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专题讨论之handbook版－第2章（讨论版）
An Illustration from the Files of Bill and Fred's Exceptional Adventures:
Bill breaks-down an older Wintel box to make another FreeBSD box for Fred. Bill installs a single SCSI drive as SCSI unit zero and installs FreeBSD on it.
Fred begins using the system, but after several days notices that the older SCSI drive is reporting numerous soft errors and reports this fact to Bill.
After several more days, Bill decides it is time to address the situation, so he grabs an identical SCSI drive from the disk drive “archive” in the back room. An initial surface scan indicates that this drive is functioning well, so Bill installs this drive as SCSI unit four and makes an image copy from drive zero to drive four. Now that the new drive is installed and functioning nicely, Bill decides that it is a good idea to start using it, so he uses features in the SCSI BIOS to re-order the disk drives so that the system boots from SCSI unit four. FreeBSD boots and runs just fine.
Fred continues his work for several days, and soon Bill and Fred decide that it is time for a new adventure -- time to upgrade to a newer version of FreeBSD. Bill removes SCSI unit zero because it was a bit flaky and replaces it with another identical disk drive from the “archive”. Bill then installs the new version of FreeBSD onto the new SCSI unit zero using Fred's magic Internet FTP floppies. The installation goes well.
Fred uses the new version of FreeBSD for a few days, and certifies that it is good enough for use in the engineering department. It is time to copy all of his work from the old version. So Fred mounts SCSI unit four (the latest copy of the older FreeBSD version). Fred is dismayed to find that none of his precious work is present on SCSI unit four.
Where did the data go?
When Bill made an image copy of the original SCSI unit zero onto SCSI unit four, unit four became the “new clone”. When Bill re-ordered the SCSI BIOS so that he could boot from SCSI unit four, he was only fooling himself. FreeBSD was still running on SCSI unit zero. Making this kind of BIOS change will cause some or all of the Boot and Loader code to be fetched from the selected BIOS drive, but when the FreeBSD kernel drivers take-over, the BIOS drive numbering will be ignored, and FreeBSD will transition back to normal drive numbering. In the illustration at hand, the system continued to operate on the original SCSI unit zero, and all of Fred's data was there, not on SCSI unit four. The fact that the system appeared to be running on SCSI unit four was simply an artifact of human expectations.
We are delighted to mention that no data bytes were killed or harmed in any way by our discovery of this phenomenon. The older SCSI unit zero was retrieved from the bone pile, and all of Fred's work was returned to him, (and now Bill knows that he can count as high as zero).
Although SCSI drives were used in this illustration, the concepts apply equally to IDE drives. |
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楼主: 剑心通明
发表于 2005-10-14 10:33
发表于 2005-10-14 11:26
