The Kangaroo Approach to Data Movement on the Grid

1. The Kangaroo Approachto Data Movementon the Grid Jim Basney, Miron Livny, Se-Chang Son, and Douglas Thain Condor Project University of Wisconsin

2. Outline • A Vision of Grid Data Movement • Architecture and Example • Semantics and Design • Necessary Mechanisms • The First Hop • What Next?

3. An Old Problem • Run programs that make use of CPUs and storage in separate locations. • There are basic, working solutions to this problem, but they do not address many of its subleties.

4. The Problem is Not Trivial • Distributed systems are subject to failures that most applications are not designed to handle. • “Oops, a router died.” • “Oops, the switch is in half-duplex mode.” • “Oops, I forgot to start one server.” • “Oops, I forgot to update my AFS tokens.” • We want to avoid wasting resources (cpu, network, disk) that charge for tenancy. • Co-allocation is a common solution, but external factors can get in the way. • Co-allocation in and of itself is wasteful! • Can’t we overlap I/O and cpu?

5. Failures: Long-distance link breaks once per day Policy: local traffic has priority over external traffic during office hours Traffic: Local apps need disk bandwidth, too. Traffic: Many CPU users on shared network Unsynchronized variations Example Compute Machines Workstation 1000 Mb/s 1 ms 100 Mb/s 1 ms 10 Mb/s 100 ms 240 Mb/s 5 ms

6. What’s inOur Toolbox? • Partial File Transfer: • Condor Remote I/O • Storage Resource Broker (SRB) • (NFS?) • Whole file transfer: • Globus GASS • FTP, GridFTP • (AFS?) • It’s not just what you move, but when you move it.

7. Off-Line On-Line Fetch files on demand. Stage in, execute, stage out. Examples: FTP+telnet, Globus GASS Partial File Get/put only the fragment immediately needed. Examples: Condor, Storage Resource Broker (SRB) NFS telnet stdout A Taxonomy ofExisting Systems Data Movement Systems Whole File Get whole file at open, and write out at close. Examples: Globus GASS in app, AFS

8. Offline I/O • Benefits: • Makes good throughput by pipelining. • Co-allocation of cpu and network not needed. • Easy to schedule. • Drawbacks: • Must know needed files in advance. • Co-use of cpu and network not possible. • Must pull/push whole file, even when only partial is needed.

9. Online I/O • Benefits: • Need not know I/O requirements up front. (Some programs compute file names.) • Gives user incremental results. • (Partial) Only moves what is actually used. • Drawbacks: • Very difficult to schedule small or un-announced operations. • (Partial) Stop-and-wait does not scale to high latency networks.

10. Problems with Both • Error handling • GASS, AFS - close fails?!? • Condor - disconnect causes rollback • The longer the distance, the worse the performance • Drop rate is multiplied with each additional link. • Latency increases with each link. • TCP throughput is limited to the slowest link. • Resource allocation • Network allocation is done end-to-end. • CPU and I/O rarely overlap.

11. Our Vision • A no-futz wide-area data movement system that provides end-to-end reliability, maximizes throughput, and adapts to local conditions and policies. • Basic idea: • Add buffers. • Add a process to oversee.

12. Our Vision Compute Machines Home Machine 1000 Mb/s 1 ms 100 Mb/s 1 ms 10 Mb/s 100 ms 300 Mb/s 5 ms RAM RAM RAM

13. Has my data arrived safely? App File System File System File System File System Our Vision: A Grid K K K Data Movement System K K K K

14. Our Vision • Requirements: • Must be “fire-and-forget.” Relieve the application of error handling! Robust wrt to machine and software crashes. (No-futz) • Must provide incremental output results. • Hide latency from applications by overlapping I/O and cpu. • Maximize use of resources (cpu, network, disk) when available, and evacuate same when required.

15. Our Vision • Concessions: • No inter-process consistency needed. • Increased latency of actual data movement is acceptable.

16. The First Hop • A working test bed that validates the core architecture. • Supports applications using standard POSIX operations. • Concentrate on write-behind because it doesn’t require speculation. • Leave room in the architecture to experiment with read-ahead. • Preview of results: • Small scale, overlapping is slower. • Large scale, overlapping is faster.

17. Outline • A Vision of Grid Data Movement • Architecture and Example • Necessary Mechanisms • Semantics and Design • The First Hop • What Next?

