Operating System - OpenVMS
cancel
Showing results for 
Search instead for 
Did you mean: 

How to speed up a shadow copy on an idle system?

 
Clark Powell
Frequent Advisor

How to speed up a shadow copy on an idle system?

I'm doing a shadow copy between two (eva5000) disks on an idle OpenVMS 8.3 system, (ES45.) The copy is going very slowly. I am happy that this process is not fast on a busy system but this system has only to do this shadow copy and nothing else. The disk IO is about 1/4 of what I see during a backup. Is there a way to speed this up?
7 REPLIES 7
Jim_McKinney
Honored Contributor

Re: How to speed up a shadow copy on an idle system?

The system logical name

SHAD$COPY_BUFFER_SIZE

controls the amount of data that can be written in a single IO by the server during a copy operation. I once read that 112 was the optimal value for EVAs though I have not done any testing with this. Values from 31 to 127 are valid. My understanding is that the default is 127 when the logical is not defined.

$ DEFINE/SYSTEM SHAD$COPY_BUFFER_SIZE 112
Robert Walker_8
Valued Contributor

Re: How to speed up a shadow copy on an idle system?

Gday,

Dont have experience with EVAs but wouldnt they have some mirroring capability between them then using OpenVMS to shadow copy the array.

Wouldnt a snapshot and a copy be faster in a quiet period or do you have to ensure updates are passed on? We sometimes killed shadow merges as they were woeful compared with shadow copies - but this was on HSJ based equip - many years ago.

Basically shadowing is a background task - unlike Smart Arrays and likely EVAs which one can set rebuild priorities to high. I dont believe there is any priority stuff with Host Based Shadowing - certainly not in sysgen anyway.

Check out http://www2.openvms.org/kparris/s2003_volshad_perf.ppt#950,18,Speeding Shadow Copies it indicates a backup/physical and shadow is faster than shadow copy.

Robert.
Volker Halle
Honored Contributor

Re: How to speed up a shadow copy on an idle system?

Clark,

if you look at page 17 of the Keith Parris presentation (see link above), you'll find that a full shadow-copy with different data on the 2 disks needs to do lots of IOs - muc more than a BACKUP/PHYSICAL.

Volker.
Paul Beaudoin
Regular Advisor

Re: How to speed up a shadow copy on an idle system?

Clark

We have used these logicals in the past to control exactly this:

SHAD$MERGE_DELAY_FACTOR ! Specifies the relative delay in shadow merges from 100 (slow merge) - 1000 (fast merge) (200 default)

SHAD$MERGE_DELAY_FACTOR_DSA* ! Sets SHAD$MERGE_DELAY_FACTOR for a specific disk

Full disclosure: I've not used this in some years so canot guarantee they work now as in VMS 7.x.

Paul
Volker Halle
Honored Contributor

Re: How to speed up a shadow copy on an idle system?

Paul,

as the logical name says, this logical only affects MERGEs, not COPIEs.

The SHADOW_COPY operation is believed to have a higher priority than merges, so these logicals were invented to somehow limit the performance impact of merges.

Volker.
Paul Beaudoin
Regular Advisor

Re: How to speed up a shadow copy on an idle system?

Volker,

I wasn't aware of that ...
Thanks - I am now a bit better educated!

Paul
Andy Bustamante
Honored Contributor

Re: How to speed up a shadow copy on an idle system?

See $HELP INIT/SHADOW for one option you might consider if this is a new shadow set.

As Volker points out, you can also use $BACKUP/PHYSICAL to cut down the first shadow copy time. Be sure to mount the target disk /over=shadow after the backup to avoid problems.

Andy Bustamante
If you don't have time to do it right, when will you have time to do it over? Reach me at first_name + "." + last_name at sysmanager net