Write Once Read Many

In elettronica l'espressione Write Once Read Many (WORM, indicata a volte come Write One Read Multiple o Write Once Read Mostly[1], lett. "scrivi una volta leggi molte") si riferisce a un tipo di memoria su cui si può scrivere soltanto una volta, ma che può essere letto diverse volte. L'utilità di tali sistemi è nell'archiviazione a lungo termine di dati fissi.

Ci sono due tipi di dispositivi di memorizzazione WORM[1]:

  • WORM fisici, su cui si può scrivere fisicamente solo una volta per le loro caratteristiche costruttive (supporti come il CD-R e il DVD-R, o circuiti elettronici come le PROM);
  • WORM logici, che forniscono capacità WORM tramite l'utilizzo di chiavi elettroniche o di altri sistemi per impedire la riscrittura, anche su supporti dove la riscrittura normalmente sarebbe possibile. La ragione dietro a questa limitazione, in quest'ultimo caso, sta nella necessità, ad esempio su richiesta delle autorità, di standard di memorizzazione mediante i quali le informazioni possano essere mantenute in sicurezza e rese disponibili per un lungo periodo di tempo.

Dispositivi che limitano intenzionalmente la scrittura a un'unica istanza includono: Super DLT (supporti Super DLT II sono usati per operazioni normali e WORM, a seconda della richiesta dell'utente), Linear Tape-Open, AIT e diversi dischi rigidi. La protezione mediante software è meno sicura, ed è possibile, in alcuni casi, riutilizzare supporti così protetti.

Unità di memoria di massa WORM fisiche per personal computer erano disponibili già prima dell'introduzione dei CD-R, ed erano viste come una tecnologia alternativa e contrapposta ai lettori di CD-ROM non scrivibili. Utilizzavano sempre dischi ottici di vari tipi come supporto, ma protetti da una cartuccia di plastica, anche a doppia faccia, e molto più costosi dei CD. Nel 1988 i produttori più noti di periferiche di questo tipo erano Maxtor, Optotech, Panasonic e Laser Magnetic Storage (una joint venture di Philips e Control Data), ciascuno con tecniche di incisione laser differenti; i primi tre utilizzavano cartucce ottiche da 5,25", LMS cartucce ottiche da 12". Le capacità andavano da 200 MB a 1 GB e oltre per facciata.[2][3][4]

Note

  1. ^ a b (EN) Write Once Read Many, in Encyclopedia of Database Systems, Springer, 2009.
  2. ^ Worm disk drives Maxtor RXT-800S e Optotech 5984 (JPG), in MCmicrocomputer, n. 75, Roma, Technimedia, giugno 1988, pp. 74-78, ISSN 1123-2714 (WC · ACNP).
  3. ^ Worming inside... (JPG), in MCmicrocomputer, n. 77, Roma, Technimedia, settembre 1988, pp. 148-150, ISSN 1123-2714 (WC · ACNP).
  4. ^ (EN) Panasonic Optical Disk Conversions, su disktransfer.co.uk.

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