Although FLEX-ES systems can be used to run standalone utilities (those which do not require a mainframe operating system), most installations will use FLEX-ES to bring up one or more mainframe operating systems. These operating systems typically require large amounts of disk space. There is a direct correspondence between the size of an emulated mainframe DASD and the amount of server disk space required to emulate that DASD.
Disk space on a node's (FLEX-ES host computer's) primary server hard disk must be allocated at the time the node's server OS is installed. Disk space on subsequent hard disks may be allocated when that disk is added to the server.
Thus, in planning your installation it is necessary to determine your disk space requirements. To do this, you should make the following calculations before attempting any installation of the server OS or of FLEX-ES.
There are two ways to look at mainframe DASD. From a programmer's point of view, DASD are seen as one or more addressable units. Each of these addressable units counts as a single disk. From a hardware installer's or field engineer's point of view, DASD are seen as physical units (which may contain several addressable units).
FLEX-ES allocates storage in terms of addressable units (that is, it employs the programmer's point of view as opposed to the hardware installer's or field engineer's).
Example:
From a hardware point of view, a 3380A seems to be a single unit with two spindles inside, each with two access mechanisms. From a programmer's point of view, however, this single physical box is actually four DASD, because each of the four "access mechanisms" of a 3380 is an addressable unit and acts as if it were a separate DASD.
Example:
A physical 3330-1 contains two 3336-1 disk packs, each of which is a separately addressable unit. In the FLEX-ES emulation, this is treated as two 3330 DASD. Note as well that in the emulation these disks are not removable in the traditional sense. (It is possible to achieve something like removability. For example, an emulated DASD might be mounted OFFLINE and moved to another location using the server OS dd(1) command. An emulated DASD mounted OFFLINE could also be on a hot-swappable server disk. An emulated DASD also be attached via a FLEX-ES network channel on another node, then mounted OFFLINE and physically removed (via hot or cold swap) from that machine.)
FLEX-ES systems do not have "physical" limitations on the number of DASD that may be installed (as a string of physical DASD might). It is possible to have more emulated DASD than certain physical controllers permit. It is also possible to have odd numbers of emulated DASD that would be impossible in a physical configuration (for instance, a single 3350, rather than a pair, or three 3380 "access mechanisms" rather than the four required in a physical 3380).
FLEX-ES allocates DASD in terms of addressable units. If you are used to thinking in terms of physical devices rather than addressable units, the following notes may help you convert. The "FLEX-ES Device Name" is the name you would use to specify each of these emulated devices in the FLEX-ES configuration file.
| Device | Model | Configuration | Addressable Units | FLEX-ES Device Name |
| 3330 | 2 | 1 3336-1 pack | 1 | 3330 |
| 3330 | 1 | 2 3336-1 packs | 2 | 3330 (use 2 entries) |
| 3330 | 11 | 2 3336-11 packs | 2 | 3330-11 (use 2 entries) |
| 3340 | A2 | 2 3348-35 packs | 2 | 3340-35 (use 2 entries) |
| 3340 | A2 | 2 3348-70 packs | 2 | 3340-70 (use 2 entries) |
| 3340 | B1 | 1 3348-35 pack | 1 | 3340-35 |
| 3340 | B11 | 3348-70 pack | 1 | 3340-70 |
| 3340 | B2 | 2 3348-35 packs | 2 | 3340-35 (use 2 entries) |
| 3340 | B2 | 2 3348-70 packs | 2 | 3340-70 (use 2 entries) |
| 3350 | (two drives) | 2 | 3350 (use 2 entries) | |
| 3380 | A | (4 acc. mech.) | 4 | 3380-A (use 4 entries) |
Refer to your mainframe operating system documentation to determine the space requirements for the mainframe operating system(s) and for applications and users under these operating system(s). This space will first be specified in terms of one or more mainframe DASD of various types. Then, using the procedures described below, these mainframe DASD requirements will be translated into amounts of space on server disks.
The table below gives the total size in bytes required by a FLEX-ES system for many types of emulated CKD DASD. To find the actual server disk allocation in terms of server disk allocation units, divide this total size by the size of the server disks allocation unit (in bytes) and round up to the next integral allocation unit. For convenience, this table also gives the results of this calculation for the common case of 1 megabyte allocation units.
