Data restore

[Uwe Zessin]

/INITIALIZE allows BACKUP to change some volume characteristics. The rest of the restore process is the same.

The problem is that a lot of meta data needs to be written on a restore and that is a synchronous operation - I doubt that BACKUP does any optimizations. Also, you cannot optimize any head movements.

Show us how to tune a restore!!

[Wim Van den Wyngaert]

Initialize your tape with the highest density possible for your drive.
Use /group=0 to reduce the overhead by 10%.

Wim

[Antoniov.]

Hi Lionel,
assuming you make restore with default /INIT option; because of this you have to work in single user mode.
Assuming no other users/processes are running while you make the restore.
Assuming in your saveset there are many sequential files.

You can temporary trick your system to avoid multiple i/o on disk.
You can turn on deferred write with
$ SET VOL DKD105: /NOWRITE
Also you can increase multiblock count (it works fine only on sequential files)
$ SHOW RMS !--read current value
$ SET RMS /BUFF=128 !-- set new value
Also you can increase two system parameters ACP_DINDXCACHE and ACP_DIRCACHE. They are both dynamic.
It is very importart you restore all modified parameters after you terminate saveset restore.

Antonio Vigliotti

[Ian Miller.]

as BACKUP creates files using XQP calls rather than RMS I don't think changing RMS parameters will help.

[Hein van den Heuvel]

Ian> as BACKUP creates files using XQP calls rather than RMS I don't think changing RMS parameters will help.

And that would be for non-image restores.

For /IMAGE restores, as discussed here, the target device is mounted /FOREIGN and it is all Backup's doing. No RMS, no XQP. Backup knows and executed the ODS layout with logical IOs, not virtual IOs.

I discussed this some with Mark H. We believe image restore does do the bitmap in a single IO, but the indexf.sys in a single block IO per file. Furthermore it is all synchroneous, in line = slow. This in contrast to the backup phase which will do concurrent IO. This could obviously be improved upon, but over the past decades there has been no business justification for that.

Hein.

[Jan van den Ende]

Uwe,


A backup design usually includes the RTO ;-)


Yes, of course. And Management should have a written, advance, realistic estimate.

But, in my experience (scarse on VMS, but witnessed from the side-line on *UX and Billyware all too frequently) the DECISION to restore always took longer than the actual resore itself.

And yes, the discussion always starts with ANY time being too long. But in the balancing between restore duration, restore reliability and backup duration usually restore duration looses priority.
Much more can be gained by a real rigid (and rehearsed) procedure for the restore-decision process.

YMMV !!

Proost.

Have one on me.

Jan

[Antoniov.]

I understand RTO means Run-Time Optimization and in my mind is stupefyingly backup takes 70 min., writing to slow device (tape), while restore takes 210 min., writing to quick device (hard disk).
I know restore takes more time than backup in vms, but the rate 3:1 seems to me too high.

Antonio Vigliotti

[Uwe Zessin]

We're talking about a backup solution,
so RTO = Recovery Time Objective.
It means that the customer specifies the time in which the data must be available again and the backup solution is planned accordingly (or the RTO must be relaxed for reasons like not enough money or other constraints ;-)


Antonio,
haven't you followed recent discussions here?

*** Modern tape drives are not slow ! ***

A tape media is just moving forward (a LTO-3 at up to 80 MegaBytes/second, uncompressed). A disk drive must move its arm to the correct cylinder and then wait until the right sector passes the head.

Re-read Hein's excellent descriptions about why a restore is limited to lots of small, synchronous I/Os, which will make the restore slow.

[Antoniov.]

Uwe,
thank you for RTO meaning.
I'm with you about new quick tape devices but, in this case, tape throughtput is less than 0,285 Mbs per minute (20Gb/70min) so I believe it is a DAT tape, no some device like DLT.
I'm still convinced the rate 3:1 is unreasonable, in this case.

Antonio Vigliotti

[Uwe Zessin]

What is "0,285 Mbs" supposed to mean? As far as I can tell, a small "b" usually indicates "Bit".

I guess we need to compare out maths:
20,000,000,000 / (70*60) = 4,761,904

Thats 4.7 MegaBytes/second to me.