A RAM disk loads an entire disk image into host memory and presents it as an Avanti SCSI disk. It occupies one target on its SCSI controller but carries no per-device license-unit charge. Because its data is memory-resident while Avanti runs, it is useful for disposable data such as OpenVMS pagefiles or Tru64 UNIX swap.
The backing image must already exist, must not be empty, and must have a size that is an exact multiple of 512 bytes. Avanti must be able to read the complete image before the RAM disk starts. A size-query, open, failed-read, or short-read error aborts startup and leaves the backing file unchanged.
The Configuration Utility does not currently display or create RAM-disk targets. Add a ramdisk component to an unused disk<bus>.<target> entry by editing the configuration file while Avanti and the Configuration Utility are closed. The mandatory image property must name the fully specified backing-image path. Avanti also needs enough available host memory and commit capacity to allocate the complete image in addition to the configured guest memory.
When write_back=true, Avanti loads the backing image at startup and stages the complete image during a graceful stop or an explicit guest SYNCHRONIZE CACHE command. It replaces the backing file only after the staged write and flush succeed; an open, failed-write, short-write, or replacement error makes the save fail and leaves the previous backing file in place. An abnormal termination can lose changes that have not yet been written. When write_back=false, runtime changes are never persisted to the backing image, including on SYNCHRONIZE CACHE.
Optional properties are vendor, model_num, rev_num, and serial_num for the guest-visible SCSI identity; cdrom for read-only CD-ROM behavior; tcq and disconnect; and heads, cylinders, and sectors for an exact geometry override. Leave read_only=false; a non-CD-ROM RAM disk cannot use it as an independent read-only mode, while cdrom=true always forces runtime read-only behavior. When model_num is empty, Avanti selects its default vendor, model, and revision, replacing any separate vendor or rev_num; set vendor, model_num, and rev_num together when overriding the automatic identity. Heads and sectors may be 0 through 255; cylinders may be 0 through 65,535. A complete nonzero override is used only when heads * cylinders * sectors equals the image byte size divided by 512; otherwise Avanti derives the geometry. Leave all geometry values zero to use automatic geometry. Enable tagged command queueing only with disconnect enabled. The debug property is reserved for a purpose-built diagnostic version and has no effect in an ordinary Avanti build.
If set, serial_num may be at most 255 bytes.
A RAM disk is not equivalent to durable storage. Keep authoritative data on storage with an appropriate independent backup and recovery plan.
