# ZFS Sector Size 512B 4096B (4K)



## Leander (Apr 16, 2014)

Hi,

I'm about to set up a new ZFS based NAS with round about 24x 2.5" HDDs. Since I have medium-like expirience with ZFS, I sure know about some trouble with the sector size of the drives. So I did some research on the HDDs I'm planning to use. Unfortunately they have 4096B sectors.

WD Green WD20NPVX
I obviously would like to get an alternative drive model with a sector size of 512B in order to prevent any issues ... but my market analysis showed that there is not realy an alternative to 2TB drives in the size of 2.5". The good news though is, that WD offers a tool which allows you to align the drive.

White paper about the "Advanced Format-Technologie": http://www.wdc.com/wdproducts/library/WhitePapers/DEU/2579-771430.pdf
Download the "WD Advanced Format" tool: http://www.wdc.com/global/products/features/?id=7&language=1

*So basically, I would appreaciate some answers & advices on the following points:*

As far as I understood WD, the tool is going to tell the controller to emulate 512B instead of 4096B sectors - BUT it is actually NOT changing the phisical sector size ... because if it would change the physical size, the drives size would shrink, right?
Would this tool solve the ZFS related sector size issues or would there still be open to dos related to the sector issue? ... or will I still suffer of the 128 Kb record issue, which throttles down the performance?
Lets assume I plan to add a SSD which is based on 512B. Will a temporry fake of sector size ONLY for adding the SSD as ZIL or L2ARC, resolve the "balance"? Closer description here: http://blog.monsted.dk/?q=node/1 or http://wiki.bsdforen.de/zfs_ashift or http://ivoras.sharanet.org/blog/tree/2011-01-01.freebsd-on-4k-sector-drives.html ... or will I start to suffer of the 128 Kb record issue, which throttles down the performance? 
This one ... http://utcc.utoronto.ca/~cks/space/blog/solaris/ZFS4KSectorDisks


> Since a 512b to 4K transition is probably inevitable in every disk drive technology, you now want to create all new vdevs with ashift=12. A vdev created with at least one 4K drive so that it gets an ashift of 12 can thereafter freely mix 512b drives and 4K drives; as far as I know you can even replace all of the 4K drives in it with 512b drives. On Illumos the only way to do this is to set the reported physical sector size of at least one disk in the new vdev to 4K (if they aren't 4K disks already), at which point you become unable to add them to existing pools created with 512-byte disks. On old versions of Solaris (such as the Solaris 10 update 8 that we're still running) this is impossible.


 ... sounds like, as if there is nothing to worry about if you are planning to use 4K drives only (even if there might be a 512B replacements comes later)
Also, I found this interesting patch. Unfortunately I don't have any further information about it yet http://lists.freebsd.org/pipermail/freebsd-hackers/2013-July/043094.html
Last but not least, I remember some setting of sysinstall for a ZFS setup: "force 4K sectors". Is there something usefull I could learn in order to set up a performant ZFS with the WD drives?


Thanks & Best Regards


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## wblock@ (Apr 16, 2014)

Have you bought those drives yet?  Because if it's not too late...

4K drives are fine, just align them *and* use gnop(8) to force ZFS to use 4K blocks.  Those are two separate things, not one.

Don't trust WD software, all that I've ever tried was ignored by the drives.  All alignment means is making sure that the start of a partition is even with the start of a block.


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