topic Re: Backup /FAST in Operating System - OpenVMS

Backup /FAST

Wim Van den Wyngaert — Thu, 04 Aug 2005 04:44:30 GMT

I was thinking of using /FAST as a backup qualifier for image backups and "almost all files on a disk" backups. Does anyone have real life test experience of how much performance benefit this would give ? Is streaming mode improved by it ?

Wim

Re: Backup /FAST

Marc Van den Broeck — Thu, 04 Aug 2005 05:02:03 GMT

Wim,

if you backup "almost all files on disk" it will not improve performance. The qualifier was intended to be used when backing up "only a small selection of files on disk".

Rgds
Marc

Re: Backup /FAST

Wim Van den Wyngaert — Thu, 04 Aug 2005 05:09:55 GMT

Marc,

OK, I know this but since the scan is done before the backup, the backup itself could go faster (don't know how selection works without /fast) and the drive could stay in stream mode. Just wanted to avoid real testing.

Wim

Re: Backup /FAST

John Gillings — Thu, 04 Aug 2005 08:08:00 GMT

Wim,

This is a classic YMMV (Your Mileage May Vary). Any benefits will depend very much on the exact circumstances.

If you want a guess, I'd say it's unlikely to make much difference with modern storage and tape drives. With fast disks, smart tape drives and caches all over the place, I'd be surprised if you could find a case that consistently gives a significant improvement with /FAST, ESPECIALLY if your input file set is, as you say "almost all files on a disk".

For the very fastest performance use a BACKUP/PHYSICAL, but you sacrifice the ability to do selective restores.

Re: Backup /FAST

Wim Van den Wyngaert — Thu, 04 Aug 2005 09:19:18 GMT

Did some testing on a 6.2 machine.

/FAST always increased the DIO count of doing an operation.

An image backup went slightly slower with /fast.

A normal all files backup went slightly faster.

A selective backup (70% of the files) went slightly slower.

But the differences are very low (1-2%), so I guess that whatever the millage, it is antenna tuning that wont speed up things.

So, I will forget the option exists.

Wim