Generic Disk Driver Enhancement

Generic Disk Driver

Enhancement

Jim Hawkins and Hari L., vCSY

Introduction

Historically MPE/iX HP e3000 customers have been advised to only use disk devices that were sold and

supported by HP. This advice was based upon the design of the MPE/iX disk drivers which depend upon

features of the device hardware and firmware to guarantee reliable storage of data. In order to assist HP

e3000 customers in their desire to run their systems for as long as possible and, as result of the Interex

System Improvement Ballot (SIB), the Generic Disk Driver Enhancement (GDD) has been designed to

remove as many of this dependencies as possible while still retaining the data durability that the HP

e3000 is famous for.

It is made available via patch MPENX24 for C.75.00.

Caution: HP advises customers to always seek to use HP branded disk devices as a first choice, then

to try the OEM versions of these devices and, only after these options are exhausted, move to devices

from other manufacturers. HP in no way guarantees that any “non-HP” SCSI disk devices will perform

adequately with this patch. HP is not liable for any data durability issues or performance issues that

result from the use of this patch or usage of any storage devices not previously certified by HP as HP

e3000 compatible.

The remainder of this article describes:

 Requirements for MPE/iX Disks of any type

 MPE/iX SCSI Disk Drivers

 Description of changes to each SCSI disk driver

MPE/iX requirements for Disks

MPE/iX depends upon some basic requirements for any disk device;

 Disks subsystems should support 256 Bytes Sector access to data. Actually in the case of SCSI

disks physical block/sector size is 512 Bytes. ALL disk access on MPE/iX is therefore done in at

least 512 Byte “chunks”, for memory mapped structures, I/O size is 8*512Bytes for 4096 Bytes

per page. Various subsystems handle “conversion” from 256 Byte sectors to 512 Byte (or larger)

I/O in their data access routines. For example in DEBUG Sub-sector reads like “dsec 1.100,20”

are converted into a single I/O to LDEV 1 for 2048 Bytes. Upon the completion of that I/O Debug

displays only the bytes requested, $100-$119, from the larger buffer.

 Disks must be durable. This may seem obvious but there is an issue with “modern” disks as they

often employ RAM buffers or caches to speed up access. Ideally MPE/iX “write” operations

should bypass disk caches to guarantee durability. Realistically it turns out that in many disks

caches cannot be avoided so we have a requirement that disks be protected by UPS to avoid

losing the data in cache due to a power failure.

 Disk access must be “atomic” – that is, all or nothing. Upper layers should be able to assume

that all data has been transferred when “good” status is returned. “bad” status indicates an

inability to be sure of the state of the data; it is expected that “bad” status should be consistently

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Generic Disk Driver Enhancement Jim Hawkins and Hari L., vCSY Introduction Historically MPE/iX HP e3000 customers have been advised to only use disk devices that were sold and supported by HP. This advice was based upon the design of the MPE/iX disk drivers which depend upon features of the device hardware and firmware to guarantee reliable storage of data.
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returned. (Disk DMs monitor “suspect” sectors and move/recover data to spare areas to avoid data loss.)  Disks must guarantee ordered access. Simply put, if I write to a location on Disk first “A” next “B” and then “C” in three distinct I/Os then when I read the data back it must be “C” not “B” or “A.” SCSI disks typically support the multiple “in flight” I/Os and often have RAM buffer/caches to store data; in both cases we want to be sure that I/Os are processed in the order that they are sent.
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Prior to this enhancement, AEN enabling was done only for HP devices. But since most of the other devices also support AEN, the enabling of AEN has been extended to these discs also provided the specific disc supports setting of the AEN fields. b) PAGE 8 (Cache settings) – Page 8 allows setting of two fields that control the use of read and write caches during disc I/O. These fields are called ‘Read Cache Disabled (RCD)’ and ‘Write Cache Enabled (WCE)’.