2 Background
To understand the continuing popularity of portable
storage, it is useful to review the strengths and weak-
nesses of portable storage and distributed file systems.
While there is considerable variation in the designs of
distributed file systems, there is also a substantial de-
gree of commonality across them. Our discussion be-
low focuses on these common themes.
Performance: A portable storage device offers uni-
form performance at all locations, independent of fac-
tors such as network connectivity, initial cache state,
and temporal locality of references. Except for a few
devices such as floppy disks, the access times and band-
widths of portable devices are comparable to those of
local disks. In contrast, the performance of a dis-
tributed file system is highly variable. With a warm
client cache and good locality, performance can match
local storage. With a cold cache, poor connectivity and
low locality, performance can be intolerably slow.
Availability: If you have a portable storage device
in hand, you can access its data. Short of device fail-
ure, which is very rare, no other common failures pre-
vent data access. In contrast, distributed file systems
are susceptible to network failure, server failure, and a
wide range of operator errors.
Robustness: A portable storage device can easily
be lost, stolen or damaged. Data on the device be-
comes permanently inaccessible after such an event.
In contrast, data in a distributed file system continues
to be accessible even if a particular client that uses it
is lost, stolen or damaged. For added robustness, the
operational staff of a distributed file system perform
regular backups and typically keep some of the back-
ups off site to allow recovery after catastrophic site
failure. Backups also help recovery from user error:
if a user accidentally deletes a critical file, he can re-
cover a backed-up version of it. In principle, a highly
disciplined user could implement a careful regimen of
backup of portable storage to improve robustness. In
practice, few users are sufficiently disciplined and well-
organized to do this. It is much simpler for professional
staff to regularly back up a few file servers, thus bene-
fiting all users.
Sharing/Collaboration: The existence of a com-
mon name space simplifies sharing of data and collab-
oration between the users of a distributed file system.
This is much harder if done by physical transfers of de-
vices. If one is restricted to sharing through physical
devices, a system such as PersonalRAID can be valu-
able in managing complexity.
Consistency: Without explicit user effort, a dis-
tributed file system presents the latest version of a file
when it is accessed. In contrast, a portable device has
to be explicitly kept up to date. When multiple users
can update a file, it is easy to get into situations where
a portable device has stale data without its owner being
aware of this fact.
Capacity: Any portable storage device has finite
capacity. In contrast, the client of a distributed file
system can access virtually unlimited amounts of data
spread across multiple file servers. Since local storage
on the client is merely a cache of server data, its size
only limits working set size rather than total data size.
Security: The privacy and integrity of data on
portable storage devices relies primarily on physical se-
curity. A further level of safety can be provided by
encrypting the data on the device, and by requiring a
password to mount it. These can be valuable as a sec-
ond layer of defense in case physical security fails. De-
nial of service is impossible if a user has a portable
storage device in hand. In contrast, the security of data
in a distributed file system is based on more fragile as-
sumptions. Denial of service may be possible through
network attacks. Privacy depends on encryption of
network traffic. Fine-grain protection of data through
mechanisms such as access control lists is possible, but
relies on secure authentication using a mechanism such
as Kerberos [19].
Ubiquity: A distributed file system requires oper-
ating system support. In addition, it may require en-
vironmental support such as Kerberos authentication
and specific firewall configuration. Unless a user is
at a client that meets all of these requirements, he
cannot access his data in a distributed file system.
In contrast, portable storage only depends on widely-
supported low-level hardware and software interfaces.
If a user sits down at a random machine, he can be
much more confident of accessing data from portable
storage in his possession than from a remote file server.
3 Lookaside Caching
Our goal is to exploit the performance and avail-
ability advantages of portable storage to improve these
same attributes in a distributed file system. The result-
ing design should preserve all other characteristics of
the underlying distributed file system. In particular,
2
