hesela.devCopy-on-close
Copy-on-close defers filling an edited shared file's unchanged regions from shared storage until all users have closed it.
Windows 2000 Single Instance Storage (SIS) keeps edits private while unchanged regions use its common store. After users close a modified file, a worker copies the remaining clean regions. Closing therefore does not certify completed materialization. If allocation runs out, the unfinished file retains its reparse point and common-store dependency. [1]
FAST 2020 contrasts this whole-file unsharing policy with copy-on-abundant-write, which batches changes before copying shared nodes. The two policies have different triggers. [2]
Relations
Advantages
- A complete overwrite avoids copying the old contents during materialization. [1]
Limitations
- A small edit can leave most of the file to be copied; background work must be included when assessing lifecycle cost. [1, 2]
- This is a historical SIS mechanism, not a claim about current Windows support or generic close-call durability. [1]
Citations
- Bolosky, Corbin, Goebel and Douceur (2000). Single Instance Storage in Windows 2000, USENIX Windows Systems Symposium, pp. 13-24, sections 2.3.3-2.3.5.
- Zhan et al. (2020). How to Copy Files, FAST 2020, section 6: file or directory clones.
Further reading
Sources & caveats
- Historical source review of the SIS paper, especially sections 2.3.3-2.3.5, cross-checked against FAST 2020 section 6. No SIS workload or storage-failure experiment was run; current platform support was not assessed.