ZFS File S"> Selected Notes on the ZFS File System


  • device: HDD, SSD, iSCSI
  • vdev: tree of: stripe, mirror, raidz1, raidz2, device
  • pool: vdevs of: data, write cache, read cache
  • transactional object layer
  • zfs volume emulator, zfs posix layer, other file systems
  • on posix layer: zfs file system, NFS, CIFS

Example of a hybrid storage pool

  • raidz array of hard drives
  • ZIL (write cache) PCIe NVRAM
  • Level 2 ARC (read cache) flash SSD

ZFS Intent Log) write cache"> ZIL (ZFS Intent Log) write cache

  • used for sync writes, transaction integrity
  • separate ZIL device usually helps performance more than L2ARC
  • write-only, except read at boot
  • helps NFS, because the NFS protocol says all writes are sync
  • no effect on read or async write workloads
  • little benefit for sequential write workload
  • max useful size is half of main memory

ARC (Adaptive Replacement Cache) read cache

  • dynamically balances most-recently-used and most-frequently-used
  • (level 1) ARC default target size is 7/8 physical memory
  • L2ARC keeps its directory in kernel memory
  • Dedup Table also uses ARC - lots of dedup objects need lots of RAM - field reports say L2ARC can help with dedup

Optimize devices

  • data drives - GB/$ - big, slow, cheap
  • ZIL - write latency/$ - fast, small
  • L2ARC - fast random read, big enough

ZIO pipeline

  • includes compress, dedup, checksum
  • compress is fast and almost always a win
  • dedup is especially good on backups and virtual machines


  • compression, dedup, and raidz complicate accounting
  • the numbers don't add up, but are correct
  • capacity planning can be challenging

Other features

  • unlimited number of snapshots
  • clones - read-write based on a snapshot
  • recently added user and group quotas


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Topic revision: r2 - 2010-12-08 - FraserGunn
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