O
device	O
,	O
while	O
a	O
Btrfs	B-Operating_System
subvolume	O
is	O
not	O
and	O
it	O
cannot	O
be	O
treated	O
or	O
used	O
that	O
way	O
.	O
</s>
<s>
Making	O
dd	O
or	O
LVM	B-Application
snapshots	B-Application
of	O
btrfs	B-Operating_System
leads	O
to	O
data	O
loss	O
if	O
either	O
the	O
original	O
or	O
the	O
copy	O
is	O
mounted	O
while	O
both	O
are	O
on	O
the	O
same	O
computer	O
.	O
</s>
<s>
Given	O
any	O
pair	O
of	O
subvolumes	O
(	O
or	O
snapshots	B-Application
)	O
,	O
Btrfs	B-Operating_System
can	O
generate	O
a	O
binary	O
diff	B-Application
between	O
them	O
(	O
by	O
using	O
the	O
btrfs	B-Operating_System
send	O
command	O
)	O
that	O
can	O
be	O
replayed	O
later	O
(	O
by	O
using	O
btrfs	B-Operating_System
receive	O
)	O
,	O
possibly	O
on	O
a	O
different	O
Btrfs	B-Operating_System
file	B-Application
system	I-Application
.	O
</s>
<s>
The	O
send/receive	O
feature	O
can	O
be	O
used	O
with	O
regularly	O
scheduled	O
snapshots	B-Application
for	O
implementing	O
a	O
simple	O
form	O
of	O
file	B-Application
system	I-Application
replication	B-Architecture
,	O
or	O
for	O
the	O
purpose	O
of	O
performing	O
incremental	B-General_Concept
backups	I-General_Concept
.	O
</s>
<s>
A	O
quota	O
group	O
(	O
or	O
qgroup	O
)	O
imposes	O
an	O
upper	O
limit	O
to	O
the	O
space	O
a	O
subvolume	O
or	O
snapshot	B-Application
may	O
consume	O
.	O
</s>
<s>
A	O
new	O
snapshot	B-Application
initially	O
consumes	O
no	O
quota	O
because	O
its	O
data	O
is	O
shared	O
with	O
its	O
parent	O
,	O
but	O
thereafter	O
incurs	O
a	O
charge	O
for	O
new	O
files	O
and	O
copy-on-write	O
operations	O
on	O
existing	O
files	O
.	O
</s>
<s>
When	O
quotas	O
are	O
active	O
,	O
a	O
quota	O
group	O
is	O
automatically	O
created	O
with	O
each	O
new	O
subvolume	O
or	O
snapshot	B-Application
.	O
</s>
<s>
These	O
initial	O
quota	O
groups	O
are	O
building	O
blocks	O
which	O
can	O
be	O
grouped	O
(	O
with	O
the	O
btrfs	B-Operating_System
qgroup	O
command	O
)	O
into	O
hierarchies	O
to	O
implement	O
quota	O
pools	O
.	O
</s>
<s>
Quota	O
groups	O
only	O
apply	O
to	O
subvolumes	O
and	O
snapshots	B-Application
,	O
while	O
having	O
quotas	O
enforced	O
on	O
individual	O
subdirectories	O
,	O
users	O
,	O
or	O
user	O
groups	O
is	O
not	O
possible	O
.	O
</s>
<s>
As	O
the	O
result	O
of	O
having	O
very	O
little	O
metadata	O
anchored	O
in	O
fixed	O
locations	O
,	O
Btrfs	B-Operating_System
can	O
warp	O
to	O
fit	O
unusual	O
spatial	O
layouts	O
of	O
the	O
backend	O
storage	O
devices	O
.	O
</s>
<s>
The	O
btrfs-convert	O
tool	O
exploits	O
this	O
ability	O
to	O
do	O
an	O
in-place	O
conversion	O
of	O
an	O
ext2/3/4	O
or	O
ReiserFS	B-Application
file	B-Application
system	I-Application
,	O
by	O
nesting	O
the	O
equivalent	O
Btrfs	B-Operating_System
metadata	O
in	O
its	O
unallocated	O
space	O
—	O
while	O
preserving	O
an	O
unmodified	O
copy	O
of	O
the	O
original	O
file	B-Application
system	I-Application
.	O
</s>
