DB2 Space calculation – from pages to tracks

DB2 Space calculation – from pages to tracks

All system z DASD I/O systems base on the IBM DASD device type IBM 3390 Model 3 and are compatible to it. This means that all calculations a DB2 DBA or sysadm can to do are on basis of this type and model characteristics. Quite typical for this DASD model: 12 pages of DB2 data fit into 1 track. For older types, such as IBM 3380, which are not considered here, only 10 pages to fit into a track. More about DB2 data and storage, see below.

IBM Type 3390 Model 3

  • 56664 bytes per track
  • 849960 bytes per cylinder
  • 15 tracks per cylinder
  • 3339 cylinders per device

3339 cylinders
x 15 trk/cyl
= 50100 tracks/device

2838 MB/Device

approx. 2.75 GB

DB2 space calcalution on basis of device type 3390

smallest unit DB2 can externalize:
1 page with 4 KB,
 which is 4096 bytes

1 track =
56664 bytes

overhead per block of records stored
= 340 Bytes

56664 /
(4096 + 340) = 12.773

which is 12 pages per track

12 pages = 12 x 4 KB = 48 KB

net user data = 48 KB
which claim 55 KB space on DASD device

DB2 data + 15% overhead
= total usage of DASD storag

1 cylinder
= 15 rracks x 12 pages
= 180 pages
= 720 KB

1 Device
= 50100 tracks
= 601200 pages
= 2.4 GB DB2 net user data

/ DB2 for z/OS

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