System hangs during Backup/Verify operations

Keywords

edge BackupEDGE verify lockup hang crash

Release

BackupEDGE for SCO UNIX ver 3.2.4.x and above with SCSI subsystems.

Problem

During Backup or Verify, and especially during Bit-Level Verify, the system hangs. More specifically, the SCSI bus hangs. Console multiscreens function, but the system does not respond to commands. All other terminals are frozen.

Cause

Backup and Verify operations are very taxing to the SCSI bus. The most taxing is a bit-level verification, which must read data from two devices (the tape drive and hard drive) at the same time. While BackupEDGE is running during the lockup, it is not really the problem; the system is failing to transfer data through the SCSI subsystem correctly.

The SCO device drivers have critical timing issues that in some instances require near perfect SCSI bus impedance and termination, and specific firmware releases in host adapters, hard drives, and tape drives to achieve reliable operation. SCO OpenServer 5 in particular is much more timing critical than previous releases. Hence some people say "this hardware worked fine until I upgraded to OpenServer 5, and now it locks up".

Solution

It is possible to increase the reliability of the SCSI bus with minimum performance degradation by disabling a few of the advanced features of the SCSI device driver. Make an archive copy of the file /etc/conf/pack.d/Sdsk/space.c Edit /etc/conf/pack.d/Sdsk/space.c Look for the following lines:

int     Sdsk_no_sg = 0;
int     Sdsk_no_tag = 0;
int     Sdsk_set_RCD = 0;           (OpenServer 5 Only)

Change the 0 in each line to a 1. Save the file, re-link the kernel, and re-boot. These three lines control Scatter/Gather Enable, Tagged Queuing, and target read cache respectively. Disabling them will slow the system slightly while increasing stability. If your system remains stable after a week or so of activity you may try enabling these features one-by-one until you fail. Then you'll know for sure which advanced feature causes instability on your system.

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