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

[Rob T. <ro...@rt...>]

Just glancing over the documentation, the AS/400 is a funny animal. It 
basically treats all storage as one image (Single Level Store). Disk is 
just slow RAM. Therefor the disk needs to be formatted with specific 
patterns to appear like initialized RAM. FORMAT needs to write that 
specific pattern.

As far as I can tell the memory layout does work with 512-byte sectors, 
the 520/522 byte requirements are just an extra couple of bytes of 
CRC/ECC checking and correction on top of parallel SCSI parity checking. 
Would be nice if you could convince the AS/400 SCSI controller to trust 
the bus.

The Skip Read/Write commands are proprietary and required. When certain 
sectors of memory are needed, the system will do a regular SCSI Read 
asking for multiple sectors, but before that read, the Skip Read sends a 
bitmap of sectors that it's not really interested in. So the read could 
be a single SCSI "Read 100 sectors starting at 1000", but the preceeding 
Skip Read might indicate "Only give me block 1, 12-15 and 78-99 of the 
next command", which will save a lot of bandwidth and enhances the 
performance. It's optimization between data transfer and command 
processing overhead.

There are some Atto SCSI-to-iSCSI bridges that may be a reasonable way 
to start experimenting. Saw one on eBay the other day for $250.

Rob

On 2/24/2022 2:02 PM, Sietse van Zanen wrote:
> Hi,
>
>
> The first thing you nbeed to check / decide is what target back-end to use. 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. 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). For the linux storage system they have to be based on 2xxx chips, because that's what SCST target driver supports.
>
>
> 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. 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. However you probably won't and will replace them with regular disks.
>
>
> 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.
>
>
> Questions that you need to answer before going ahead:
>
> Are the SCSI commands crtitical for operation of AS/400? Can the AS/400 deal with devices that don't support these commands?
>
> Are disks with 520/522 bytes sector required by AS/400, or can normal 512/4096 bytes sectore devices be used?
>
> Is the FORMAT UNIT behaviour crtitical to operation of the AS/400, can disks be formatted in another way or on another system?
>
>
> -Sietse
>
>