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TRACK-ALIGNED EXTENTS: MATCHING ACCESS PATTERNS TO DISK DRIVE CHARACTERISTICS

TRACK-ALIGNED EXTENTS: MATCHING ACCESS PATTERNS TO DISK DRIVE CHARACTERISTICS. STILL INCOMPLETE. J. Schindler J.-L.Griffin C. R. Lumb G. R. Ganger Carnegie Mellon University. Key Ideas. Track-based disk access could improve performance of modern disk drives by up to 50%

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TRACK-ALIGNED EXTENTS: MATCHING ACCESS PATTERNS TO DISK DRIVE CHARACTERISTICS

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  1. TRACK-ALIGNED EXTENTS:MATCHING ACCESS PATTERNSTO DISK DRIVE CHARACTERISTICS STILL INCOMPLETE J. SchindlerJ.-L.GriffinC. R. LumbG. R. Ganger Carnegie Mellon University

  2. Key Ideas • Track-based disk access could improve performance of modern disk drives by up to 50% • Track boundaries can be detected at a reasonable cost • This knowledge can be used by existing file systems without creating hardware dependencies • The technique results significant improvements of application performance

  3. MOTIVATION • Modern disk drives virtualize their storage space as a “flat array of fixed-size blocks” • Prevents OS from taking into account actual characteristics of disk drive • Sole feasible optimization technique is increasing block size • Introduces several problems • Has limited benefits

  4. Increasing block size

  5. Increasing block sizes (cont’d) • Achieving good disk efficiency requiresvery large blocks: • Only 50% efficiency for random writes with 256K blocks • Aligning blocks with track boundaries achieves much higher efficiencies: • Best results obtained when block size is multiple of track size (264KB in example)

  6. TRACK-BASED DISK ACCESS • Cannot keep increasing disk request sizes because large disk requests • Have higher latency • Require more I/O buffer space • Work best when applications access sequentially very large files • Require collocation of related small files

  7. Disk characteristics • Head switches occur anytime a single request accesses blocks that span two tracks • Cause a delay of 0.6 to 1ms • Are not likely to diminish in the near future • Zero-latency access allows disk firmware to read data blocks in the order they can be read rather than in the requested order

  8. Example • Disk firmware wants to read sector 200-299 • Disk head located before sector 250 • Firmware will read • Fist sectors 250 to 299 • Then sectors 200 to 249

  9. TRACK-AWARE SYSTEM DESIGN • Locating track boundaries is not easy because • Outer tracks have more sectors than inner tracks • Tracks have spare blocks • Defect handling schemes vary widely among disk makes and models • Need be performed once • Defects only appear during first 48 hours of operation

  10. Allocation and access (I) • To use track boundary information, file system must support variable extents • Extent-based file systems specify ranges of LBNs allocated to each file (extents) • Should always specify extents that fit track boundaries

  11. Allocation and access (II) • Block-based file systems, such as FFS, group LBNs into fixed-size groups of 2n sectors called blocks • Must ensure that blocks will never span track boundaries • Wastes less than 5% of disk space • This space could still be used for other purposes

  12. Allocation and access (III) • Should also • Extend or clip prefetch and write back requests based on track boundaries • Use disk command queuing to fully take advantage of zero-latency disks • Current SCSI and IDE/ATA controllers do not allow out-of-order delivery to or from the host

  13. Prototype implementation • Prototype includes • Two techniques for detecting track boundaries • A modified version of FreeBSD FFS

  14. Detecting track boundaries (I) • First technique: • Identifies discontinuities in access efficiency • Could be done by linearly increasing number of sector in each I/O request • Uses instead binary search algorithm and tries to predict size of next extent • Still very slow (4 hours for 9GB disk)

  15. Detecting track boundaries (II) • Second technique: • Specific to SCSI disks • Extract disk information through SCSI commands • Exploits the regularity of disk geometry • Much faster ( < one minute per disk) • Does not always work when disk mapping scheme is not knwon

  16. FreeBSD FFS overview • Each block has • An lblkno (logical block number) specifying its offset from the beginning of the file • A blkno (physical block number) , which is an abstract representation of disk addresses used by the OS • Each blkno corresponds to a range of contiguous disk sector numbers

  17. Free BSD modifications • Exclude from all allocation decisions all blocks that span track boundaries • Mark them as used in the free block map • FFS clustered read-ahead algorithm • Accesses runs of blocks between excluded blocks with a single request • Must still take care of track boundaries without excluded blocks

  18. Evaluation • Using • Two disks supporting zero-latency access • Two disks supporting it • Also used Disksim simulator to simulate disks

  19. Disk performance • Measured request headtime, that is, amount of time that the head is dedicated to a request • Biggest improvements were for disks supporting zero-delay access • Efficiency improvements of up to 50% • Also reduced standard-deviation of response times

  20. Other experiments • With a modified FFS file system • Not that different timings • With a video server • Track extents resulted in much lower startup latency at high arrival rates • A log-structured file system

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