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Here's a tip: Look at the number of total 512 byte sectors available for the disk drive device or storage system as an indicator of a storage device's actual raw capacity. Some storage systems use 520 byte or 528 byte low-level sectors for data consistency, yet report usable sectors as 512 bytes; thus you may see a lower total raw capacity for the device. Most vendors document how many bytes, sometimes both in base 2 and base 10, as well as the number of 512 byte sectors supported on their storage devices and storage systems (though it might be in the small print).
Base 2 and base 10 numbering account for only part of the missing storage capacity compared to what you expect to see. Rounding up or down can mean the difference between a 146GB and 147GB disk drive, for example. A storage system's internal overhead space needs (snapshots, replication disk-based buffers, storage system software, cache memory de-stage or scram space), as well as RAID levels, spare drives, and operating system or file-system formatting will impact your actual storage space.
When adding storage capacity, be clear with vendors so that all parties understand what the other is talking about: Is the discussion about raw or usable (formatted, RAID level, file-system and storage system overhead) capacity, and what unit of measure (base 2 or base 10) will be used.
This was first published in December 2007