Hi Radek,
You said: "... which is usually close to half of the track (sometimes
third...) ..."
When is SDB 1/3 Track?
Thanks and regards,
David
On 2021-03-02 07:25, Radoslaw Skorupka wrote:
General comment about LBI, large blocks and performance:
1. Blocksize for tapes, especially for real tapes is very important
for performance. LBI is something to legalize previously used large
blocks - I mean blocks larger than officially supported 32760. Even
3490E tapes supported larger blocks and it was used by ADRDSSU (read
HSM backups). AFAIK LBI was proviced with OS/390 V2R10, long time
after MAGSTAR family GA.
2. Blocksize on DASD *does affect performance*, but it is not so
strong as with tapes. More: on tape there is no big reason not to use
large blocks, but on DASD the blocksize is strictly related to track
size. For 3390 blocksize of, let's imagine 160kB has no sense, because
block has to fit in track. And even 32760 is not very reasonable
(assuming same consecutive blocks), because waste of space. Yes, it
would be nice to have blocksize limit at 56664B, but there is no such
option (we talk about z/OS, not z/VSE). Optimal blocksize is SDB,
which is usually close to half of the track (sometimes third...).
3. However "big enough" blocksize is good enough! In other words you
will not observe big performance difference between BLKSIZE=32000 and
BLKSIZE=8000. On DASD, of course.
4. So, in this case, hypothetical LBI=track would not provide
important relief. That could mean maybe there are other ways to skin a
cat. I don't know the application, but some ideas to consider:
- use VSAM, SMB, BLSR, etc.
- use index if applicable
- use LDS and DIV
- use DB2 table
My €0,02
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