<s>
The	O
conversion	O
involves	O
creating	O
a	O
copy	O
of	O
the	O
whole	O
ext2/3/4	O
metadata	O
,	O
while	O
the	O
Btrfs	B-Operating_System
files	O
simply	O
point	O
to	O
the	O
same	O
blocks	O
used	O
by	O
the	O
ext2/3/4	O
files	O
.	O
</s>
<s>
This	O
makes	O
the	O
bulk	O
of	O
the	O
blocks	O
shared	O
between	O
the	O
two	O
filesystems	B-Application
before	O
the	O
conversion	O
becomes	O
permanent	O
.	O
</s>
<s>
Thanks	O
to	O
the	O
copy-on-write	O
nature	O
of	O
Btrfs	B-Operating_System
,	O
the	O
original	O
versions	O
of	O
the	O
file	B-Operating_System
data	O
blocks	O
are	O
preserved	O
during	O
all	O
file	B-Operating_System
modifications	O
.	O
</s>
<s>
Until	O
the	O
conversion	O
becomes	O
permanent	O
,	O
only	O
the	O
blocks	O
that	O
were	O
marked	O
as	O
free	O
in	O
ext2/3/4	O
are	O
used	O
to	O
hold	O
new	O
Btrfs	B-Operating_System
modifications	O
,	O
meaning	O
that	O
the	O
conversion	O
can	O
be	O
undone	O
at	O
any	O
time	O
(	O
although	O
doing	O
so	O
will	O
erase	O
any	O
changes	O
made	O
after	O
the	O
conversion	O
to	O
Btrfs	B-Operating_System
)	O
.	O
</s>
<s>
All	O
converted	O
files	O
are	O
available	O
and	O
writable	O
in	O
the	O
default	O
subvolume	O
of	O
the	O
Btrfs	B-Operating_System
.	O
</s>
<s>
A	O
sparse	O
file	B-Operating_System
holding	O
all	O
of	O
the	O
references	O
to	O
the	O
original	O
ext2/3/4	O
filesystem	B-Application
is	O
created	O
in	O
a	O
separate	O
subvolume	O
,	O
which	O
is	O
mountable	B-Application
on	O
its	O
own	O
as	O
a	O
read-only	O
disk	O
image	O
,	O
allowing	O
both	O
original	O
and	O
converted	O
file	B-Application
systems	I-Application
to	O
be	O
accessed	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
Deleting	O
this	O
sparse	O
file	B-Operating_System
frees	O
up	O
the	O
space	O
and	O
makes	O
the	O
conversion	O
permanent	O
.	O
</s>
<s>
In	O
4.x	O
versions	O
of	O
the	O
mainline	O
Linux	B-Application
kernel	O
,	O
the	O
in-place	O
ext3/4	O
conversion	O
was	O
considered	O
untested	O
and	O
rarely	O
used	O
.	O
</s>
<s>
However	O
,	O
the	O
feature	O
was	O
rewritten	O
from	O
scratch	O
in	O
2016	O
for	O
btrfs-progs	O
4.6	O
.	O
and	O
has	O
been	O
considered	O
stable	O
since	O
then	O
.	O
</s>
<s>
In-place	O
conversion	O
from	O
ReiserFS	B-Application
was	O
introduced	O
in	O
September	O
2017	O
with	O
kernel	O
4.13	O
.	O
</s>
<s>
When	O
creating	O
a	O
new	O
Btrfs	B-Operating_System
,	O
an	O
existing	O
Btrfs	B-Operating_System
can	O
be	O
used	O
as	O
a	O
read-only	O
"	O
seed	O
"	O
file	B-Application
system	I-Application
.	O
</s>
<s>
The	O
new	O
file	B-Application
system	I-Application
will	O
then	O
act	O
as	O
a	O
copy-on-write	O
overlay	O
on	O
the	O
seed	O
,	O
as	O
a	O
form	O
of	O
union	O
mounting	B-Application
.	O
</s>
<s>
The	O
seed	O
can	O
be	O
later	O
detached	O
from	O
the	O
Btrfs	B-Operating_System
,	O
at	O
which	O
point	O
the	O
rebalancer	O
will	O
simply	O
copy	O
over	O
any	O
seed	O
data	O
still	O
