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I have just obtained two hard drives which use 4k sectors, but report their physical sector size to the OS as only 512 bytes.

Does this mean that the OS must send eight requests to the drive to read 4k of data, whereas a true 4k drive could transfer the same amount of data with a single request?

Is there any performance benefit with that? Will it decrease system load, or improve NCQ performance, having to send fewer requests for the same amount of data?

Or do "real" 4k drives still use 512-byte logical sectors, so data will always be split into 512 byte chunks for transfer?

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1 Answer 1

ALL harddisks with 4k sectors report 512 byte to the OS for compatibility reasons: Think of BIOS, DOS or Windows.

The "cached" performance does not change, as 512 Byte sector devices can already read and write multiple sectors at once. Most filesystems also use data block sizes bigger than 512 byte - many use 4k and bigger.

On the disk a single 512 byte write is emulated by a 4k read-and-write cycle. You don't want that to happen too often. This will be also executed if a "mutliple write" does not start and/or end on a 4k boundary.

With Windows XP (and older Linux) you have to pay attenttion to the partitioning. The "Dos compatible" partinioning scheme will result in a non-aligned primary partition - which will result in a big performance hit.

The 4k sectors are a bit more efficient with surface data density, as there is less overhead involved. This also means that you can read more data on each "turn" of the platters, but its only a slight increase in data rate.

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So what you are saying is that even with drives that happily tell the OS they have 4k sectors, all read and write operations are still split into 512 byte chunks to be sent over the SATA cable? –  Malvineous Jun 12 '11 at 1:11
    
"All harddisks with 4K sectors report 512 byte to the OS" - that's true only of "512e" drives. These will have an "AF" logo. But the first "4k native" drives are already on the market. ("4kn" logo) These have no 512-byte compatibility mode. Windows 8 and later have full support for this. So do recent versions of UEFI. –  Jamie Hanrahan Aug 27 at 11:27

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