Ron Hawkins wrote:
Brian,
We are quoting the same reference - your quote is directly below mine.
IBM are putting the cost of space as a priority, whereas Mainstar are
putting performance. It really depends on just how busy the MCDS is.
How long does a CA split take? It depends on the CI size, but it's about
30-100 times longer than an insert without a CA split. You'll be read and
rewriting those CIs one at a time.
Ron
Even with a large number of buffers to improve physical I/O efficiency,
the overhead of a CA-split is significant. This is a good argument for
allocating enough CA free space to reduce likelihood of a CA-split,
especially if consistent performance after a reorganization is a
concern. It is not a valid argument for allocating CI free space,
because a CI split (with a reasonable, non-default number of buffers) is
cheap. Also CI free space can actually cost you performance by
spreading active data over more CIs, requiring more physical reads,
buffers, and more channel time if large numbers of records are read. CA
free space costs you nothing but disk space - the reserved CIs contain
no data and never have to be read until they are actually used in
performing a CI split.
If disk space is your major concern, then leave no free space when the
file is reorganized. (This is of course also optimum for performance if
the file won't be updated, or if updates can be batched while the file
is "off line" and it can be reorganized right after updates are made)
If performance of a frequently-updated, on-line KSDS file is your major
concern, I would recommend experimenting with various values of CA free
space to keep CA splits low, but leaving no CI free space. Allow those
areas of the file undergoing inserts to do CI splits as necessary,
producing underutilized CIs; then these updated areas of the file will
utilize the available CA free space, making it unlikely that a CI split
will force a high-overhead CA split. That way only those portions of
the file that have to be updated will see degraded I/O throughput from
having fewer records per CI, rather than incurring that overhead on all
CIs by leaving CI free space.
-----Original Message-----
From: IBM Mainframe Discussion List [mailto:[EMAIL PROTECTED]
Behalf Of FRASER, Brian
Sent: Thursday, 8 February 2007 12:41 PM
To: [email protected]
Subject: Re: FIXCDS to delete records from mcds
What IBM actually say is:
" IBM also recommends that installations define their CDSs with
FREESPACE(0) so VSAM can figure out where space is needed and perform the
necessary CI/CA splits when doing inserts.
This will create space where needed and eliminate the need for the extra
space that would be required if the CDSs were defined with FREESPACE(50
50)"
--
Joel C. Ewing, Fort Smith, AR [EMAIL PROTECTED]
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