referenced	O
by	O
the	O
new	O
file	B-Application
system	I-Application
before	O
detaching	O
.	O
</s>
<s>
Mason	O
has	O
suggested	O
this	O
may	O
be	O
useful	O
for	O
a	O
Live	O
CD	O
installer	O
,	O
which	O
might	O
boot	O
from	O
a	O
read-only	O
Btrfs	B-Operating_System
seed	O
on	O
an	O
optical	O
disc	O
,	O
rebalance	O
itself	O
to	O
the	O
target	O
partition	B-Application
on	O
the	O
install	O
disk	O
in	O
the	O
background	O
while	O
the	O
user	O
continues	O
to	O
work	O
,	O
then	O
eject	O
the	O
disc	O
to	O
complete	O
the	O
installation	O
without	O
rebooting	O
.	O
</s>
<s>
In	O
his	O
2009	O
interview	O
,	O
Chris	O
Mason	O
stated	O
that	O
support	O
for	O
encryption	O
was	O
planned	O
for	O
Btrfs	B-Operating_System
.	O
</s>
<s>
In	O
the	O
meantime	O
,	O
a	O
workaround	O
for	O
combining	O
encryption	O
with	O
Btrfs	B-Operating_System
is	O
to	O
use	O
a	O
full-disk	O
encryption	O
mechanism	O
such	O
as	O
dm-crypt/	O
LUKS	O
on	O
the	O
underlying	O
devices	O
and	O
to	O
create	O
the	O
Btrfs	B-Operating_System
filesystem	B-Application
on	O
top	O
of	O
that	O
layer	O
.	O
</s>
<s>
the	O
developers	O
were	O
working	O
to	O
add	O
keyed	O
hash	O
like	O
HMAC	B-Algorithm
(	O
SHA256	B-Algorithm
)	O
.	O
</s>
<s>
Unix	O
systems	O
traditionally	O
rely	O
on	O
"	O
fsck	B-Device
"	O
programs	O
to	O
check	O
and	O
repair	O
filesystems	B-Application
.	O
</s>
<s>
This	O
functionality	O
is	O
implemented	O
via	O
the	O
btrfs	B-Operating_System
check	O
program	O
.	O
</s>
<s>
Since	O
version	O
4.0	O
this	O
functionality	O
is	O
deemed	O
relatively	O
stable	O
.	O
</s>
<s>
However	O
,	O
as	O
of	O
December	O
2022	O
,	O
the	O
btrfs	B-Operating_System
documentation	O
suggests	O
that	O
its	O
--repair	O
option	O
be	O
used	O
only	O
if	O
you	O
have	O
been	O
advised	O
by	O
"	O
a	O
developer	O
or	O
an	O
experienced	O
user	O
"	O
.	O
</s>
<s>
There	O
is	O
another	O
tool	O
,	O
named	O
btrfs-restore	O
,	O
that	O
can	O
be	O
used	O
to	O
recover	O
files	O
from	O
an	O
unmountable	O
filesystem	B-Application
,	O
without	O
modifying	O
the	O
broken	O
filesystem	B-Application
itself	O
(	O
i.e.	O
,	O
non-destructively	O
)	O
.	O
</s>
<s>
In	O
normal	O
use	O
,	O
Btrfs	B-Operating_System
is	O
mostly	O
self-healing	O
and	O
can	O
recover	O
from	O
broken	O
root	O
trees	O
at	O
mount	O
time	O
,	O
thanks	O
to	O
making	O
periodic	O
data	O
flushes	O
to	O
permanent	O
storage	O
,	O
by	O
default	O
every	O
30	O
seconds	O
.	O
</s>
<s>
Thus	O
,	O
isolated	O
errors	O
will	O
cause	O
a	O
maximum	O
of	O
30	O
seconds	O
of	O
filesystem	B-Application
changes	O
to	O
be	O
lost	O
at	O
the	O
next	O
mount	O
.	O
</s>
<s>
Ohad	O
Rodeh	O
's	O
original	O
proposal	O
at	O
USENIX	B-Operating_System
2007	O
noted	O
that	O
B+	O
trees	O
,	O
which	O
are	O
widely	O
used	O
as	O
on-disk	O
data	O
structures	O
for	O
databases	O
,	O
could	O
not	O
efficiently	O
allow	O
copy-on-write-based	O
snapshots	B-Application
because	O
its	O
