Is it okay to run I/O Express while I am doing a backup?

I/O Express can be actively caching while a backup is running. However, there are some things you should keep in mind when running I/O Express and Backup simultaneously.

As you know, I/O Express uses physical memory from the Free List that is not needed by any other process. Some major changes were made to the Backup utility in VMS V5.2 and above. These changes improved the speed of Backup, but also required the use of more physical memory than previously. Therefore, with I/O Express and Backup running at the same time, you wll have two applications that use a large portion of physical memory. Each process utilizes memory in a different way.

I/O Express

I/O Express adjusts its use of memory to the current needs of the system. It allocates pages from the free list only if the number of pages on the free list is above the amount specified by I/O Express. When the size of the free list drops below the specified size, I/O Express will purge its cache and release that memory for any other process on the system that might need it. I/O Express checks for a memory crisis every five seconds and as soon as one is detected, returns pages of memory back to the operating system.

Backup

Most system managers complain about how slow Backup is. To address this problem, Digital changed Backup in VMS v5.2. With the new Backup, Digital recommends that several SYSUAF parameters be set higher than a regular user would normally require. Therefore, in VMS v5.2 and above, Backup now uses physical memory to create a large buffer, or working set, to store the files being backed up. Backup will store the information from each file fragment into the buffer and then, when the buffer is full, all the data is written to the backup media. Using such a large chunk of memory is not normally a problem because backups are usually run at night when there are few users on the system and memory is plentiful.

I/O Express improves performance in repetitive read situations; Backup is a sequential read operation. Therefore, I/O Express will not improve the speed of your backup. However, if you have users accessing the disk while your backup is running, they may receive a benefit from caching. The amount of benefit that users receive from I/O Express while Backup is running will depend upon the type of reads being done and how much memory is available for the cache. If your users log off just before Backup begins, I/O Express will begin flushing its cache while Backup is extending its working set. This situation might cause a temporary slowdown on your system.

See also the FAQ Does I/O Express speed up Backup?

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