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OceanStore: An Architecture for Global-Scale Persistent Storage John Kubiatowicz, et al ASPLOS 2000
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OceanStore Global scale information storage. Mobile access to information in a uniform and highly available way. Servers are untrusted. Caches data anywhere, anytime. Monitors usage patterns. 10 10 users, each with 10,000 files
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[Rhea et al. 2003] OceanStore
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Main Goals Untrusted infrastructure Nomadic data
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Example applications Groupware and PIM Email Digital libraries Scientific data repository Personal information management tools: calendars, emails, contact lists
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Example applications Groupware and PIM Email Digital libraries Scientific data repository Challenges: scaling, consistency, migration, network failures
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Storage Organization OceanStore data object ~= file Ordered sequence of read-only versions Every version of every object kept forever Can be used as backup An object contains metadata, data, and references to previous versions
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Storage Organization A stream of objects identified by AGUID Active globally-unique identifier Cryptographically-secure hash of an application-specific name and the owner’s public key Prevents namespace collisions
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Storage Organization Each version of data object stored in a B- tree like data structure Each block has a BGUID Cryptographically-secure hash of the block content Each version has a VGUID Two versions may share blocks
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[Rhea et al. 2003] Storage Organization
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Access Control Restricting readers: Symmetric encryption key distributed to allowed readers. Restricting writers: ACL. Signed writes. ACL for object chosen with signed certificate.
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Location and Routing Attenuated Bloom FiltersBloom Filters Find 11010
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Location and Routing Plaxton-like trees
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Updating data All data is encrypted. A set of predicates is evaluated in order. The actions of the earliest true predicate are applied. Update is logged if it commits or aborts. Predicates: compare-version, compare-block, compare-size, search Actions replace-block, insert-block, delete-block, append
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Application-Specific Consistency An update is the operation of adding a new version to the head of a version stream Updates are applied atomically Represented as an array of potential actions Each guarded by a predicate
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Application-Specific Consistency Example actions Replacing some bytes Appending new data to an object Truncating an object Example predicates Check for the latest version number Compare bytes
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Application-Specific Consistency To implement ACID semantic Check for readers If none, update Append to a mailbox No checking No explicit locks or leases
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Application-Specific Consistency Predicate for reads Examples Can’t read something older than 30 seconds Only can read data from a specific time frame
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Replication and Consistency A data object is a sequence of read-only versions, consisting of read-only blocks, named by BGUIDs No issues for replication The mapping from AGUID to the latest VGUID may change Use primary-copy replication
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Serializing updates A small primary tier of replicas run a Byzantine agreement protocol. A secondary tier of replicas optimistically propagate the update using an epidemic protocol. optimistically Ordering from primary tier is multicasted to secondary replicas.
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The Full Update Path
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Deep Archival Storage Data is fragmented. Each fragment is an object. Erasure coding is used to increase reliability. Erasure coding
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Introspection computation observation optimization Uses: Cluster recognition Replica management Other uses
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Software Architecture Java atop the Staged Event Driven Architecture (SEDA) Each subsystem is implemented as a stage With each own state and thread pool Stages communicate through events 50,000 semicolons by five graduate students and many undergrad interns
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Software Architecture
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Language Choice Java: speed of development Strongly typed Garbage collected Reduced debugging time Support for events Easy to port multithreaded code in Java Ported to Windows 2000 in one week
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Language Choice Problems with Java: Unpredictability introduced by garbage collection Every thread in the system is halted while the garbage collector runs Any on-going process stalls for ~100 milliseconds May add several seconds to requests travel cross machines
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Experimental Setup Two test beds Local cluster of 42 machines at Berkeley Each with 2 1.0 GHz Pentium III 1.5GB PC133 SDRAM 2 36GB hard drives, RAID 0 Gigabit Ethernet adaptor Linux 2.4.18 SMP
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Experimental Setup PlanetLab, ~100 nodes across ~40 sites 1.2 GHz Pentium III, 1GB RAM ~1000 virtual nodes
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Storage Overhead For 32 choose 16 erasure encoding 2.7x for data > 8KB For 64 choose 16 erasure encoding 4.8x for data > 8KB
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The Latency Benchmark A single client submits updates of various sizes to a four-node inner ring Metric: Time from before the request is signed to the signature over the result is checked Update 40 MB of data over 1000 updates, with 100ms between updates
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The Latency Benchmark Update Latency (ms) Key Size Update Size 5% Time Median Time 95% Time 512b 4kB394041 2MB103710861348 1024b 4kB9899100 2MB109811501448 Latency Breakdown PhaseTime (ms) Check0.3 Serialize6.1 Apply1.5 Archive4.5 Sign77.8
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The Throughput Microbenchmark A number of clients submit updates of various sizes to disjoint objects, to a four- node inner ring The clients Create their objects Synchronize themselves Update the object as many time as possible for 100 seconds
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The Throughput Microbenchmark
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Archive Retrieval Performance Populate the archive by submitting updates of various sizes to a four-node inner ring Delete all copies of the data in its reconstructed form A single client submits reads
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Archive Retrieval Performance Throughput: 1.19 MB/s (Planetlab) 2.59 MB/s (local cluster) Latency ~30-70 milliseconds
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The Stream Benchmark Ran 500 virtual nodes on PlanetLab Inner Ring in SF Bay Area Replicas clustered in 7 largest P-Lab sites Streams updates to all replicas One writer - content creator – repeatedly appends to data object Others read new versions as they arrive Measure network resource consumption
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The Stream Benchmark
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The Tag Benchmark Measures the latency of token passing OceanStore 2.2 times slower than TCP/IP
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The Andrew Benchmark File system benchmark 4.6x than NFS in read-intensive phases 7.3x slower in write-intensive phases
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[Koloniari and Pitoura] Bloom Filters Compact data structures for a probabilistic representation of a set Appropriate to answer membership queries
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Bloom Filters (cont’d) Query for b: check the bits at positions H 1 (b), H 2 (b),..., H 4 (b). back
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[Kang et al. 2003] Site ASite BSite C V 0 : (x 0 ) V 1 : (x 1 ) write x V 0 : (x 0 ) V 4 : (x 4 ) V 5 : (x 5 ) V 2 : (x 2 ) write x V 3 : (x 3 ) write x Pair-Wise Reconciliation
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Site ASite BSite C V0V0 V1V1 V2V2 V3V3 V4V4 H0H0 H0H0 H1H1 H0H0 H2H2 H0H0 H3H3 H0H0 H1H1 H2H2 H4H4 H i = hash (V i ) V5V5 H3H3 H5H5 H0H0 H1H1 H2H2 H4H4 Hash History Reconciliation back
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[Mitzenmacher] Erasure Codes Message Encoding Received Message Encoding Algorithm Decoding Algorithm Transmission n cn n n back
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