leaf	O
nodes	O
were	O
linked	O
together	O
:	O
if	O
a	O
leaf	O
was	O
copied	O
on	O
write	O
,	O
its	O
siblings	O
and	O
parents	O
would	O
have	O
to	O
be	O
as	O
well	O
,	O
as	O
would	O
their	O
siblings	O
and	O
parents	O
and	O
so	O
on	O
until	O
the	O
entire	O
tree	O
was	O
copied	O
.	O
</s>
<s>
He	O
suggested	O
instead	O
a	O
modified	O
B-tree	B-Architecture
(	O
which	O
has	O
no	O
leaf	O
linkage	O
)	O
,	O
with	O
a	O
refcount	B-General_Concept
associated	O
to	O
each	O
tree	O
node	O
but	O
stored	O
in	O
an	O
ad	O
hoc	O
free	O
map	O
structure	O
and	O
certain	O
relaxations	O
to	O
the	O
tree	O
's	O
balancing	O
algorithms	O
to	O
make	O
them	O
copy-on-write	O
friendly	O
.	O
</s>
<s>
The	O
result	O
would	O
be	O
a	O
data	O
structure	O
suitable	O
for	O
a	O
high-performance	O
object	O
store	O
that	O
could	O
perform	O
copy-on-write	O
snapshots	B-Application
,	O
while	O
maintaining	O
good	O
concurrency	B-Architecture
.	O
</s>
<s>
At	O
Oracle	B-Application
later	O
that	O
year	O
,	O
Chris	O
Mason	O
began	O
work	O
on	O
a	O
snapshot-capable	O
file	B-Application
system	I-Application
that	O
would	O
use	O
this	O
data	O
structure	O
almost	O
exclusively	O
—	O
not	O
just	O
for	O
metadata	O
and	O
file	B-Operating_System
data	O
,	O
but	O
also	O
recursively	O
to	O
track	O
space	O
allocation	O
of	O
the	O
trees	O
themselves	O
.	O
</s>
<s>
This	O
allowed	O
all	O
traversal	O
and	O
modifications	O
to	O
be	O
funneled	O
through	O
a	O
single	O
code	O
path	O
,	O
against	O
which	O
features	O
such	O
as	O
copy	O
on	O
write	O
,	O
checksumming	O
and	O
mirroring	O
needed	O
to	O
be	O
implemented	O
only	O
once	O
to	O
benefit	O
the	O
entire	O
file	B-Application
system	I-Application
.	O
</s>
<s>
Btrfs	B-Operating_System
is	O
structured	O
as	O
several	O
layers	O
of	O
such	O
trees	O
,	O
all	O
using	O
the	O
same	O
B-tree	B-Architecture
implementation	O
.	O
</s>
<s>
Their	O
data	O
is	O
a	O
location	O
key	O
,	O
or	O
the	O
key	O
of	O
the	O
inode	B-Application
item	O
it	O
points	O
to	O
.	O
</s>
<s>
Directory	O
items	O
together	O
can	O
thus	O
act	O
as	O
an	O
index	O
for	O
path-to-inode	O
lookups	O
,	O
but	O
are	O
not	O
used	O
for	O
iteration	O
because	O
they	O
are	O
sorted	O
by	O
their	O
hash	O
,	O
effectively	O
randomly	B-Algorithm
permuting	I-Algorithm
them	O
.	O
</s>
<s>
This	O
means	O
user	O
applications	O
iterating	O
over	O
and	O
opening	O
files	O
in	O
a	O
large	O
directory	O
would	O
thus	O
generate	O
many	O
more	O
disk	O
seeks	O
between	O
non-adjacent	O
files	O
—	O
a	O
notable	O
performance	O
drain	O
in	O
other	O
file	B-Application
systems	I-Application
with	O
hash-ordered	O
directories	O
such	O
as	O
ReiserFS	B-Application
,	O
ext3	B-Application
(	O
with	O
Htree-indexes	O
enabled	O
)	O
and	O
ext4	B-Application
,	O
all	O
of	O
which	O
have	O
TEA-hashed	O
filenames	O
.	O
</s>
<s>
Files	O
with	O
hard	B-Application
links	I-Application
in	O
multiple	O
directories	O
have	O
