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jonna
New User
Joined: 18 Apr 2005
Posts: 9
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HI,
If I will male BLKSIZE is ZERO in JCL for perticular dataset will it loose any data on output file.
Thanks,
Mahee |
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dick scherrer
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Joined: 23 Nov 2006
Posts: 19254
Location: Inside the Matrix
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Hello,
No. But why would you ask? Has something strange happened? |
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jonna
New User
Joined: 18 Apr 2005
Posts: 9
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| dick scherrer wrote: |
Hello,
No. But why would you ask? Has something strange happened? |
We are planing some performance improvements from my existing datasets which having BLKSIZE greater than zero. If you will change BLKSIZE zero it will take less CPU time. Just I need to confirm if I will do this change it will cause any problem like data loss in output datasets. Thanks for your reply. |
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gcicchet
Senior Member
Joined: 28 Jul 2006
Posts: 1703
Location: Australia
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Hi,
if data is lost it will definitely run quicker. 
Gerry |
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dick scherrer
Site Director
Joined: 23 Nov 2006
Posts: 19254
Location: Inside the Matrix
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Hello,
| Quote: |
| If you will change BLKSIZE zero it will take less CPU time. |
What source provided this information?
| Quote: |
| if data is lost it will definitely run quicker. |
That's just not right. . .  |
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Garry Carroll
Senior Member
Joined: 08 May 2006
Posts: 1069
Location: Dublin, Ireland
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| Quote: |
| If you will change BLKSIZE zero it will take less CPU time. |
It's more likely to reduce elapsed time by reducing the number of EXCPs.
| Quote: |
Quote:
if data is lost it will definitely run quicker.
That's just not right. . . icon_wink.gif |
I agree. The process of losing data (which I doubt would happen) can take just as long.
Garry  |
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Robert Sample
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Joined: 06 Jun 2008
Posts: 8547
Location: Dubuque, Iowa, USA
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| Quote: |
| We are planing some performance improvements from my existing datasets which having BLKSIZE greater than zero. If you will change BLKSIZE zero it will take less CPU time. Just I need to confirm if I will do this change it will cause any problem like data loss in output datasets. Thanks for your reply. |
Performance improvements will depend on what your current block size is. SMS-controlled datasets will use half-track blocking when BLKSIZE=0 is used in JCL. As long as the program can handle this (COBOL needs BLOCK CONTAINS 0 RECORDS, for example), there's no difference to the system whether BLKSIZE=????? or BLKSIZE=0 is coded. If the block size is already half-track blocking, you won't see any performance advantage to BLKSIZE=0. And unless the program expects a specific block size you won't lose any data no matter what block size is used.
What's the buffers set to (DCB=BUFNO=??)? In my experience, buffering has a much bigger impact on performance than block size (other than one record per block), and the default QSAM 5 buffers provides really bad performance. BUFNO=30 is typically a decent number and the memory increase usually doesn't impact the program. |
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Pedro
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Joined: 01 Sep 2006
Posts: 2185
Location: Silicon Valley
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None of you made his point clear:
blksize=0 does not mean the blocksize will be zero. It tells the system to determine what the best blocksize is and use that. After the job runs, the dataset will have a proper blocksize. |
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Anuj Dhawan
Senior Member
Joined: 22 Apr 2006
Posts: 6254
Location: Mumbai, India
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Hi,
Said bit differently..
BLKSIZE=0 can often cause problems if the dataset is not opened and closed unless your SMS environment has been correctly established.
If it has not, then the BLKSIZE=0 will be honoured and the dataset will have that attribute, which makes the dataset invalid for processing by DFHSM, or whatever other ILM software you use.
The correct coding is RECFM=xx,LRECL=nnn. |
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Anuj Dhawan
Senior Member
Joined: 22 Apr 2006
Posts: 6254
Location: Mumbai, India
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Hi,
| jonna wrote: |
| We are planing some performance improvements from my existing datasets which having BLKSIZE greater than zero. If you will change BLKSIZE zero it will take less CPU time. Just I need to confirm if I will do this change it will cause any problem like data loss in output datasets. |
Check this article, might provide you some insght regarding the BLKSIZE
http://esj.com/article.aspx?ID=5220025647PM |
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expat
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Joined: 14 Mar 2007
Posts: 8794
Location: Welsh Wales
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Mmmmmmmmmmmmmm, BUFNO, with the modern DASD subsystems of today the BUFNO parameter is not really that useful, asuming that the DASD subsystems are set up correctly.
