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.

View the full question and answer on Server Fault.

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