512e vs 4Kn: Sector Formats, Performance, ZFS and Which Hard Drive to Buy

512e vs 4Kn vs 512n: what the three sector formats are

A sector is the smallest unit a drive reads or writes, and a drive reports two sizes: the logical size the operating system addresses and the physical size the drive actually writes. Microsoft's Advanced Format compatibility page defines the three formats you will meet on hard drive data sheets.

• 512n (512-byte native): 512 bytes logical and 512 bytes physical. Microsoft lists it as supported on all Windows versions.
• 512e (512-byte emulation): exposes 512 bytes as its logical sector size, similar to a regular disk, but makes its 4 KB physical sector size available to software that asks.
• 4Kn (4 KB native): no emulation layer; the drive exposes 4 KB as both its logical and its physical sector size.

512n has not disappeared. WD's Ultrastar DC HC310 data sheet offers a 512n version of the 4TB model to support legacy systems with native 512-byte block sizes, alongside its 512e and 4Kn models.

512e vs 4Kn performance: what the documents say

The performance difference comes from writes that do not fill a whole physical sector. On a 512e drive, software can send a 512-byte write to a 4 KB physical sector. Microsoft explains that the drive then has to read the sector, change part of it and write it back, and that this work by the storage device can result in a performance loss. OpenZFS makes the same point from the file system side: partial sector writes incur a penalty.

Switching a drive to 4Kn does not remove the problem on its own. Seagate's Exos X24 data sheet notes that data must be aligned to 4K sectors to see improved performance in 4Kn format. If the partitions and the file system already write in aligned 4 KB units, a 512e drive avoids the extra work too.

On sequential transfer, WD's Ultrastar DC HC310 data sheet splits its figures by format and shows no gap between the two: up to 255 MB/s sustained for both its 512e and 4Kn models, and 233 MB/s for its 512n models. We have not found a maker benchmark that compares 512e and 4Kn throughput on the same drive under small random writes, so this page puts no number on that difference.

Alignment is where most of the cost hides. Microsoft says partitions that are not aligned to a 4 KB physical sector boundary make later writes unaligned, and that Windows since Vista SP1 places the first partition at 1024 KB on disks of 4 GB or larger, which is aligned to a 4 KB boundary. OpenZFS warns that poorly aligned disk accesses can greatly degrade pool performance.

512e vs 4Kn on Windows

Microsoft's support table is the clearest compatibility statement we have found. 512n is supported on all Windows versions. 4Kn is supported on all versions from Windows 8 onwards, and Windows 8 added the ability to install Windows to, and boot from, a 4K native disk. Windows XP, Windows Server 2003 and Windows Server 2003 R2 support neither 512e nor 4Kn media.

The operating system is only half of it. Microsoft warns that the majority of apps and operating systems do not query for, and align their I/O to, the physical sector size, which on 4K native media can result in unexpected failed I/Os. A 512e drive keeps presenting 512-byte sectors to that software, which is the main reason it stays the safer choice where you cannot vouch for every program that touches the disk.

512e vs 4Kn for ZFS and TrueNAS: set ashift before the pool exists

ZFS records sector size in a pool property called ashift, a power of two. OpenZFS documents values from 9 to 16 as valid, with 0 meaning auto-detect, and says that for optimal performance the pool sector size should be greater than or equal to the sector size of the underlying disks. Its FAQ says ZFS automatically detects the sector size of the drive by default.

That default is where 512e matters. OpenZFS's tuning guide says ZFS assumes the sector size a drive reports is correct and calculates ashift from it. A 512e drive reports 512 bytes, so auto-detection can choose a sector size smaller than the 4 KiB the drive really writes. For disks that use 4 KiB sectors but report 512 bytes, the manual page says to set ashift=12, which is 1<<12, or 4096.

Timing is the trap. OpenZFS says this tuning can only be set when block devices are first added to a pool, and that changing the ashift value later will not modify any existing vdev, not even on disk replacement. So a pool built at the wrong value stays that way until it is rebuilt. With 4Kn drives the reported size is already 4 KiB, so auto-detection and the recommended value agree.

We have not found a TrueNAS document on 512e or 4Kn, so this page rests its ZFS answer on the OpenZFS documentation and repeats no forum advice. Where a TrueNAS system creates ZFS pools, the OpenZFS rules above are the ones to check before you create the pool.

Synology 512e vs 4Kn

Synology's own drives answer half the question. Its HAT3300 data sheet gives the sector size as 512e for the whole range, and its HAT5300 data sheet gives 512e too. Seagate's IronWolf Pro data sheet lists 512E as the logical sector size for the NAS drives in its first capacity table.

The other half is whether a Synology NAS accepts 4Kn drives at all, and we have not found a Synology document that states it. Synology's guide to choosing a drive for its NAS does not mention sector size. If you are buying for a Synology box, the drives Synology itself sells are 512e, and a 4Kn drive is a question to put to Synology before you buy.

HDD 512e vs 4Kn: how to tell from the part number

Some makers sell the same drive in both formats under different part numbers, so a listing title that gives only the capacity and the line does not tell you which format you will receive.

• Toshiba MG10: the overview lists 512e or 4Kn Advanced Format as a feature. On the 20 TB SATA models the last letter marks it: MG10ACA20TA is 4Kn and MG10ACA20TE is 512e.
• Seagate Exos X24: FastFormat models ship in 512e and can be switched to 4Kn, and the data sheet lists a 4096-byte 4Kn sector size for the SATA models.
• WD Ultrastar DC HC310: 512e models can be converted to 4Kn and the reverse, and a 512n version exists on the 4TB model.
• Synology HAT3300 and HAT5300: 512e across both ranges.
• Seagate IronWolf Pro: 512E logical sectors on the models in the data sheet's first table.

Find the full part number on the listing, then find that exact part number on the maker's data sheet. A seller's description of the format is a claim until the sheet confirms it.

Can you change a drive from 512e to 4Kn?

On some drives, yes, at a cost. Seagate says its Exos X24 FastFormat models ship in the 512e format state, and that when switching from 512e to 4Kn by executing the FastFormat routine, all data on the drive will be deleted. WD says the Ultrastar DC HC310's 512e models can be converted to 4Kn and back.

Treat a conversion as part of setting a drive up, not something to do to a drive in service: back up first, convert, then build the pool or volume. On ZFS that order matters twice, because ashift is fixed when the drive first joins a pool.

512e or 4Kn: which to buy

• Mixed or older software, or a system you cannot fully vouch for: 512e. It presents 512-byte sectors to software that never asks about the physical size.
• Windows older than Windows 8 that must boot from the drive: not 4Kn. Windows XP and the Server 2003 family support neither 512e nor 4Kn.
• A new ZFS pool: either format works if ashift is 12 from the start.
• A Synology NAS: 512e, the format of Synology's own HAT3300 and HAT5300 drives, unless Synology confirms 4Kn for your model.
• A legacy system that needs native 512-byte blocks: a 512n model, such as the 4TB HC310 option.

Live Amazon.co.uk prices sit on the pages for the lines named above: Toshiba MG10, Toshiba MG09, Seagate Exos X24 and Ultrastar DC HC550. Their tables do not filter by sector format, so check each row's part number against the sheet. If a SAS drive formatted 4Kn is not showing up behind a controller, the troubleshooting page 4Kn SAS drive not detected covers that case, and SAS vs SATA covers the interface question.