18. Architecture • Layers • Application • Adaptation • Consistency • Transport • Example

19. Blocking procedure call Non-blocking message Architecture Application File System open, read, write, close, fsync Adaptation get, put, push, abort open, read, write, close, fsync Consistency Consistency put ack ack put ack ack Transport Transport Transport put put

20. Transport Layer • Interface • Send message, query route, query status • Semantics • Ordering - None (or worse!) • Reliability - Likely, but not guaranteed. • Duplication - Unlikely, but possible. • Performance • Uses all available resources (net, mem, disk) to maximize throughput. • Subject to local conditions (traffic, failures) and policies (priority, bw limits)

21. Transport Layer In Out Transport 1 Gb/s 1 Gb/s If output is blocked, then save input to disk until it is full. When output is ready again, read from disk, memory, or input? RAM 300 Mb/s The freedom to reorder transported blocks may allow us to improve throughput.

22. Consistency Layer • Interface • Get block, put block, sync file, abort file • Semantics • Ordering - Order preserving or not? • Reliability - Detects success • Duplication - Delivers at most once • Performance • Must cache dirty blocks until delivered • Might cache clean blocks • Might speculatively read clean blocks

23. Consistency Layer Receiver: Keeps records to enforce ordering and supress duplicates. Sender: Keeps records to detect success, cache writes. Consistency Consistency Transport Transport Transport

24. Adaptation Layer • Converts POSIX operations into Kangaroo operations • Open • O_CREAT, always succeeds • Otherwise, checks for existence with a ‘get’ • Read = kangaroo get • Write = kangaroo put • Close = NOP • Fsync = kangaroo sync

25. Open Close Write Read Has ‘file’ arrived? Or: Abort this transfer. Put Get Read Write Get Put Ack Ack Put Ack Ack Put Put Example Blocking procedure call Non-blocking message Application File System Adaptation Consistency Consistency Transport Transport Transport

26. Outline • A Vision of Grid Data Movement • Architecture and Example • Semantics and Design • Necessary Mechanisms • The First Hop • What Next?

27. Semantics and Design • A data movement system is a bridge between file systems. • It addresses many of the same issues as file systems: • Consistency • Committal • Ordering • Replication

28. Consistency • Single Node • A put/get blocks until the local server has atomically accepted it. • Multiple processes that are externally synchronized will see a consistent view. • Multiple Nodes • No guarantees unless you use an explicit sync. • This is reasonable in a Grid environment, because most users make use of a wide-area scheduler to partition jobs and data.

29. Commital • Possible meanings of commit: • Force this data to the safest medium available. • Make these changes visible to others. • Make this data safe from a typical crash. • Possible implementations in Kangaroo: • Push all the way to target, and force to disk (tape?) • Push to the target server. • Push to the nearest disk.

30. Commital • Safest choice is to implement the most conservative -- push all the way to the server, and force it to disk there. • Some applications may want the more relaxed meanings. • POSIX only provides one interface: fsync(). • Easy solution: implement all three, and provide a flexible binding in the Adaptation layer.

31. Ordering • Does the system commit operations in the same order they were sent? • Relaxed -- no ordering • Satisifies large majority of apps that do not overlap writes. • Interesting case of output log files. • Need to wait max TTL before re-using an output file name • Strict -- exact ordering, enforced at recvr • Increases queue lengths everywhere. • Doesn’t burden user with determining if application is safe to relax.

32. Strict Ordering Algorithm • Much like TCP: • Sender keeps copies of data blocks until they are acknowledged. • Receiver sends cumulative acks and commits unbroken sequences.

33. Strict Ordering Algorithm • But some differences from TCP: • No connection semantics. • Block ID is (birthday,sequence). • Receiver keeps on disk last ack’d ID of all senders it has ever talked to. • If sender reboots: • Compute the next ID from blocks on disk • If none, reset b to current time, s to 0 • If receiver reboots: • Last recvd ID of all senders is on disk. • Garbage problem: fix with a long receiver timeout + reset message causes sender to start over.

34. Replication Issues • We would like to delete data stored at the sender ASAP, but… • Do I Trust this Disk? • Buffer Storage - Could disappear at any time. • Reliable Storage - No deliberate destruction. • Reliability is not everything • If delivery is highly likely and recomputation is relatively cheap, then losing data is acceptable… but only if delivery failure is detectable! • Reliability = More copies. • User should be able to configure a range from “most reliable” to “fewest copies.”

