<s>
Bitcask	B-Application
is	O
an	O
Erlang	B-Operating_System
application	O
that	O
provides	O
an	O
API	O
for	O
storing	O
and	O
retrieving	O
key/value	O
data	O
into	O
a	O
log-structured	O
hash	B-Algorithm
table	I-Algorithm
.	O
</s>
<s>
The	O
design	O
owes	O
a	O
lot	O
to	O
the	O
principles	O
found	O
in	O
log-structured	B-Application
file	I-Application
systems	I-Application
and	O
draws	O
inspiration	O
from	O
a	O
number	O
of	O
designs	O
that	O
involve	O
log	O
file	O
merging	O
.	O
</s>
<s>
Bitcask	B-Application
has	O
a	O
number	O
of	O
advantages	O
owing	O
to	O
its	O
write-once	O
,	O
append-only	O
on-disk	O
data-format	O
and	O
its	O
use	O
of	O
an	O
in-memory	O
hash-table	B-Algorithm
of	O
keys	O
for	O
lookups	O
:	O
</s>
<s>
Single	O
seek	O
to	O
retrieve	O
any	O
value	O
:	O
Bitcask	B-Application
's	O
in-memory	O
hash-table	B-Algorithm
of	O
keys	O
points	O
directly	O
to	O
the	O
locations	O
on	O
disk	O
where	O
the	O
data	O
resides	O
.	O
</s>
<s>
Bitcask	B-Application
never	O
needs	O
more	O
than	O
one	O
disk-seek	O
to	O
read	O
a	O
value	O
,	O
and	O
the	O
operating	O
system	O
's	O
file-system	O
caching	O
can	O
obviate	O
the	O
need	O
for	O
disk-seeks	O
entirely	O
for	O
some	O
lookups	O
.	O
</s>
<s>
Fast	O
,	O
bounded	O
crash	O
recovery	O
:	O
Bitcask	B-Application
's	O
disk	O
format	O
makes	O
recovery	O
straightforward	O
.	O
</s>
<s>
Recovery	O
need	O
only	O
review	O
the	O
last	O
record	O
or	O
two	O
written	O
and	O
verify	O
checksums	B-Algorithm
to	O
ensure	O
that	O
the	O
data	O
is	O
consistent	O
.	O
</s>
<s>
Easy	O
backup	O
:	O
Bitcask	B-Application
's	O
disk	O
format	O
means	O
that	O
any	O
utility	O
that	O
archives	O
or	O
copies	O
files	O
in	O
disk-block	O
order	O
will	O
properly	O
backup	O
or	O
copy	O
a	O
Bitcask	B-Application
database	O
.	O
</s>
<s>
Because	O
Bitcask	B-Application
keeps	O
all	O
keys	O
in	O
memory	O
at	O
all	O
times	O
,	O
the	O
system	O
must	O
have	O
enough	O
memory	O
to	O
contain	O
the	O
entire	O
keyspace	O
in	O
addition	O
to	O
other	O
operational	O
components	O
and	O
the	O
operating	O
system	O
's	O
file-system	O
buffers	B-General_Concept
.	O
</s>
