Bug#486526: Please enhance zipl to support booting from the RECOMP area of a CMS minidisk
Package: s390-tools
Version: 1.5.3-1
Severity: wishlist
Please enhance zipl to support booting from the RECOMP area of a CMS minidisk. (This will also require an enhancement to mke2fs to respect the RECOMP area of a CMS minidisk when making a file system on it.)
Rationale: Currently, only ECKD DASD devices formatted with the Linux disk layout (ldl) or the z/OS-compliant compatible disk layout (cdl) are bootable: ECKD DASD devices formatted with the CMS disk layout are not bootable. But CMS minidisks formatted with the RECOMP option are potentially bootable under z/VM. That is how the CMS operating system is booted: from the RECOMP area. This enhancement would make it possible to implement Linux under z/VM using only CMS minidisks. This has a number of advantages for z/VM shops, such as exploitation of existing z/VM DASD
management tools, better exploitation of minidisk caching, etc. Currently, at least the /boot partition must be cdl or ldl.
Example:
Assume a minidisk of device type 3390 for 75 cylinders exists at virtual device number 101. Assume that the CMS operating system is currently running:
format 101 b ( blksize 4096
DMSFOR603R FORMAT WILL ERASE ALL FILES ON DISK B(101). DO YOU WISH TO CONTINUE?
ENTER 1(YES) OR 0(NO).
1
DMSFOR605R ENTER DISK LABEL:
sb0101
DMSFOR733I FORMATTING DISK B
DMSFOR732I 75 CYLINDERS FORMATTED ON B(101)
READY;
format 101 b 74 ( recomp
LABEL VDEV M STAT CYL TYPE BLKSZ FILES BLKS USED-(%) BLKS LEFT BLK TOTAL
SB0101 101 B R/W 74 3390 4096 0 7-00 13313 13320
A CMS file system for a 74-cylinder disk has now been set up, with cylinders numbered 0-73. The last cylinder, cylinder number 74, which is actually the 75th cylinder, is the RECOMP area. It is reserved for a bootable program. The RECOMP area is part of the minidisk, but it is not a part of the CMS filesystem space. CMS is too small for a 1-cylinder RECOMP area. It would need about 7 cylinders or so. But one cylinder is plenty large enough for a boot loader, such as z/VM's Stand Alone Program Loader (SAPL) or the zipl bootstrap loader for Linux. In fact, the SAPL can be written to and tested from a CMS minidisk. Such a minidisk is not bootable by the hardware. I.e., it cannot be IPLed in an LPAR, even if cylinder 0 of the minidisk corresponds to cylinder 0 of a real DASD volume. But it can be booted under z/VM by CP.
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