Bug#486526: marked as done (Please enhance zipl to support booting from the RECOMP area of a CMS minidisk)
Your message dated Sun, 24 Jan 2010 19:57:37 -0500 (EST)
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and subject line Canceling enhancement request
has caused the Debian Bug report #486526,
regarding Please enhance zipl to support booting from the RECOMP area of a CMS minidisk
to be marked as done.
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--- Begin Message ---
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.
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).
DMSFOR605R ENTER DISK LABEL:
DMSFOR733I FORMATTING DISK B
DMSFOR732I 75 CYLINDERS FORMATTED ON B(101)
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.
--- End Message ---
--- Begin Message ---
- To: firstname.lastname@example.org
- Subject: Canceling enhancement request
- From: Stephen Powell <email@example.com>
- Date: Sun, 24 Jan 2010 19:57:37 -0500 (EST)
- Message-id: <471587805.7473501264381057459.JavaMail.firstname.lastname@example.org>
The assumption behind this enhancement request was incorrect.
Contrary to my understanding at the time, zipl *does* support
booting from a CMS-format disk. It just doesn't do it in the
way I expected it to based on my background and experience with
the CMS operating system. The only place an IPL-able CMS
nucleus can be written is on the RECOMP area of a CMS minidisk.
Since the Linux kernel does not (or did not at the time anyway)
recognize the RECOMP area as a separate partition, I assumed
(incorrectly as it turned out) that one could not IPL zipl
from a CMS minidisk. But that is not the case. The portion
of the boot loader which goes beyond the first two IPL records
(for a CKD DASD) or the first IPL record (for an FBA DASD) is
written to a special file on the regular partition. This method
is actually superior to the method used by CMS.
Therefore, I withdraw the enhancement request.
--- End Message ---