Illegal Request SCSI errors with ext4

Environment

• Red Hat Enterprise Linux 5
• iSCSI storage enclosure
• EXT4 filesystem

Issue

• System using ext4 filesystem experiences regular scsi errors.

kernel: FAILED
kernel:   status = 1, message = 00, host = 0, driver = 08
kernel:   <6>sd: Current: sense key: Illegal Request
kernel:     <<vendor>> ASC=0x94 ASCQ=0x1ASC=0x94 ASCQ=0x1

Resolution

• Disabling barrier usage prevents the errors by not causing the SYNCHRONIZE_CACHE command to be sent. Add barrier=0 as a mount option to ext4 mounts.

Root Cause

• The error messages are due to a storage enclosure rejecting the SYNCHRONIZE_CACHE scsi command. The SYNCHRONIZE_CACHE command is used for scsi devices to function as a write barrier. If the storage hardware doesn't accept or understand this command, an error will be generated every time one is sent.

Diagnostic Steps

• The exact error message is from sd_sync_cache() in driver/scsi/sd.c. It will have the sd: prefix on the first line with the sense information.

printk(KERN_WARNING "FAILED\n  status = %x, message = %02x, "
                        "host = %d, driver = %02x\n  ",
                        status_byte(res), msg_byte(res),
                        host_byte(res), driver_byte(res));
            if (driver_byte(res) & DRIVER_SENSE)
                    scsi_print_sense_hdr("sd", &sshdr);

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