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

[Richard Elling]

> On Mar 22, 2016, at 7:41 AM, Chris Siebenmann <cks at cs.toronto.edu> wrote:
> 
>>> This implicitly assumes that the only reason to set ashift=12 is
>>> if you are currently using one or more drives that require it. I
>>> strongly disagree with this view. Since ZFS cannot currently replace
>>> a 512n drive with a 512e one, I feel [...]
>> 
>> *In theory* this replacement should work well if the lie works *correctly*.
>> In ZoL, for the "-o ashift" is supported in "zpool replace", the
>> replacement should also work in mixed sector sizes.
>> And in illumos the whitelist will do the same.
>> What errors have you ever seen?
> 
> We have seen devices that changed between (claimed) 512n and
> (claimed) 512e/4k *within the same model number*; the only thing that
> distinguished the two was firmware version (which is not something that
> you can match in sd.conf). This came as a complete surprise to us the
> first time we needed to replace an old (512n) one of these with a new
> (512e) one.
> 
> 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.)

What is missing from
http://wiki.illumos.org/display/illumos/ZFS+and+Advanced+Format+disks <http://wiki.illumos.org/display/illumos/ZFS+and+Advanced+Format+disks>
is:

1. how to change the un_phy_blocksize for any or all uns
2. how to set a default setting for all drives in sd.conf by setting attributes to
    the "<vid+pid>" of ""  (see sd(7d))

I am aware of no new HDDs with 512n, so this problem will go away for HDDs.
However, there are many SSDs that work better with un_phy_blocksize = 8192
and some vendors set sd.conf or source appropriately.
 -- richard

> 
>>> For many usage cases, somewhat more space usage and perhaps
>>> somewhat slower pools are vastly preferable to a loss of pool
>>> redundancy over time. I feel that OmniOS should at least give you
>>> the option here (in a less crude way than simply telling it that
>>> absolutely all of your drives are 4k drives, partly because such
>>> general lies are problematic in various situations).
>> 
>> The whitelist (sd.conf) should fit into this consideration. But not
>> sure how mixed sector sizes impact the performance.
> 
> Oh, 512e disks in a 512n pool will probably have not great performance.
> ZFS does a lot of unaligned reads and writes, unlike other filesystems;
> if you say your disks are 512n, it really believes you and behaves
> accordingly.
> 
> 	- cks

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