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Glossary

  • Array — the set of data disks and parity disks one config protects together.
  • Block — the fixed-size chunk files are divided into for parity and hashing; 256 KiB by default.
  • Check — the command that re-reads and re-hashes the whole array now, verifying data and parity.
  • Content file — SnapRAID's index: every file's name, size, timestamp and block hashes. Kept in several copies on different disks. Lose them all and the parity cannot be used.
  • Diff — the command listing what a sync would change: added, removed, updated, moved. The pre-flight check.
  • Fix — the command that rebuilds corrupted, deleted or lost files from parity and the content file.
  • Hash — a short fingerprint of a block's contents; how corruption is detected, as opposed to repaired.
  • Parity — recovery information computed from the data blocks, from which any one missing block in a stripe can be rebuilt. Also the disk, or the file, holding it.
  • Parity level — one independent parity computation. parity, 2-parity up to 6-parity; each level survives one more simultaneous failure.
  • Pool — SnapRAID's built-in merged view of the data disks, made of symlinks; a lighter alternative to a merging filesystem.
  • Scrub — hash verification of a rotating slice of the array, remembering each block's last verification date.
  • Silent corruption (bit rot) — data changed on disk without any error being reported; only a hash comparison notices.
  • SMART — disks' built-in self-monitoring data; SnapRAID's smart command reads it to estimate disk health.
  • Stripe — the set of blocks at the same position across all data disks, plus their parity block(s).
  • Sync — the command that brings parity and the content file up to date with the disks as they are now — and, in doing so, makes the current state the protected one, deletions included.

Sources

Checked against, in this guide's lab:

  • The SnapRAID manual, on the project site: https://www.snapraid.it/manual
  • The project and its releases: https://github.com/amadvance/snapraid
  • The community all-in-one script: https://github.com/auanasgheps/snapraid-aio-script
  • The MergerFS project: https://github.com/trapexit/mergerfs