Memory Disk - Concept ?

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Memory Disk - Concept ?

Post by blackcat » Wed Feb 28, 2024 3:48 am

Recently during an installation I was confronted with the following output:
There is not enough space on the memory disk.
You must take these steps now to complete this installation:

- Reboot the system
Duplicate entry for <SYS$LDR>TR$DEBUG.EXE is being ignored

Created memory disk WEB03X$DKA0:[VMS$COMMON.SYS$LDR]SYS$MD.DSK;3
- using 221568 blocks in 1 extent with 25631 spare blocks
- mounted on WEB03X$LDM1550: with volume label MD240598807C
- contains OpenVMS V9.2-1

Can someone explain the concept of a memory disk and why OpenVMS/x86 needs/uses this.

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Re: Memory Disk - Concept ?

Post by m_detommaso » Wed Feb 28, 2024 4:03 am

Refer to the manual "VSI OpenVMS x86-64 Boot Manager User Guide", in particular, chapter 2 : ... de-922.pdf

- A brief description of Memory Disk -

Memory Disk
Unlike prior releases of VSI OpenVMS for Alpha, VAX, or Integrity systems, all of the files that comprise the core VSI OpenVMS kernel are pre-packaged into a Logical Disk (LD) container file, referred to as the MemoryDisk. Whether booting from a local drive or booting an installation kit over a network, the MemoryDisk is loaded into memory by the VSI OpenVMS Boot Manager using UEFI physical Block IO drivers. This greatly simplifies and speeds up the boot process while eliminating the need for VSI OpenVMS to provide Boot Drivers for every supported type of boot device. During installation, the MemoryDisk will contain the full ISO image, regardless of whether it is downloaded from a web server or loaded from a virtual or physical DVD. After installation, on subsequent boots, the MemoryDisk contains only the minimal kernel files that are required to achieve the ability to switch to runtime drivers.

Dump Kernel
During boot, after the Boot Manager loads the MemoryDisk into memory and initiates boot of the primary kernel, the MemoryDisk remains memory-resident. In the event of a primary kernel shutdown or crash, the same MemoryDisk is booted a second time into separate memory space, forming what is known as the dump kernel. The dump kernel is a limited-functionality VSI OpenVMS instance which processes a crash dump using runtime drivers and upon completion, initiates a system reset. Because the MemoryDisk is already resident in memory, the dump kernel boots nearly instantly and processes the crash data much faster than prior implementations that required primitive device drivers.

Boot Blocks
Because the embedded MemoryDisk is a separately bootable entity from the system disk, VSI OpenVMS V9.2-2 supports additional “inner” boot blocks around the MemoryDisk itself. When booting a system disk, the “outer” boot blocks are used to locate these “inner” boot blocks. For the most part, this added complexity is transparent to the user. Related utilities such as BACKUP, PCSI, SYSGEN, etc. have been modified to maintain the integrity and security of the embedded MemoryDisk. The use of symlinks allows access to MemoryDisk-resident files from system components.

More info at chapter 2.3.1


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