RAID SCSI Hard Drive: 73GB vs 72.8GB

Jacob S asked:

I know that this may be a stupid question to some but I don’t often work with SCSI drives with what seems to be non-standardized sizes compared to SATA drives. I currently have a RAID1 of two drives on an old Dell server and one of the drives has failed.

The current drives are Maxtor 73GB SCSI Ultra320 80-pin 10k (8J073J002075E). When searching for replacement drives of similar specs, it seems that 72.8GB drives are most commonly listed and 73GB drives are more rare.

Is this a case of a manufacturer rounding up on the capacity or is there a real 200MB difference going to cause an issue?

(Note: I realize that the general rule of thumb is to replace with all the same specifications or higher — this question is whether the difference between 72.8 versus 73 is just labeling versus actual technical difference in size.)

My answer:

What the marketing people slap on the front of a drive doesn’t matter. What matters is how many logical blocks the drive actually has usable.

# fdisk -l /dev/sda
Disk /dev/sda: 223.6 GiB, 240057409536 bytes, 468862128 sectors

In this drive I have 468862128 logical blocks, so if I need to replace the drive, the replacement must have the same number, or more.

With a smaller number of blocks, it will simply be impossible to recover the array to the disk, because there won’t be enough space. With a larger number of blocks, the remaining space will go unused, but it will be possible to rebuild the array onto the drive.

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