z/OS performs data erasure by telling the control unit to erase
tracks. That results in the track having only an R0 record, and the
rest of the track is overwritten with binary zeros. No program that
uses normal CCW interfaces to the DASD can then read anything on the
track.
Writing an EOF is different. That replaces just one block on the
track, leaving the rest of the data (in the subsequent blocks)
intact. ZAP and other programs can read the rest of the data in that
case, but cannot read the original block that you overwrote.
Sorry, Walt, you are wrong.
The effect of the ERASE commands varies by vendor. Some vendors do
overwrite the track with zeros or other values, some do not. However,
the logical effect is the same in all cases; the original data on the
track cannot be accessed from the mainframe host (CKD commands).
Writes come in 2 flavors. UPDATE writes simply update the contents of
an existing block. But FORMAT writes are used to write new blocks, and
must be use to change the length of existing blocks (actually by
overwriting them). So writing an EOF is a format write. Format writes
always erase the rest of the track, which will make all previous blocks
unavailable. Again, it varies by vendor as to whether the data is
physically ovewritten, but the old records are NOT retrieveable by ZAP
or any other program.
--
Bruce A. Black
Senior Software Developer for FDR
Innovation Data Processing 973-890-7300
personal: [EMAIL PROTECTED]
sales info: [EMAIL PROTECTED]
tech support: [EMAIL PROTECTED]
web: www.innovationdp.fdr.com
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