Crossing the Chasm: Sneaking a parallel file system into Hadoop Wittawat Tantisiriroj Swapnil Patil, Garth Gibson PARALLEL DATA LABORATORY Carnegie Mellon University
In this work Compare and contrast large storage system architectures Internet services High performance computing Can we use a parallel file system for Internet service applications? Hadoop, an Internet service software stack HDFS, an Internet service file system for Hadoop PVFS, a parallel file system http://www.pdl.cmu.edu/ 2
Today s Internet services Applications are becoming data-intensive Large input data set (e.g. the entire web) Distributed, parallel application execution Distributed file system is a key component Define new semantics for anticipated workloads Atomic append in Google FS Write-once in HDFS Commodity hardware and network Handle failures through replication http://www.pdl.cmu.edu/ 3
The HPC world Equally large applications Large input data set (e.g. astronomy data) Parallel execution on large clusters Use parallel file systems for scalable I/O e.g. IBM s GPFS, Sun s Lustre FS, PanFS, and Parallel Virtual File System (PVFS) http://www.pdl.cmu.edu/ 4
Why use parallel file systems? Handle a wide variety of workloads High concurrent reads and writes Small file support, scalable metadata Offer performance vs. reliability tradeoff RAID-5 (e.g., PanFS) Mirroring Failover (e.g., LustreFS) Standard Unix FS interface & POSIX semantics pnfs standard (NFS v4.1) http://www.pdl.cmu.edu/ 5
Outline A basic shim layer & preliminary evaluation Three add-on features in a shim layer Evaluation http://www.pdl.cmu.edu/ 6
HDFS & PVFS: high level design Meta-data servers Store all file system metadata Handle all metadata operations Data servers Store actual file system data Handle all read and write operations Files are divided into chunks Chunks of a file are distributed across servers http://www.pdl.cmu.edu/ 7
PVFS shim layer under Hadoop Hadoop applications Hadoop framework Extensible file system API HDFS client library PVFS shim layer Unmodified PVFS client library (C) Unmodified PVFS HDFS servers servers Forward requests to and respond from PVFS client library using Java Native Interface (JNI) Client Server http://www.pdl.cmu.edu/ 8
Preliminary Evaluation Text search ( grep ) common workloads in Internet service applications Search for a rare pattern in 100-byte records 64GB data set 32 nodes Each node servers as storage and compute nodes http://www.pdl.cmu.edu/ 9
Vanilla PVFS is disappointing 2.5 times slower http://www.pdl.cmu.edu/ 10
Outline A basic shim layer & preliminary evaluation Three add-on features in a shim layer Readahead buffer File layout information Replication Evaluation http://www.pdl.cmu.edu/ 11
Read operation in Hadoop Typical read workload: Small (less than 128 KB) Sequential through an entire chunk HDFS prefetches an entire chunk No cache coherence issue with its write-once semantic http://www.pdl.cmu.edu/ 12
Readahead buffer PVFS has no client buffer cache Avoid a cache coherence issue with concurrent writes Readahead buffer can be added to PVFS shim layer In Hadoop, a file can become immutable after it is closed No need for cache coherence mechanism http://www.pdl.cmu.edu/ 13
PVFS with 4MB buffer still quite slow http://www.pdl.cmu.edu/ 14
Outline A basic shim layer & preliminary evaluation Three add-on features in a shim layer Readahead buffer File layout information Replication Evaluation http://www.pdl.cmu.edu/ 15
Collocation in Hadoop File layout information Describe where chunks are located Collocate computation and data Ship computation to where data is located Reduce network traffic http://www.pdl.cmu.edu/ 16
Hadoop without collocation Computation Chunk1 Chunk2 Chunk3 Compute Node 3 data transfers over network Storage Node Chunk1 Chunk2 Chunk3 Chunk 3 Node A Chunk 1 Node B Chunk 2 Node C http://www.pdl.cmu.edu/ 17
Hadoop with collocation Computation Chunk1 Chunk2 Chunk3 Compute Node no data transfer over network Storage Node Chunk3 Chunk1 Chunk2 Chunk 3 Node A Chunk 1 Node B Chunk 2 Node C http://www.pdl.cmu.edu/ 18
Expose file layout information File layout information in PVFS Stored as extended attributes Different format from Hadoop format A shim layer converts file layout information from PVFS format to Hadoop format Enable Hadoop to collocate computation and data http://www.pdl.cmu.edu/ 19
