[parted-devel] [PATCH 1/2] fdasd: geometry handling updated from upstream s390-tools

[Viktor Mihajlovski]

On 24.09.2015 09:29, Phil Susi wrote:

> So it is kind of like that object storage stuff the SCSI committee has
> been toying with for years?  Where you give it a key and an arbitrary
> data set to store associated with that key?  And instead of using the
> disk like that, you have to have a translation layer to make it behave
> more like a regular disk?
it is not exactly like that. If you think of traditional SCSI disks as a 
string of same sized blocks, a DASD can be thought of as a string of 
same sized tracks. Each of the tracks will contain records consisting of 
a count (length) field, an optional key field, and the data itself.
Nothing a Linux filesystem would be able to consume.
The formatting makes sure that all of the records in a track have the 
same length and no key field. A DASD formatted this way can be 
represented as a regular block device by the DASD driver.
>
> So instead of simply writing the VTOC to the first track, some crazy
> person decided to make an ioctl for it, and when it is attached via
> virtio, you skip this useless ioctl and just write to the first track
> like a sane person?  Why keep the ioctl for direct attached disks?  Why
> not just get rid of it in both cases?
>
OK, I was simplifying a bit, so let me be more precise. The first track 
contains meta-data (the so called VOLSER), pointing to the actual VTOC 
record.
The architectural compliant way (only possible for a natively attached 
DASD) to locate the VTOC is to use native s390 I/O which makes it 
necessary to use ioctls.
For practical purposes, the VTOC of disks formatted by the Linux dasdfmt 
tool will be located in track 2, which is therefore the only valid 
layout for virtio attached DASD.
In neither case (traditional or virtio) an ioctl is used to read or 
write the VTOC content.

-- 

Mit freundlichen Grüßen/Kind Regards
    Viktor Mihajlovski

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