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

[Fred Liu]

> -----Original Message-----
> From: OmniOS-discuss [mailto:omnios-discuss-bounces at lists.omniti.com] On
> Behalf Of Chris Siebenmann
> Sent: 星期二, 三月 22, 2016 3:27
> To: Richard Elling
> Cc: omnios-discuss at lists.omniti.com
> Subject: Re: [OmniOS-discuss] 4kn or 512e with ashift=12
> 
> > > Adding the ashift argument to zpool was discussed every few years
> > > and so far was always deemed not enterprisey enough for the Solaris
> > > heritage, so the setup to tweak sd driver reports and properly rely
> > > on that layer was pushed instead.
> >
> > The issue is that once a drive model lies, then the Solaris approach
> > is to encode the lie into a whitelist, so that the lie is always
> > handled properly. The whitelist is in the sd.conf file.
> >
> > By contrast, the ZFSonLinux approach requires that the sysadmin knows
> > there is a lie and manually corrects for every invocation. This is
> > unfortunately a very error-prone approach.
> 
>  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?

> that you really want to force-create all pools with ashift=12 even on 512n drives
> unless you're absolutely confident that you will be able to obtain replacement
> 512n drives over the lifetime of your pool (or unless the impact of using
> ashift=12 is so drastic on your usage case that you will need to totally rethink
> your pool if you become unable to get 512n replacement drives).

Same as my comments above.
> 
>  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.


Thanks.

Fred