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

Fred Liu Fred_Liu at issi.com
Tue Mar 22 10:00:50 UTC 2016

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    > -----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

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