Filesystems, RPC and HDFS


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Comparison between traditional filesystems and HDFS writes

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Filesystems, RPC and HDFS

  1. 1. February 2012Filesystems, RPC and HDFSAlexander Lorenz
  2. 2. Agenda1 Linux Kernel I/O Scheduler2 I/O Stack in Linux3 VFS Implementation4 NFS RFC Model5 RPC6 HDFS7 Limitations / Problems (Discussion)2 ©2011 Cloudera, Inc. All Rights Reserved. Confidential. Reproduction or redistribution without written permission is prohibited.
  3. 3. Linux Kernel I/O Scheduler• Disk seek is the slowest operation in a computer• I/O scheduler arranges the disk head to move in a single direction to minimize seeks• Prevent Starvation• Improve overall disk throughput by • Reorder requests to reduce the disk seek time • Merge requests to reduce the number of requests3
  4. 4. Kernel I/O Scheduler Framework Block layer • Linux elevator is an abstract layer to which different I/O scheduler can attach • Merging mechanisms are provided by request queues enqueue • Front or back merge of a request and a bio Merge, sort IO Scheduler • Merge two requests Internal queues • Sorting policy and merge decision are done in elevators • Pick up a request to be merged with a bio prioritize • Add a new request to the request queue dequeue • Select next request to be processed by block drivers external queue device driver4
  5. 5. I/O Stack in Linux Application Userland Sys Calls Kernelspace Filesystem Access Locking Cache Prefetch Flush Bulk writes Meta Disk Layout Data HDD Driver5
  6. 6. VFS Implementation Application Userland Sys Calls Kernelspace VFS ext3 ext2 NFS CIFS6
  7. 7. NFS RFC Model Application with NFS Access Filesystem Filesystem NFS Client NFS Server File Handler RPC RPC Kernelspace TCP/IP TCP/IP Kernelspace UDP UDP Local HDD Local HDD7
  8. 8. NFS - OSI Model8
  9. 9. RPC Client Server Process starts Server waits RPC Message Server start Client waits PC Server waits PE PR RPC Return Process continued Termination9
  10. 10. HDFS Layer Local Client HDFS Application POSIX API HDFS API VFS NFS Driver Network HDFS10
  11. 11. HDFS Model Client add Blck (src) HDFS Cluster Namenode write DN Block received Pipeline DN Block received DN Block received10
  12. 12. HDFS Write Model RPC (ClientProtocol) RPC Client DFS NN rcv only FSData stream (socket) RPC (DFSClient.DFSInputStream) RPC proxy DN intern DN RPC (DataNodeProtocol) RPC xceiver IPC Proxy VFS RPC proxy RPC HDD DN (DataNodeProtocol)11
  13. 13. Links / Resources The Hadoop Distributed File System Konstantin Shvachko, Hairong Kuang, Sanjay Radia, Robert Chansler,Yahoo! NFS and RPC Chavalit Srisathapornphat, CISC856 Linux I/O Schedulers Hao-Ran Liu13