PVFS with file layout information comparable performance http://www.pdl.cmu.edu/ 20
Outline A basic shim layer & preliminary evaluation Three add-on features in a shim layer Readahead buffer File layout information Replication Evaluation http://www.pdl.cmu.edu/ 21
Replication in HDFS Rack-awareness replication By default, 3 copies for each file (triplication) 1.Write to a local storage node 2.Write to a storage node in the local rack 3.Write to a storage node in the other rack http://www.pdl.cmu.edu/ 22
Replication in PVFS No replication in the public release of PVFS Rely on hardware based reliability solutions Per server RAID inside logical storage devices Replication can be added in a shim layer Write each file to three servers No reconstruction/recovery in the prototype http://www.pdl.cmu.edu/ 23
PVFS with replication Hadoop applications Hadoop framework Extensible file system API PVFS shim layer Unmodified PVFS client library (C) Unmodified PVFS server Unmodified PVFS server Unmodified PVFS server http://www.pdl.cmu.edu/ 24
PVFS shim layer under Hadoop Hadoop applications Hadoop framework Extensible file system API ~1,700 lines of code PVFS shim layer Readahead buffer HDFS client library HDFS servers PVFS shim layer Unmodified PVFS client library (C) Unmodified PVFS servers File layout info Replication Client Server http://www.pdl.cmu.edu/ 25
Outline A basic shim layer & preliminary evaluation Three add-on features in a shim layer Evaluation Micro-benchmark (non MapReduce) MapReduce benchmark http://www.pdl.cmu.edu/ 26
Micro-benchmark Cluster configuration 16 nodes Pentium D dual-core 3.0GHz 4 GB Memory One 7200 rpm SATA 160 GB (8 MB buffer) Gigabit Ethernet Use file system API directly without Hadoop involvement http://www.pdl.cmu.edu/ 27
N clients, each reads 1/N of single file Round-robin file layout in PVFS helps avoid contention http://www.pdl.cmu.edu/ 28
Why is PVFS better in this case? Without scheduling, clients read in a uniform pattern Client1 reads A1 then A4 Client2 reads A2 then A5 Client3 reads A3 then A6 PVFS Round-robin placement HDFS Random placement A1 A4 A2 A5 A3 A6 A1 A3 A2 A5 A4 A6 Contention http://www.pdl.cmu.edu/ 29
HDFS with Hadoop s scheduling Example 1: Client1 reads A1 then A4 Client2 reads A2 then A5 Client3 reads A6 then A3 A1 A3 A2 A5 A4 A6 Example 2: Client1 reads A1 then A3 Client2 reads A2 then A5 Client3 reads A4 then A6 A1 A3 A2 A5 A4 A6 http://www.pdl.cmu.edu/ 30
Read with Hadoop s scheduling Hadoop s scheduling can mask a problem with a non-uniform file layout in HDFS http://www.pdl.cmu.edu/ 31
N clients write to n distinct files By writing one of three copies locally, HDFS write throughout grows linearly http://www.pdl.cmu.edu/ 32
Concurrent writes to a single file By allowing concurrent writes in PVFS, copy completes faster by using multiple writers http://www.pdl.cmu.edu/ 33
Outline A basic shim layer & preliminary evaluation Three add-on features in a shim layer Evaluation Micro-benchmark (non MapReduce) MapReduce benchmark http://www.pdl.cmu.edu/ 34
MapReduce benchmark setting Yahoo! M45 cluster Use 50-100 nodes Xeon quad-core 1.86 GHz with 6GB Memory One 7200 rpm SATA 750 GB (8 MB buffer) Gigabit Ethernet Use Hadoop framework for MapReduce processing http://www.pdl.cmu.edu/ 35
MapReduce benchmark Grep: Search for a rare pattern in hundred million 100-byte records (100GB) Sort: Sort hundred million 100-byte records (100GB) Never-Ending Language Learning (NELL): (J. Betteridge, CMU) Count the numbers of selected phrases in 37GB data-set http://www.pdl.cmu.edu/ 36
Read-Intensive Benchmark PVFS s performance is similar to HDFS http://www.pdl.cmu.edu/ 37
Write-Intensive Benchmark By writing one of three copies locally, HDFS does better than PVFS http://www.pdl.cmu.edu/ 38
Summary PVFS can be tuned to deliver promising performance for Hadoop applications Simple shim layer in Hadoop No modification to PVFS PVFS can expose file layout information Enable Hadoop to collocate computation and data Hadoop application can benefit from concurrent writing supported by parallel file systems http://www.pdl.cmu.edu/ 39
Acknowledgements Sam Lang and Rob Ross for help with PVFS internals Yahoo! for the M45 cluster Julio Lopez for help with M45 and Hadoop Justin Betteridge, Le Zhao, Jamie Callan, Shay Cohen, Noah Smith, U Kang and Christos Faloutsos for their scientific applications http://www.pdl.cmu.edu/ 40