multiple	O
reference	O
items	O
,	O
one	O
for	O
each	O
parent	O
directory	O
.	O
</s>
<s>
Files	O
with	O
multiple	O
hard	B-Application
links	I-Application
in	O
the	O
same	O
directory	O
pack	O
all	O
of	O
the	O
links	O
 '	O
filenames	O
into	O
the	O
same	O
reference	O
item	O
.	O
</s>
<s>
This	O
was	O
a	O
design	O
flaw	O
that	O
limited	O
the	O
number	O
of	O
same-directory	O
hard	B-Application
links	I-Application
to	O
however	O
many	O
could	O
fit	O
in	O
a	O
single	O
tree	O
block	O
.	O
</s>
<s>
(	O
On	O
the	O
default	O
block	O
size	O
of	O
4KiB	O
,	O
an	O
average	O
filename	O
length	O
of	O
8	O
bytes	O
and	O
a	O
per-filename	O
header	O
of	O
4	B-Application
bytes	O
,	O
this	O
would	O
be	O
less	O
than	O
350	O
.	O
)	O
</s>
<s>
Applications	O
which	O
made	O
heavy	O
use	O
of	O
multiple	O
same-directory	O
hard	B-Application
links	I-Application
,	O
such	O
as	O
git	B-Application
,	O
GNUS	B-Operating_System
,	O
GMame	B-Device
and	O
BackupPC	B-Language
were	O
observed	O
to	O
fail	O
at	O
this	O
limit	O
.	O
</s>
<s>
The	O
limit	O
was	O
eventually	O
removed	O
(	O
and	O
as	O
of	O
October	O
2012	O
has	O
been	O
merged	O
pending	O
release	O
in	O
Linux	B-Application
3.7	O
)	O
by	O
introducing	O
spillover	O
extended	O
reference	O
items	O
to	O
hold	O
hard	B-Application
link	I-Application
filenames	O
which	O
do	O
not	O
otherwise	O
fit	O
.	O
</s>
<s>
File	B-Operating_System
data	O
is	O
kept	O
outside	O
the	O
tree	O
in	O
extents	B-Application
,	O
which	O
are	O
contiguous	O
runs	O
of	O
disk	O
data	O
blocks	O
.	O
</s>
<s>
Extent	O
blocks	O
default	O
to	O
4KiB	O
in	O
size	O
,	O
do	O
not	O
have	O
headers	O
and	O
contain	O
only	O
(	O
possibly	O
compressed	O
)	O
file	B-Operating_System
data	O
.	O
</s>
<s>
In	O
compressed	O
extents	B-Application
,	O
individual	O
blocks	O
are	O
not	O
compressed	O
separately	O
;	O
rather	O
,	O
the	O
compression	B-General_Concept
stream	O
spans	O
the	O
entire	O
extent	O
.	O
</s>
<s>
Files	O
have	O
extent	O
data	O
items	O
to	O
track	O
the	O
extents	B-Application
which	O
hold	O
their	O
contents	O
.	O
</s>
<s>
This	O
makes	O
for	O
efficient	O
seeks	O
in	O
large	O
files	O
with	O
many	O
extents	B-Application
,	O
because	O
the	O
correct	O
extent	O
for	O
any	O
given	O
file	B-Operating_System
offset	O
can	O
be	O
computed	O
with	O
just	O
one	O
tree	O
lookup	O
.	O
</s>
<s>
Snapshots	B-Application
and	O
cloned	O
files	O
share	O
extents	B-Application
.	O
</s>
<s>
When	O
a	O
small	O
part	O
of	O
a	O
large	O
such	O
extent	O
is	O
overwritten	O
,	O
the	O
resulting	O
copy-on-write	O
may	O
create	O
three	O
new	O
extents	B-Application
:	O
a	O
small	O
one	O
containing	O
the	O
overwritten	O
data	O
,	O
and	O
two	O
large	O
ones	O
with	O
unmodified	O
data	O
on	O
either	O
side	O
of	O
the	O
overwrite	O
.	O
</s>
<s>
To	O
avoid	O
having	O
to	O
re-write	O
unmodified	O
data	O
,	O
the	O
copy-on-write	O
may	O
instead	O
create	O
bookend	O
extents	B-Application
