Samsara:%20Honor%20Among%20Thieves%20in%20Peer-to-Peer%20Storage - PowerPoint PPT Presentation
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Samsara:%20Honor%20Among%20Thieves%20in%20Peer-to-Peer%20Storage
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Samsara: Honor Among Thieves in Peer-to-Peer Storage Landon P. Cox and Brian D. Noble University of Michigan Samsara From Wikipedia, the free encyclopedia Sa s ra ... – PowerPoint PPT presentation
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Title: Samsara:%20Honor%20Among%20Thieves%20in%20Peer-to-Peer%20Storage
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Samsara Honor Among Thieves in Peer-to-Peer
Storage
- Landon P. Cox and Brian D. Noble
- University of Michigan
2
Samsara
- From Wikipedia, the free encyclopedia
- Sa?sara or Sa?sara (Sanskrit ?????)
- Literally means "continuous flow"
- Is the cycle of birth, life, death, rebirth or
reincarnation within many Eastern religions
3
Paper overview
- Proposes an incentive mechanism motivating
participants in a P2P distributed file system to
contribute as much space as they consume - Addresses the tragedy of the commons
- Requires each peer that requests storage from
another peer to hold a claim for same amount of
storage - Claims can be exchanged
4
The tragedy of the commons
- Assume a group of herders that a common pasture,
on which they are entitled to let their cows
graze - To maximize his/her personal benefit, each herder
will put as many cows as it can on the common
pasture - As a result, the common pasture becomes
overgrazed and useless - Happened to the Boston Common
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Boston common
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Introduction
- P2P file systems have many advantages
- Require users to consume storage according to
their contribution - Otherwise system will collapse
- Solution is a mechanism enforcing"storage
fairness" - Incentive mechanism
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Extant solutions
- A trusted third-party enforcing quotas
- Requires a centralized administration
- Letting people buy and sell storage space
- Requires a trusted clearance infrastructure
- Using certified identities and trusted keys
- Requires a trusted certification authority
- Enforcing total symmetry within pairs of peers
- Unpractical
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Samsara key idea (I)
- Manufacture symmetric relations
- through claim forwarding
- All exchanges of data for claims form symmetric
contracts - Each node periodically checks the other for
compliance - Done in a probabilistic fashion
- When a node breaches the contract, other node is
free to drop the data of its partner
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Samsara key idea (II)
- Nodes can forward claims rather than honoring
them - Still remain responsible for the claims they have
forwarded - Mechanism penalizes unresponsive nodes in a
probabilistic fashion - A node suffering a short outage may lose some
replicas of its data
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Background
- Samsara is an add-on to Pastiche a P2P
cooperative backup system - To be discussed later
- Built itself on top of Pastry network
Pastiche Pastiche
Samsara Pastry
OS Disks OS Disks
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Overall design
- Objective is equal exchange
- If A stores data for B then B must store an
equal-size claim for B - If B discards As claim then A can discard Bs
data - Equal exchange is enforced by periodic queries
- Not answering a query is a sufficient reason to
have you data dropped
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The problem
- This simple claim model punishes nodes too
severely for transient failures - New approach
- Is probabilistic
- Takes into account transient failures
- When a node fails to answer a query, each of is
replica sites drops data with some probability
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Claim construction (I)
- Claims are incompressible placeholders
- Computing a claim requires
- a secret passphrase P
- a secret symmetric key K
- and a location in storage space
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Claim construction (II)
- Assuming we have 512-byte claims
- The first claim C0 would contain
- Twenty-five 20-bit hashes hi SHA1(P, i) where
P is the secret pass phrase and i the hash
index - First 12 bits of next hash in sequence
- all encrypted with the symmetric key K
- C0 h0, h1, , first 12 bits of h25K
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Claim construction (III)
- Successive claims are built using repeating the
process - C1 h26, h27, , first 12 bits of h51K
- Ci hj, hj1, , first 12 bits of hj25K
- where j 26i
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Answering claim queries
- Can be done with a single SHA1 hash
- Querying party provides
- Unique value h0
- List of objects to verify
- Responding party
- Append h0 to first object O0 in list and compute
h1 SHA1(O0, h0) - Recursively computes hi1 SHA1(Oi, hi)
- Returns last hj
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Example (I)
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Example
B has claim ß1 on A and B has claim ?1 on B
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Example
Node B does not have enough space to hold claim
?1
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Example
Node B forwards its claim for space on node A to
node C
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Claim forwarding
- If a node X
- has a claim ? on another node Y and
- owns a claim ? to a third node Z
- It can forward its claim ? to node Y
- Everything works fine until a node fail
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Failures in dependency chains
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Failures in dependency chains
- Before failure,
- B stores data for A,
- C stores data for B
- E stores data for D and hold a claim e1 on A
- When C fails and stop answering queries from B,
- B uses it storage rights on A and replaces claim
e1 by its own claim ß1
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Failures in dependency chains
- After that we have a cascade of damaging actions
- A fails to answer queries from E
- E holds D responsible for loss of claim e1 and
discards the data it had stored for D - D loses its backup data on E even though it had
always operated in a correct fashion - Forwarding claims increases the risk of data
losses
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Failures in dependency cycles
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Failures in dependency cycles
- The effect of a failure is much less dramatic
when we have a dependency cycle, where - B stores data for A,
- C stores data for B
- E stores data for D
- A stores data for E
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Failures in dependency cycles
- When C fails and stop answering queries from B,
- B uses it storage rights on A and requests it to
store its claim ß1 - Since A stores data for E, it can forward claim
ß1 to E - Since E stores data for D, it can forward claim
ß1 to E - E keeps claim ß1 because it has data on E
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Evaluation
- Samsara is faster than scp
- Most chain are short as long as there is free
space - Great news!
- Nodes should forward claims in a very
conservative fashion to minimize data losses
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