<s>
HAMMER	B-Application
is	O
a	O
high-availability	O
64-bit	B-Device
file	B-Application
system	I-Application
developed	O
by	O
Matthew	O
Dillon	O
for	O
DragonFly	B-Application
BSD	I-Application
using	O
B+	O
trees	O
.	O
</s>
<s>
Its	O
major	O
features	O
include	O
infinite	O
NFS-exportable	O
snapshots	B-Application
,	O
master	B-Operating_System
–	I-Operating_System
multislave	I-Operating_System
operation	O
,	O
configurable	O
history	O
retention	O
,	O
fsckless-mount	O
,	O
and	O
checksums	B-Algorithm
to	O
deal	O
with	O
data	O
corruption	O
.	O
</s>
<s>
HAMMER	B-Application
also	O
supports	O
data	O
block	O
deduplication	B-General_Concept
,	O
meaning	O
that	O
identical	O
data	O
blocks	O
will	O
be	O
stored	O
only	O
once	O
on	O
a	O
file	B-Application
system	I-Application
.	O
</s>
<s>
A	O
successor	O
,	O
HAMMER2	B-Application
,	O
was	O
announced	O
in	O
2011	O
and	O
became	O
the	O
default	O
in	O
Dragonfly	O
5.2	O
(	O
April	O
2018	O
)	O
.	O
</s>
<s>
HAMMER	B-Application
file	I-Application
system	I-Application
provides	O
configurable	O
fine-grained	O
and	O
coarse-grained	O
filesystem	B-Application
histories	O
with	O
online	O
snapshots	B-Application
availability	O
.	O
</s>
<s>
Up	O
to	O
65536	O
master	B-Operating_System
(	O
read	O
–	O
write	O
)	O
and	O
slave	O
(	O
read-only	O
)	O
pseudo	B-Application
file	I-Application
systems	I-Application
(	O
PFSs	O
)	O
,	O
with	O
independent	O
individual	O
retention	O
parameters	O
and	O
inode	B-Application
numbering	O
,	O
may	O
be	O
created	O
for	O
each	O
file	B-Application
system	I-Application
;	O
PFS	O
may	O
be	O
mirrored	O
to	O
multiple	O
slaves	O
both	O
locally	O
or	O
over	O
network	O
connection	O
with	O
near	O
real-time	O
performance	O
.	O
</s>
<s>
No	O
file	B-Application
system	I-Application
checking	O
is	O
required	O
on	O
remount	B-Application
.	O
</s>
<s>
HAMMER	B-Application
supports	O
volumes	O
up	O
to	O
1EiB	O
of	O
storage	O
capacity	O
.	O
</s>
<s>
File	B-Application
system	I-Application
supports	O
CRC	O
checksumming	O
of	O
data	O
and	O
metadata	O
,	O
online	O
layout	O
correction	O
and	O
data	B-General_Concept
deduplication	I-General_Concept
,	O
and	O
dynamic	O
inodes	B-Application
allocation	O
with	O
an	O
effectively	O
unlimited	O
number	O
of	O
inodes	B-Application
.	O
</s>
<s>
Currently	O
,	O
regular	O
maintenance	O
is	O
required	O
to	O
keep	O
the	O
file	B-Application
system	I-Application
clean	O
and	O
regain	O
space	O
after	O
file	O
deletions	O
.	O
</s>
<s>
By	O
default	O
,	O
a	O
cron	B-General_Concept
job	I-General_Concept
performs	O
the	O
necessary	O
actions	O
on	O
DragonFly	B-Application
BSD	I-Application
daily	O
.	O
</s>
<s>
HAMMER	B-Application
does	O
not	O
support	O
multi-master	O
configurations	O
.	O
</s>
<s>
HAMMER	B-Application
is	O
optimized	O
to	O
reduce	O
the	O
number	O
of	O
physical	O
I/O	O
operations	O
to	O
cover	O
the	O
most	O
likely	O
path	O
,	O
ensuring	O
sequential	B-General_Concept
access	I-General_Concept
for	O
optimal	O
performance	O
.	O
</s>
<s>
HAMMER	B-Application
was	O
developed	O
specifically	O
for	O
DragonFly	B-Application
BSD	I-Application
to	O
provide	O
a	O
feature-rich	O
yet	O
better	O
designed	O
analogue	O
of	O
the	O
then	O
increasingly	O
popular	O
ZFS	B-Application
.	O
</s>
<s>
HAMMER	B-Application
was	O
declared	O
production-ready	O
with	O
DragonFly	O
2.2	O
in	O
2009	O
;	O
in	O
2012	O
,	O
design-level	O
work	O
shifted	O
onto	O
HAMMER2	B-Application
,	O
which	O
was	O
declared	O
stable	O
with	O
DragonFly	O
5.2	O
in	O
2018	O
.	O
</s>
<s>
,	O
HAMMER	B-Application
is	O
now	O
often	O
referred	O
to	O
as	O
HAMMER1	O
to	O
avoid	O
confusion	O
with	O
HAMMER2	B-Application
,	O
although	O
an	O
official	O
renaming	O
has	O
not	O
happened	O
.	O
</s>
<s>
Both	O
filesystems	B-Application
are	O
independent	O
of	O
each	O
other	O
due	O
to	O
different	O
on-disk	O
formats	O
,	O
and	O
continue	O
to	O
receive	O
separate	O
updates	O
and	O
improvements	O
independently	O
.	O
</s>