35. Replication Issues • End-to-End Argument: • Regardless of whatever duplication is done internally for performance or reliability, only the end points can be responsible for ensuring (or detecting) correct delivery. • So, the sender must retain a record of what was sent, even if it does not retain the actual data.

36. Replication Techniques • Pass the Buck • Hold the Phone • Don’t Trust Strangers

37. D D D D Pass the Buck • Delete the local copy after a one-hop ack. Requires atomic accept and sync. (Similar to email) K K K K R

38. D D D Hold the Phone • Sender keeps a copy of local data until the end-to-end ack is received. Midway hops need not immediately flush to disk. K K K K D R

39. D I trust you. R D No, you are just buffer. D I trust you. Don’t Trust Strangers • If the sender determines the receiver to be reliable, then delete, otherwise hold. K K K K R D

40. Replication Comparison • Pass the Buck • Evacuates source ASAP. One copy of data. • Dirty reads must hop through all nodes. • No retry of failures. (Success still likely.) • Hold the Phone • Evacuates source more slowly. Two copies. • Dirty reads always satisfied at source. • Sender can retry failures. • Don’t Trust Strangers • Evacuates source like PTB, but still 2 copies. • Dirty reads hop. • Retries done midway.

41. Outline • A Vision of Grid Data Movement • Architecture and Example • Necessary Mechanisms • Semantics and Design • The First Hop • What Next?

42. Necessary Mechanisms • Adaptation Layer • Needs a tool for trapping and rerouting an application’s I/O calls without special privileges: Bypass • Transport Layer • Needs a tool for detecting network conditions and enforcing policies: Cedar

43. Bypass • General-purpose tool for trapping and redirecting standard library procedures. • Trap all I/O operations. Those involving Kangaroo are sent to Adaptation layer. Otherwise, execute without modification. • Can be applied at run-time to any dynamically-linked program: • vi kangaroo://home.cs.wisc.edu/tmp/file • grep thain gsiftp://ftp.cs.wisc.edu/etc/passwd • gcc http://www/example.c -o kangaroo://home/output

44. Cedar • Standard socket abstraction. • Enforces limits on how much bandwidth can be consumed across multiple times scales. • Also measures congestion and reports to locally-determined manager. • Example: • If conditions are good, do not exceed 10Mb/s. • If there is competition for the link, fall back to no more than 1Mb/s.

45. Why Limit Bandwidth? • Isn’t TCP flow control sufficient? • An overloaded receiver can squelch a sender with back-pressure. • Competing TCPs will tend to split the available bw equally. • No. Three reasons: • To enforce local policies on resources consumed by visiting processes. • To clamp processes competing for a single resource. • To leave some bandwidth available for small-scale unscheduled operations.

46. Outline • A Vision of Grid Data Movement • Architecture and Example • Semantics and Design • Necessary Mechanisms • The First Hop • What Next?

47. The First Hop • We have implemented a kangaroo testbed which has most of the critical features: • Each node runs a kangaroo_server process which accepts messages on TCP and UNIX-domain sockets. • Outgoing data is placed into a spool dir in the file system for a kangaroo_mover process to pick it up and send it out. • Bypass is used to attach unmodified UNIX applications to a libkangaroo.a which contacts the local server to execute puts and gets.

48. The First Hop • Several important elements are yet to be implemented: • Only one sync algorithm • push to server but not to disk • Only one replication algorithm: • hold the phone • Consistency layer detects delivery success, but does not timeout and retry. • Receiver implements only relaxed ordering. • Reads are implemented simply as minimal blocking RPCs to the target server.

49. Measurements • Micro: How fast can an app write output? • Plain file • Plain file through Kangaroo • Kangaroo • Mini: How fast can output be moved? • Online: Stream from memory to network. • Offline: Stage to disk, then write to network. • Kangaroo • Macro: How fast can we run an event-processing program? • Online: Read and write over network. • Offline: Stage input, run program, stage output. • Kangaroo

50. Measurements • Two types of machines used: • Disk>Network (Linux Workstations) • 100 Mb/s switched Ethernet • 512 MB RAM • 10.2 GB Quantum Fireball Plus LM • Ultra ATA/66, 7200 RPM, 2MB cache • 650 MHz P3 • Network>Disk (Linux Cluster Nodes) • 100 Mb/s switched Ethernet • 1024 MB RAM • 9.1 GB IBM 08L8621 • Ultra2 Wide SCSI-3, 10000 RPM, 4MB cache • 2 * 550 MHz P3 Xeon