Note that when calculating the space required for a multi-chunk emulated DASD you should for each chunk add not less than the space equivalent to the FLEX-ES requirements for an additional cylinder of the DASD in question. In practice, since server disk space is generally allocated in units of 1 megabyte, or in units not smaller than 1 megabyte, and since no DASD cylinder exceeds 1 megabyte in required size, it is usually best simply to add 1 megabyte per chunk. Thus, an emulated 3390-9 (8,382 megabytes) which is to employ five chunks for its backing storage would require a total of 8,387 megabytes.
(Note that in this table the cylinder count of the emulated DASD is given inclusive of all alternate, CE and SA cylinders or blocks. Non-standard sized CKD emulated DASD, not shown in this table, do not have alternate, CE, or SA cylinders or blocks. Note also that 1 megabyte is 1024 Kilobytes (1,048,576 bytes), not one million bytes.)
| DASD | Track Size (K) | * Tracks/Cyl | = Cyl Size (K) | * NumCyls | = Total Size (K) | Round Up(M) |
| 2305-2 | 15 | 8 | 120 | 97 | 11,640 | 12 |
| 2314 | 8 | 20 | 160 | 203 | 32,480 | 32 |
| 3330 | 13 | 19 | 247 | 411 | 101,517 | 100 |
| 3330-11 | 13 | 19 | 247 | 815 | 201,305 | 197 |
| 3340-35 | 9 | 12 | 108 | 351 | 37,908 | 38 |
| 3340-70 | 9 | 12 | 108 | 701 | 75,708 | 74 |
| 3350 | 19 | 30 | 570 | 562 | 320,340 | 313 |
| 3375 | 36 | 12 | 432 | 962 | 415,584 | 406 |
| 3380-A,D,J | 47 | 15 | 705 | 888 | 626,040 | 612 |
| 3380-E | 47 | 15 | 705 | 1,774 | 1,250,670 | 1,222 |
| 3380-K | 47 | 15 | 705 | 2,660 | 1,875,300 | 1,832 |
| 3390-1 | 57 | 15 | 855 | 1,117 | 955,035 | 933 |
| 3390-2 | 57 | 15 | 855 | 2,230 | 1,906,650 | 1,862 |
| 3390-3 | 57 | 15 | 855 | 3,343 | 2,858,265 | 2,792 |
| 3390-9 | 57 | 15 | 855 | 10,038 | 8,582,490 | 8,382 |
| 6280 | 27 | 20 | 540 | 842 | 454,680 | 445 |
| 9345 | 47 | 15 | 705 | 1,454 | 1,025,070 | 1,002 |
| 9345-2 | 47 | 15 | 705 | 2,170 | 1,529,850 | 1,494 |
Space calculations for FBA DASD are simpler. An FBA DASD has a fixed block size of 512 bytes. It is necessary only to calculate the total size of the FBA DASD, in terms of 512-byte blocks, add the number of 512 byte CE blocks and the number of 512 byte SA blocks, and then round up to the next integral number server disk allocation units. For convenience, the table below also gives the results of this calculation for the common case of 1 megabyte allocation units.
(Note that 1 megabyte is 1024 Kilobytes (1,048,576 bytes), not one million bytes.)
| DASD | Model (Comment) | FLEX-ES Name | Total 512 Byte Blocks | Total 1M Units |
| 3310 | 3310 | 126,368 | 62 | |
| 3370 | 1 | 3370 | 559,488 | 274 |
| 3370 | 1 | 3370-1 | 559,488 | 274 |
| 3370 | 2 | 3370-2 | 714,240 | 349 |
| 9332 | 200 | 9332-200 | 360,328 | 176 |
| 9332 | 400 | 9332-400 | 360,328 | 176 |
| 9332 | 600 | 9332-600 | 550,092 | 272 |
| 9335 | 9335 | 805,140 | 394 | |
| 9336 | 10 | 9336-10 | 920,595 | 450 |
| 9336 | 20 | 9336-20 | 1,673,371 | 818 |