[an error occurred while processing this directive]

Screen Version

School of Computer Science & Engineering
University of New South Wales
 Advanced Operating Systems 
 COMP9242 2002/S2 

SGI's XFS

• Scalable FS for high-performance computing[SDH$^+$96]:
  • many files,
  • big directories,
  • huge files,
  • sparse files.
  • big file systems (TB),
  • fast crash recovery,
  • high bandwidth (uncompressed video), 500MB/s!

• Extensive use of B trees:
  • directories (keyed by 4-byte hash of file name)
  • indirect block maps (using block ranges, extents),
  • free-block maps,
  • inode maps containing:
    • pointers to ranges of inodes, and
    • bitmap for range.
• Allocation groups (large sub-partitions) to localise metadata.
  • Limits size of free space and inode maps.
  • Allows use of 32-bit pointers.
  • Limits bottlenecks due to single-threaded updates.
  • Used similar to FFS CGs (cluster files, spread directories).

• Write-ahead logging for metadata updates:
  • inodes,
  • directory blocks,
  • free block maps,
  • inode maps,
  • file block maps,
  • AG headers,
  • superblock
• Batch and asynchronously write log.
• Buffer log to avoid blocking further metadata updates.
• Support separate log device.
• Dynamically allocated inodes.
• Block size up to 64kB.

• Block allocation as contiguous as possible:
  • Extents: large (up to ) unaligned contiguous block ranges.
  • Physical pointers are extent descriptors:
    • file offset (for sparsity),
    • block address,
    • extent size.
  • Actual allocation of disk blocks is delayed as much as possible.
    • Allows maximising extents.
    • Prevents short-lived files from ever going to disk.
    • Requires extensive buffering in RAM.
• Aggressive read-ahead and write clustering.
• Direct I/O: DMA to user buffers.
• Multiple concurrent readers and writers supported to same file.
• Log is only shared data structure on SMP system.

[an error occurred while processing this directive]