,	O
or	O
extents	B-Application
which	O
are	O
simply	O
slices	B-Application
of	O
existing	O
extents	B-Application
.	O
</s>
<s>
The	O
extent	O
allocation	O
tree	O
acts	O
as	O
an	O
allocation	O
map	O
for	O
the	O
file	B-Application
system	I-Application
.	O
</s>
<s>
The	O
file	B-Application
system	I-Application
divides	O
its	O
allocated	O
space	O
into	O
block	O
groups	O
which	O
are	O
variable-sized	O
allocation	O
regions	O
that	O
alternate	O
between	O
preferring	O
metadata	O
extents	B-Application
(	O
tree	O
nodes	O
)	O
and	O
data	O
extents	B-Application
(	O
file	B-Operating_System
contents	O
)	O
.	O
</s>
<s>
They	O
are	O
intended	O
to	O
use	O
concepts	O
of	O
the	O
Orlov	B-Application
block	I-Application
allocator	I-Application
to	O
allocate	O
related	O
files	O
together	O
and	O
resist	O
fragmentation	O
by	O
leaving	O
free	O
space	O
between	O
groups	O
.	O
</s>
<s>
(	O
Ext3	B-Application
block	O
groups	O
,	O
however	O
,	O
have	O
fixed	O
locations	O
computed	O
from	O
the	O
size	O
of	O
the	O
file	B-Application
system	I-Application
,	O
whereas	O
those	O
in	O
Btrfs	B-Operating_System
are	O
dynamic	O
and	O
created	O
as	O
needed	O
.	O
)	O
</s>
<s>
Inode	B-Application
items	O
in	O
the	O
file	B-Application
system	I-Application
tree	O
include	O
a	O
reference	O
to	O
their	O
current	O
block	O
group	O
.	O
</s>
<s>
Extent	O
items	O
contain	O
a	O
back-reference	O
to	O
the	O
tree	O
node	O
or	O
file	B-Operating_System
occupying	O
that	O
extent	O
.	O
</s>
<s>
There	O
may	O
be	O
multiple	O
back-references	O
if	O
the	O
extent	O
is	O
shared	O
between	O
snapshots	B-Application
.	O
</s>
<s>
This	O
makes	O
it	O
possible	O
to	O
find	O
which	O
extents	B-Application
or	O
tree	O
nodes	O
are	O
in	O
any	O
region	O
of	O
space	O
by	O
doing	O
a	O
B-tree	B-Architecture
range	O
lookup	O
on	O
a	O
pair	O
of	O
offsets	O
bracketing	O
that	O
region	O
,	O
then	O
following	O
the	O
back-references	O
.	O
</s>
<s>
For	O
relocating	O
data	O
,	O
this	O
allows	O
an	O
efficient	O
upwards	O
traversal	O
from	O
the	O
relocated	O
blocks	O
to	O
quickly	O
find	O
and	O
fix	O
all	O
downwards	O
references	O
to	O
those	O
blocks	O
,	O
without	O
having	O
to	O
scan	O
the	O
entire	O
file	B-Application
system	I-Application
.	O
</s>
<s>
This	O
,	O
in	O
turn	O
,	O
allows	O
the	O
file	B-Application
system	I-Application
to	O
efficiently	O
shrink	O
,	O
migrate	O
,	O
and	O
defragment	B-Application
its	O
storage	O
online	O
.	O
</s>
<s>
The	O
extent	O
allocation	O
tree	O
,	O
as	O
with	O
all	O
other	O
trees	O
in	O
the	O
file	B-Application
system	I-Application
,	O
is	O
copy-on-write	O
.	O
</s>
<s>
Writes	O
to	O
the	O
file	B-Application
system	I-Application
may	O
thus	O
cause	O
a	O
cascade	O
whereby	O
changed	O
tree	O
nodes	O
and	O
file	B-Operating_System
data	O
result	O
in	O
new	O
extents	B-Application
being	O
allocated	O
,	O
causing	O
the	O
extent	O
tree	O
itself	O
to	O
change	O
.	O
