Re: [Scst-devel] About IBM iSeries|AS/400 support

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Hello Sietse,

thanks for your valuable input!


Am 24.02.2022 um 14:02 schrieb Sietse van Zanen <si...@wi...>:

> SCST supports: iSCSI, SRPT, Qlogic FC and FCOE. There used to be a SAS target driver but that's been long since removed. Ancient 8 / 16 bit (W/UW) SCSI bus/controller as target has never been an option with SCST.

Interestingly, the documentation in the www subdirectory of the current version being available says differently. But I understand that this documentation might not have been updated for a long time. Someone else already has answered me in a private email, and pointed out some things.

Without support for parallel SCSI, effort to achieve my goal is probably beyond my capabilities. Sigh. Would have been too nice.

> I think FC would be your best bet, with some old IBM Qlogic FC cards, which should be supported on AS/400. You can get these from ebay for next to nothing these days (regular qlogic, don't know about specific IBM ones).

I'm talking about really old machines, predating the broad availability of FC. ;-)

> This howver is only the physical connection part between you AS/400 and Linux system. More difficult will be to establish what storage backend to use. Passthrough will only work if the SCSI controller / devices on the linux system support the SCSI commands required by AS/400.

Passthrough is uninteresting for my goal. I'm seeking a substitute for old parallel SCSI disks on not so old hardware. Probably with the additional benefit of higher performance.

> So, probaly you'll qnat to use either fileio or blockio as storage backend, for sure if the 520/522 byte secotrs are T10-PI based, and you want to keep using these disks in the linux system.

Fileio is what I'm thinking would be best.

> Sounds like a fun project, but a lot is uncertain and you will probably have to find things out as you go along. My gut feeling says that it should be perfectly possible to connect an AS/400 to SCST.

Maybe with FC this could really be a feasible project. Newer machines supporting FC (and running a more current OS release), being backed with Fileio storage could indeed use the additional space without special disks. Might be worth to look into with the current code base.

> Questions that you need to answer before going ahead:

Btw., the answers are independent of the transport being used, and still valid for current machines. Perhaps, a two-step approach might be feasible, if the SCST devs are interested in supporting the peculiarities of IBM Midrange machinery in general: Implementing the required extras, and (I guess this is mainly my part) reviving Parallel SCSI support.

Parallel-SCSI support would be in general favorable, because many older machines generally suffer from similar fates: Older disks break, becoming rare to acquire, or electrical power becoming too expensive. RaSCSI fills that gap in a way, but its performance isn't exactly overwhelming.

One can run IBM i (aka OS/400) in a primary (or sole) LPAR on a system. Then the storage layer requires the disks being "special". Or, the primary LPAR runs IBM VIOS ("Virtual I/O Server"), a storage server backend based on AIX, offering virtual SCSI with in the system to a secondary IBM i LPAR. Then, the disks do not need to support 520/522 Bytes/sector. I'm not sure about the other dependencies: I have no experience with VIOS myself, but I think, the virtualization layer within VIOS handles that.

An LPAR is roughly equivalent to a VMware ESXi VM, while the Hypervisor itself isn't installable but sits in firmware.

> Are the SCSI commands crtitical for operation of AS/400?

Yes.

> Can the AS/400 deal with devices that don't support these commands?

No.

> Are disks with 520/522 bytes sector required by AS/400

Yes.

> or can normal 512/4096 bytes sectore devices be used?

No.

> Is the FORMAT UNIT behaviour crtitical to operation of the AS/400, can disks be formatted in another way or on another system?

As far as I've understood, FORMAT UNIT supports a "pattern" parameter. After the FORMAT UNIT being started on OS/400 is complete, it compares (how much?) of the pattern being read from disk with the expected pattern being submitted with the FORMAT UNIT command.

Does that help?

:wq! PoC