IBM s/390 manual Differences

￿A raw disk (or several raw disks) is required for each S/390 DASD volume being emulated. For example, if an OS/390 system requires 15 3390 volumes (for the system and user volumes), then FLEX-ES would need at least 15 raw disks in UnixWare.

￿A raw disk has no UNIX file system. It can be used (by UNIX programs) as a single, sequential file. Multiple raw disks are tedious to administer and can lead to fragmentation issues if not well planned. This disk management has often been the most complex element of EFS installation on other platforms.

￿Raw disks are used by FLEX-ES because UnixWare does not buffer access to them.

Instead, FLEX-ES code does all the buffering using designs that are optimized for emulated S/390 volumes. This provides a major performance boost for FLEX-ES.3

￿In principle, FLEX-ES could use standard UNIX files to emulate S/390 volumes. However,

the normal UNIX buffering is not well suited to this emulation and the resulting performance is poor.4

Linux has added more factors to these elements:

￿Linux has raw disks, but buffers I/O for them. This means that the unique FLEX-ES coding for raw disks is not very effective.

￿Linux I/O handling for normal file systems is generally faster than that of traditional UNIX systems.

￿Linux directly supports large files (larger than 2 GB), as required for 3390-3 and 3390-9 emulation.

For these reasons, FLEX-ES uses normal Linux files to emulate S/390 DASD. It is possible to use Linux raw disks, but the performance benefits are slight and not worth the administration efforts involved. Linux has recently begun to support raw devices. It is possible that future versions of FLEX-ES may use these if performance benefits warrant it. For this redbook, and for current ThinkPad/EFS systems, we will use only normal Linux files for emulating S/390 disk volumes. This substantially simplifies installation and administration of the EFS system.

We plan to use a simple naming convention with names such as /s390/OS39RA, for the Linux file containing an emulated 3390-3 with volser OS39RA.

Differences

Typically, under Linux, an emulated S/390 DASD volume is a single Linux file. In this case (a single file), the file can have any convenient name. No FLEX-ES naming convention is required.

Under UnixWare (for a Netfinity/EFS system) emulated S/390 DASD volumes may occupy several raw disks.5 In this case (multiple UNIX files per S/390 volume) a special FLEX-ES naming convention must be followed. This naming convention requires UNIX file names to end with a lowercase ASCII “s” followed by a numeric digit.

3This is not unique to FLEX-ES. Other major UNIX middleware packages, such as some relational data base managers, also use raw disks for the same reasons.

4This does not imply that UnixWare disk handling is poor. It is very good for normal UNIX applications. Emulation of S/390 volumes is a very specific, narrow application that does not match typical UNIX file usage.

5This is not required, can be done to reduce potential fragmentation issues with raw disk space.

8S/390 PID: ThinkPad Enabled for S/390