Relationship between BACKUP qualifiers, /CRC and /GROUP

[Tom O'Toole]

There is a thread where this has been discussed at great length, including the latest recommended values - it's not clear that they are absolutely correct:

http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=658938

[comarow]

Here's what is actually in the documentation.


Table 4 BACKUP Qualifiers That Affect Performance Qualifier Description
/BLOCKSIZE
To write a save set to a tape device, always use /BLOCKSIZE = 65,024, the largest block size you can use with save sets on tape.

Note: To be able to copy save sets from tape to disk, use a maximum block size of 32,768.

/GROUP
Today's tape and disk drive technology makes the XOR group feature of BACKUP obsolete. Use /GROUP=0. (If you do not specify the /GROUP qualifier, the default of 10 is used, which adds 10% more data to a save set).

/CRC
Keep the default of /CRC, which does not add extra data to the save set. A 32-bit field is always reserved in a save set whether you enable CRC or not. However, adding a small amount of CPU time helps to ensure the integrity of a backup save set.

[Ian Miller.]

I belive a slightly smaller block size (multiple of 2k or 4k - I can't remember) is recommended when using FC storage.

The big change in the new recommendations is to have a smaller DIOLM when using modern storage.

[comarow]

Absolutely correct. A high diolm causes the cache in the San controllers to thrash.

However, engineering is aware of the problem.

While interactive jobs require a diolm of 100, if you have batch jobs, not just backup, but other i/o intensive operation try reducing your diolm to 32 or even less.

Look at the accounting report for results.

Bob

[Wim Van den Wyngaert]

Can someone of HP confirm that all SCSI DLT drives have some kind of raid protection ?
E.g. TZ8* ?

Wim

[comarow]

A customer of mine did some testing with and without CRC testing.



He did do the test with the TL891 tape drives



Backing up a 20,000,006 block file (512 Byte blocks) I got the following results



Blocksize = 32256

WSQuota = 16384

Compression - off



CRC GROUP Elapsed (seconds)

Y 10 2328

Y 10 2305

Y 10 2407



Y 0 2205

Y 0 2172

Y 0 2172



N 10 2370

N 10 2399

N 10 2396



N 0 2202

N 0 2168

N 0 2167



The biggest gain is from turning off Group (i.e. group=0). This causes less tape to be written.

I guess that puts the issue to bed!

Bob Comarow