[OmniOS-discuss] 4kn or 512e with ashift=12

[Richard Elling]

> On Mar 23, 2016, at 7:36 AM, Chris Siebenmann <cks at cs.toronto.edu> wrote:
> 
>>> 	The sd.conf whitelist also requires a reboot to activate if you need
>>> 	to add a new entry, as far as I know.
>>> 
>>>    (Nor do I know what happens if you have some 512n disks and
>>>    some 512e disks, both correctly recognized and in different
>>>    pools, and now you need to replace a 512n disk with a spare 512e
>>>    disk so you change sd.conf to claim that all of the 512e disks
>>>    are 512n. I'd like to think that ZFS will carry on as normal,
>>>    but I'm not sure.  This makes it somewhat dangerous to change
>>>    sd.conf on a live system.)
>> 
>> There are two cases if we don't use the remedy (whitelist in illumos
>> or -o ashift in ZoL) here:
>> a): 512n <---> 512e. This replacement should work *in theory* if the
>> lie works *correctly*.
> 
> This will not work without the sd.conf workaround in Illumos.
> 
> All 512e disks that I know of correctly report their actual physical
> disk size to Illumos (and to Linux/ZoL). When a disk reports a 4K
> physical sector size, ZFS will refuse to allow it into an ashift=9
> vdev *regardless* of the fact that it is 512e and will accept reads
> and writes in 512-byte sectors.
> 
> In Illumos, you can use sd.conf to lie to the system and claim that
> this is not a 512e but a 512n disk (ie, it has a 512 byte physical
> sector size). I don't believe there's an equivalent on ZoL, but I
> haven't looked.
> 
> This absolute insistence on ZFS's part is what makes ashift=9 vdevs so
> dangerous today, because you cannot replace existing 512n disks in them
> with 512e disks without (significant) hackery.
> 
> (Perhaps I'm misunderstanding what people mean by '512e' here; I've
> been assuming it means a disk which reports 512 byte logical sectors and
> 4k physical sectors. Such disks are what you commonly get today.)

Yes. 512e means:
 un_phy_blocksize = 4096 (or 8192)
 un_tgt_blocksize = 512
for disks that don't lie. Lying disks claim un_phy_blocksize = 512 when it isn't.

At this point, before the discussion degenerates further, remember that George
covered this in detail at the OpenZFS conference and in his blog.
http://blog.delphix.com/gwilson/2012/11/15/4k-sectors-and-zfs/ <http://blog.delphix.com/gwilson/2012/11/15/4k-sectors-and-zfs/>

http://www.youtube.com/watch?v=TmH3iRLhZ-A&feature=youtu.be


 -- richard

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://omniosce.org/ml-archive/attachments/20160323/a8abbbcf/attachment.html>