W dniu 2013-09-12 16:13, Chip Grantham pisze:
I've finally taken the time to try to understand the numbers behind the way EAVs were
implemented. I found a great discussion in the redbook "z/OS v1.12
Implimentation" SG24-7853-00 manual, chapter 20. Any time spend you happen to
spend here is worth it. (not unlike all redbooks). Thanks to those that wrote it.
I did happen into a segment that makes me go hmmm. 20.4.3 Multicylinder unit
section says the 21-cylinder value for the MCU is derived from being the
smallest unit that can map out the largest possible EAV and stay within the
index architecture (with a block size of 8192), as follows:
* It is also a value that divides evenly into the 1 GB storage segments of an
IBM DS8000,
* These 1 GB segments are the allocation unit in the IBM DS8000 and are
equivalent to 1,113 cylinders.
I'm sure the "index architecture" references the index vtoc architecture, which
has always been a curious archeture to me. Has this design ever been made open? Just
curious as to why it made 21 the magic number?
I also ran into a math issue when I divided 21 into 1GB (or 1,073,741,824/21 =
51,130,563.0476...). I suspect that's because the 1GB storage segment is a
number used in the DS8000 degisn, and its really close to the 1GB value.
Wondering if that's true or some other reason.
Just makes me go hmmm.
Not mentioned above, but IMHO important for MVS: CA size.
The smallest portion of data (known as "cluster size" in distributed
systems) is CA. VSAM dataset consist of natural number (1,2,3,4,5,..) of
CA's.
Before EAV CA size was any number of tracks within single cylinder - for
3390 it was 1,2,3,4,...15. (fine print: I excluded striping from the
consideration).
Before EAV the allocation was made in tracks, so even the smallest CA
was equal chunk of allocation (or multiplicity).
Now ther is a need to use bigger chunks. What numer to choose? 10 cyl?
100 cyl? 53 cyl? 19 cyl?
What criteria to use?
It would be good the CA size is multiplicity of chunk size, with no
remainder.
21 cyl means 315 trk, it yields quite lot of previous CA sizes: 1,3,7,9,
15. The largest one (15) is the most important here, because it is
suggested CA size.
Of course 210 cyl also allows for the above CA sizes, but the goal is
also to have the chunk size not to large.
--
Radoslaw Skorupka
Lodz, Poland
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