</s>
<s>
To	O
avoid	O
creating	O
a	O
feedback	O
loop	O
,	O
extent	O
tree	O
nodes	O
which	O
are	O
still	O
in	O
memory	O
but	O
not	O
yet	O
committed	O
to	O
disk	O
may	O
be	O
updated	O
in	O
place	O
to	O
reflect	O
new	O
copied-on-write	O
extents	B-Application
.	O
</s>
<s>
In	O
theory	O
,	O
the	O
extent	O
allocation	O
tree	O
makes	O
a	O
conventional	O
free-space	B-Application
bitmap	I-Application
unnecessary	O
because	O
the	O
extent	O
allocation	O
tree	O
acts	O
as	O
a	O
B-tree	B-Architecture
version	O
of	O
a	O
BSP	O
tree	O
.	O
</s>
<s>
In	O
practice	O
,	O
however	O
,	O
an	O
in-memory	O
red	O
–	O
black	O
tree	O
of	O
page-sized	O
bitmaps	O
is	O
used	O
to	O
speed	O
up	O
allocations	O
.	O
</s>
<s>
These	O
bitmaps	O
are	O
persisted	O
to	O
disk	O
(	O
starting	O
in	O
Linux	B-Application
2.6.37	O
,	O
via	O
the	O
space_cache	O
mount	O
option	O
)	O
as	O
special	O
extents	B-Application
that	O
are	O
exempt	O
from	O
checksumming	O
and	O
copy-on-write	O
.	O
</s>
<s>
CRC-32C	O
checksums	B-Algorithm
are	O
computed	O
for	O
both	O
data	O
and	O
metadata	O
and	O
stored	O
as	O
checksum	B-Algorithm
items	O
in	O
a	O
checksum	B-Algorithm
tree	O
.	O
</s>
<s>
There	O
is	O
room	O
for	O
256bits	O
of	O
metadata	O
checksums	B-Algorithm
and	O
up	O
to	O
a	O
full	O
node	O
(	O
roughly	O
4KB	O
or	O
more	O
)	O
for	O
data	O
checksums	B-Algorithm
.	O
</s>
<s>
Btrfs	B-Operating_System
has	O
provisions	O
for	O
additional	O
checksum	B-Algorithm
algorithms	O
to	O
be	O
added	O
in	O
future	O
versions	O
of	O
the	O
file	B-Application
system	I-Application
.	O
</s>
<s>
There	O
is	O
one	O
checksum	B-Algorithm
item	O
per	O
contiguous	O
run	O
of	O
allocated	O
blocks	O
,	O
with	O
per-block	O
checksums	B-Algorithm
packed	O
end-to-end	O
into	O
the	O
item	O
data	O
.	O
</s>
<s>
If	O
there	O
are	O
more	O
checksums	B-Algorithm
than	O
can	O
fit	O
,	O
they	O
spill	O
into	O
another	O
checksum	B-Algorithm
item	O
in	O
a	O
new	O
leaf	O
.	O
</s>
<s>
If	O
the	O
file	B-Application
system	I-Application
detects	O
a	O
checksum	B-Algorithm
mismatch	O
while	O
reading	O
a	O
block	O
,	O
it	O
first	O
tries	O
to	O
obtain	O
(	O
or	O
create	O
)	O
a	O
good	O
copy	O
of	O
this	O
block	O
from	O
another	O
device	O
if	O
internal	O
mirroring	O
or	O
RAID	O
techniques	O
are	O
in	O
use	O
.	O
</s>
<s>
Btrfs	B-Operating_System
can	O
initiate	O
an	O
online	O
check	O
of	O
the	O
entire	O
file	B-Application
system	I-Application
by	O
triggering	O
a	O
file	B-Application
system	I-Application
scrub	O
job	O
that	O
is	O
performed	O
in	O
the	O
background	O
.	O
</s>
<s>
The	O
scrub	O
job	O
scans	O
the	O
entire	O
file	B-Application
system	I-Application
for	O
integrity	O
and	O
automatically	O
attempts	O
to	O
report	O
and	O
repair	O
any	O
bad	O
blocks	O
it	O
finds	O
along	O
the	O
way	O
.	O
</s>
<s>
An	O
fsync	B-Language
request	O
commits	O
modified	O
data	O
immediately	O
to	O
stable	O
storage	O
.	O
</s>
<s>
fsync-heavy	O
