chattr in Linux can be used to set the append-only flag to files and directories. This corresponds to theO_APPEND flag inopen().[1]
NTFS ACL has a control for "Create Folders / Append Data", but it does not seem to keep data immutable.[2]
Manycloud storage providers provide the ability to limit access as append-only.[3] This feature is especially important to mitigate the risk ofdata loss forbackup policies in the event that the computer being backed-up becomes infected withransomware capable of deleting or encrypting the computer's backups.[4][5]
The prototypical append-only data structure is thelog file. Log-structured data structures found inLog-structured file systems and databases work in a similar way: every change (transaction) that happens to the data is logged by the program, and on retrieval the program must combine the pieces of data found in this log file.[9]Blockchains addcryptography to the logs so that every transaction is verifiable.
Append-only data structures may also be mandated by the hardware or software environment:
All objects are immutable inpurely functional programming languages, where every function is pure and global states do not exist.[10]
Flash storage cells can only be written to once before erasing. Erasing on a flash drive works on the level of pages which cover many cells at once, so each page is treated as an append-only set of cells until it fills up.[9][11]
Hard drives that useshingled magnetic recording cannot be written to randomly because writing on a track would clobber a neighboring, usually later, track. As a result, each "zone" on the drive is append-only.[12][6]
Append-only data structures grow over time, with more and more space dedicated to "stale" data found only in the history and more time wasted on parsing these data. A number of append-only systems implementrewriting (copyinggarbage collection), so that a new structure is created only containing the current version and optionally a few older ones.[7][13]
^Eugene Kolodenker; William Koch; Gianluca Stringhini; Manuel Egele (April 2017). "PayBreak: Defense Against Cryptographic Ransomware".Proceedings of the 2017 ACM on Asia Conference on Computer and Communications Security. pp. 599–611.doi:10.1145/3052973.3053035.Due to the threat of ransomware targeting the key vault, our implementation stores the harvested key material into an append-only file protected with Administrator privileges.
^"NVMe Zoned Namespace".ZonedStorage.io. Archived fromthe original on 2020-01-29. Retrieved2020-04-25.The internals of Solid State Drives are such that they implement a log-structured data structure, where data is written sequentially to the media.
^Brewer, Eric; Ying, Lawrence; Greenfield, Lawrence; Cypher, Robert; T'so, Theodore (2016)."Disks for Data Centers".Proceedings of USENIX FAST 2016.Because of the write restrictions imposed by SMR, when data is deleted, that deleted capacity can not be reused until the system copies the remaining live data in that SMR zone to another part of the disk, a form of garbage collection (GC).
