PersonalRAID: Mobile Storage for Distributed and Disconnected Computers

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PersonalRAID Mobile Storage for Distributed and
Disconnected Computers

• Jun Feng
• CS851 Fall 2002

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Outline

• Introduction
• Basic Ideas and Issues
• Solutions
• Experimental Results
• Conclusion Discussion

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Storage Technology Trends

• Capacity and speed grows
• 80GB single disk, 30MB/s
• Cost and physical size reduce
• 0.2 cents/MB, Pocket Disk, MicroDrive
• Mobile storage devices emerge
• MicroDrive, SmartMedia, CompactFlash

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Share data a trivial problem
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Problems

• Inconvenience involved in manual movement of data
• Example ftp
• The poor performance in terms of both latency and
  throughput
• Example floppy disk, email
• Network connection is not always available
• Example ftp, email, NFS

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Key idea in PersonalRAID

• Use a mobile storage as a floppy disk (Virtual A)
  to synchronize contents among disconnected
  computers

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Issues - Performance
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Performance goals

• Recording not impose excessive overhead
• During disconnection, user should not be forced
  to wait for long before VA removed
• During connection, user should not be forced to
  wait for long before perform I/O
• Replaying should not impose excessive overhead
• Replaying should proceed quickly

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Issues (cont.)

• Reliability
• Users should have some degree of confidence over
  data reliability
• Transparency
• Users should not be involved in data update and
  propagation manually
• Users should see a single global name space
  regardless which device they use

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Attack problems (preview)

• Performance
• Distributed log-structured design
• Memory buffer
• V2A
• Reliability
• Physical redundancy
• Replaying, checkpoints
• Transparency
• Single coherent storage name space
• No human involvement

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Solutions

• Performance

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Log-Structured File System

• Store data in a single, contiguous log
• Never seek between writes (because youre always
  writing at the end of contiguous space)
• Read is achieved by index
• Large memory is assumed to buffer writes and also
  satisfy read requests
• Segment and segment cleaner

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Log-structured file system (cont.)
LFS
Only 1 write
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Log-structured file system (cont.)
Traditional FS
4 writes 4 disk head moves
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Log-structured file system (cont.)

• Advantages
• Improve write performance by single sequential
  write
• Fast recovery in case of failure
• Disadvantages
• Read performance is not good
• Segment cleaning cost (or disk garbage
  collection) is high

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Why LFS in PersonalRAID?

• Incremental update to diminish performance gap
• Fast recovery in case of failure
• Typical office workload
• Frequently small files read/write
• Example software developing

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The Logical Disk

• Separation of concerns
• File Management
• Example file cache, directory, file, inode
• Disk Management
• Example disk layout, blocks, cylinders, tracks

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The Logical Disk (cont.)
Read(bid, buf, cnt) Write(bid, buf,
cnt) NewBlock() DeleteBlock()
Small block size, optimized for office workload
bigger block size, optimized for scientific
computing workload
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Advantages of Logical Disk

• Makes file systems easier to develop, maintain
  and modify
• Makes file systems more flexible
• Allows efficient use of I/O bandwidth
• Example Reorganize the layout of blocks on the
  disk on the fly

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Log-structured Logical Disk
User Space
write(fd, buf, len)
write(logical block number, buf, len)
write(segment number, buf)
Kernel Space
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Data Structures in LLD
in-memory mapping table is checkpointed to disk
Contains logical address and the time stamp of
each data block
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Implementations
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Inside PRS

• PRS maintains several main-memory segments for
  both the local disk and the VA device
• Segment cleaning is begun when the number of
  clean segment falls below a threshold
• PRS works sequentially

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Data Structures
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Solutions

• Reliability

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Recovering from Failures

• Crash problem
• Tolerant single device failure
• Host disk loss
• VA device loss
• PersonalRAIDs backup ability
• Data is recorded on host disk and VA
• Metadata is checkpointed

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Crash problem

• During recording
• Writes are buffered in memory
• Segment summary is updated
• In-memory map is also updated
• During disconnection
• Flush data to disk and VA sequentially
• Flush in-memory map to host disk and VAs
  checkpoint region

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Crash Problem (cont.)

• Crash in recording
• Simply restore to last checkpoint
• Crash in disconnection
• Data inconsistence on VA and host disk
• Segment summary and checkpoint region are also
  inconsistent
• Goal is to make VA and host disk mutually
  consistent

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Crash Recovery

• read all segment summaries from host disk
• read VA's checkpoint region
• for (each block)
• compare the time stamp in segment summary and
  VA's checkpoint
• if (less)
• synchronize this data block
• update the timestamp on VA's checkpoint and
  segment summary
• update propagation bits on VA's checkpoint

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Host Disk Loss

• Synchronize As disk with VA
• Make a disk mirror C of As disk
• Replace Bs disk with C

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VA Device Loss
Metadata is constructed
First tour
Data blocks are restored
Second tour
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VA Device Loss (cont)
Metadata is constructed
A
Data blocks are restored
Tour
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Solutions

• Transparency

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Transparency

• Single global name space
• Every participating host has a separate partition
• User has to work on this partition
• All operations are done automatically
• Except crash recovery, and disk failure

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Performance Evaluation

• Benchmarks
• Andrew Benchmark
• Mixed big files and small files, recursive
  directory operations
• Evaluate almost all performance aspects for a
  file system
• Mozilla source tree
• Small files creation, read, and write
• Software development workload

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Andrew Results
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Recording Performance
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Disconnection, connection, and replaying
performance
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Conflicts Solution

• Target on personal data usage
• Conflicts are system-level or app-level events,
  must be addressed at higher levels
• PersonalRAID is a storage-level solution

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Use Virtual VAs

• Physical mobile storage is not necessary in the
  design
• Example a file, a local disk partition
• Improve recording efficiency
• Make a copy of VA for purpose of reconstructing
  of a lost VA

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Critiques

• VA device has to be present when the file system
  is accessed
• Implementation decisions
• Achieving global name space is awkward
• Security issues