Using FASTWRITE and the DASD subsystem cache efficiently then the majority of the work will be handled by the subsystem itself ratherthan by z/OS. |
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Garry Carroll
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Joined: 08 May 2006
Posts: 1069
Location: Dublin, Ireland
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| Quote: |
Mmmmmmmmmmmmmm, BUFNO, with the modern DASD subsystems of today the BUFNO parameter is not really that useful, asuming that the DASD subsystems are set up correctly.
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I would have thought that BUFNO can be quite useful where the BLKSIZE is less than optimum as it can facilitate optimization of the dataflow on the channel(s). Where, say, BLKSIZE is half the optimum size, doubling the BUFNO should result in the movement of that amount of data in a single operation as would have been moved had the BLKSIZE been optimized.
Garry. |
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expat
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Joined: 14 Mar 2007
Posts: 8794
Location: Welsh Wales
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| Garry Carroll wrote: |
| Quote: |
Mmmmmmmmmmmmmm, BUFNO, with the modern DASD subsystems of today the BUFNO parameter is not really that useful, asuming that the DASD subsystems are set up correctly.
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I would have thought that BUFNO can be quite useful where the BLKSIZE is less than optimum as it can facilitate optimization of the dataflow on the channel(s). Where, say, BLKSIZE is half the optimum size, doubling the BUFNO should result in the movement of that amount of data in a single operation as would have been moved had the BLKSIZE been optimized.
Garry. |
Yeah, silly me .............. I had made the rather rash assumption that all of your above points would have been done anyway. I have found that the default of BUFNO=5 hasn't caused me any problems yet  |
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revathitcs
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Joined: 09 Jul 2008
Posts: 1
Location: chennai
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| BLKSIZE =0 |
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dick scherrer
Site Director
Joined: 23 Nov 2006
Posts: 19254
Location: Inside the Matrix
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Hello revathitcs and welcome to the forums,
Is your post a question or are you offering some kind of information?
When you post, you need to use complete sentences. . . |
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Anuj Dhawan
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Joined: 22 Apr 2006
Posts: 6254
Location: Mumbai, India
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| revathitcs wrote: |
| BLKSIZE =0 |
And you win the smallest answer of the month award... |
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Guru Bob
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Joined: 31 Jan 2008
Posts: 21
Location: Malaysia
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My understanding is that 240K (old channel data xfer size if not FICON or ESCON) is the optimum BUFFEr size required to cause no delay in READ/WRITEs for QSAM files.
240K is for a 3390 is 9 buffers at the maximum optimum BLKSize of 27998. Hence coding BUFNO=10 for any files with >24,000 bytes for a BLKSIZE will be cool. I believe that hence allocating twice the number of buffers from the 240K size will allocate you two sets of buffers one seat being read/written and the other still haveing records read or written to them simultaneoulsy.
WIth FICON and SANs etc I am not sure how tru this is now.
DATA (QSAM ) buffers reside for COBOL program where ever you have specified your DATA(31) above the line or DATA(24) below the line (hence a severe limit on the number of BUFNO values (S80A,S878 abends) ). AMODE and RMODE of the program also influence this. |
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Terry Heinze
JCL Moderator
Joined: 14 Jul 2008
Posts: 1249
Location: Richfield, MN, USA
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| Agree with expat. I also thought increasing BUFNO would be beneficial, but notso according to a performance analyst I once talked to. Increasing BUFNO will not decrease EXCPs which is the elapsed-time-hogger. BLKSIZE has much more of an impact on reducing EXCPs, and therefore elapsed time, than BUFNO. You can see this for yourself by testing with various combinations of both. Also, half-track blocking is usually what SDB picks, but not always. The bigger the LRECL, the better the chances are that something like 1/3-track blocking might be chosen. |
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Suresh Shankarakrishnan
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Joined: 11 Jul 2008
Posts: 42
Location: USA
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http://www.ibmmainframes.com/about22889.html
Although the above link states 27998, we use 27997 to get the block size of a dataset.
Example , lrecl = 133.
27997/133 = 210 (get the integer value by truncating the decimal point, not rounding off)
210 * 133 = 27930
This relates specifically to creating blocksize, not sure if this helps though. |
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gcicchet
Senior Member
Joined: 28 Jul 2006
Posts: 1703
Location: Australia
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Hi,
may I ask why you are using 27997 and not 27998.
If you have a file with LRECL=13999 RECFM=FB you would be wasting a fair bit of space.
Blocksize of 27997 would give you 3 records per track
Blocksize of 27998 would give you 4 records per track.
I picked LRECL of 13999 as an example as it is half of 27998.
Gerry |
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