workloads	O
(	O
like	O
a	O
database	O
or	O
a	O
virtual	B-Architecture
machine	I-Architecture
whose	O
running	O
OS	O
fsyncs	B-Language
frequently	O
)	O
could	O
potentially	O
generate	O
a	O
great	O
deal	O
of	O
redundant	O
write	O
I/O	O
by	O
forcing	O
the	O
file	B-Application
system	I-Application
to	O
repeatedly	O
copy-on-write	O
and	O
flush	O
frequently	O
modified	O
parts	O
of	O
trees	O
to	O
storage	O
.	O
</s>
<s>
To	O
avoid	O
this	O
,	O
a	O
temporary	O
per-subvolume	O
log	O
tree	O
is	O
created	O
to	O
journal	B-Application
fsync-triggered	O
copies	O
on	O
write	O
.	O
</s>
<s>
Log	O
trees	O
are	O
self-contained	O
,	O
tracking	O
their	O
own	O
extents	B-Application
and	O
keeping	O
their	O
own	O
checksum	B-Algorithm
items	O
.	O
</s>
<s>
These	O
logical	O
chunks	O
are	O
combined	O
into	O
a	O
single	O
logical	O
address	O
space	O
that	O
the	O
rest	O
of	O
the	O
filesystem	B-Application
uses	O
.	O
</s>
<s>
If	O
N	O
is	O
not	O
large	O
enough	O
for	O
the	O
chosen	O
mirroring/mapping	O
,	O
then	O
the	O
filesystem	B-Application
is	O
effectively	O
out	O
of	O
space	O
.	O
</s>
<s>
Defragmentation	B-Application
,	O
shrinking	O
,	O
and	O
rebalancing	O
operations	O
require	O
extents	B-Application
to	O
be	O
relocated	O
.	O
</s>
<s>
However	O
,	O
doing	O
a	O
simple	O
copy-on-write	O
of	O
the	O
relocating	O
extent	O
will	O
break	O
sharing	O
between	O
snapshots	B-Application
and	O
consume	O
disk	O
space	O
.	O
</s>
<s>
Then	O
,	O
by	O
following	O
backreferences	O
upward	O
through	O
the	O
affected	O
subvolume	O
's	O
file	B-Application
system	I-Application
tree	O
,	O
metadata	O
pointing	O
to	O
the	O
old	O
extent	O
is	O
progressively	O
updated	O
to	O
point	O
at	O
the	O
new	O
one	O
;	O
any	O
newly	O
updated	O
items	O
are	O
stored	O
in	O
the	O
relocation	O
tree	O
.	O
</s>
<s>
All	O
the	O
file	B-Application
system	I-Application
's	O
trees	O
—	O
including	O
the	O
chunk	O
tree	O
itself	O
—	O
are	O
stored	O
in	O
chunks	O
,	O
creating	O
a	O
potential	O
bootstrapping	B-Application
problem	O
when	O
mounting	B-Application
the	O
file	B-Application
system	I-Application
.	O
</s>
<s>
All	O
superblock	O
mirrors	O
are	O
updated	O
in	O
tandem	O
,	O
except	O
in	O
SSD	B-Device
mode	O
which	O
alternates	O
updates	O
among	O
mirrors	O
to	O
provide	O
some	O
wear	B-Application
levelling	I-Application
.	O
</s>
<s>
Btrfs	B-Operating_System
was	O
included	O
as	O
a	O
"	O
technology	O
preview	O
"	O
in	O
Red	O
Hat	O
Enterprise	O
Linux	B-Application
6	O
and	O
7	O
;	O
it	O
was	O
removed	O
in	O
RHEL	O
8	O
in	O
2018	O
.	O
</s>
<s>
Btrfs	B-Operating_System
was	O
layered	O
over	O
mdadm-raid	O
arrays	O
in	O
Netgear	O
ReadyNAS	O
arm-based	O
1xx	O
and	O
intel-based	O
3xx	O
around	O
2013	O
(	O
probably	O
to	O
exploit	O
btrfs	B-Operating_System
checksums	B-Algorithm
and	O
snapshots	B-Application
while	O
avoiding	O
still	O
unstable	O
btrfs-raid	O
functionalities	O
)	O
.	O
</s>
