[OmniOS-discuss] slices & zfs

[Gordon Ross]

One thing to watch out for when using slices is that you can easily
end up with a slices that are not 4KB aligned.  That doesn't matter
when the underlying device truly uses 512 byte sectors, but these days
a _lot_ of devices "lie" about that, and advertise 512 byte sectors
when in reality you pay a (possibly severe) performance penalty for
doing I/O that's not 4K aligned (or larger).
Unfortunately, the format tools and the default "fake" geometry
(cylinders, heads, sectors) typically gives you un-aligned slices.

A while back I took a stab at making format do "megabyte aligned
partitions" (which is what most OS's with slices or partitions do now)
but that work never quite made it to completion.
(It did get far enough so I could setup my dual boot SSD with aligned
partitions:)

Gordon


On Wed, Jun 8, 2016 at 1:28 PM, Richard Elling
<richard.elling at richardelling.com> wrote:
>
> On Jun 8, 2016, at 12:24 AM, Jim Klimov <jimklimov at cos.ru> wrote:
>
> 8 июня 2016 г. 7:00:50 CEST, Martijn Fennis <martijn at fennis.tk> пишет:
>
> Hi,
>
>
> Does someone have some info about setting up ZFS on top of slices ?
>
> Having 10 drives which i would like to have the outer ring configured @
> raid 10 for about 1 TB with dedup, the rest of the drives in Raid 50.
>
>
> What i find on google is to use complete devs and difficult to
> interpret slicing info..
>
>
>
> Thanks,
> Martijn
>
> Verzonden vanuit Mail voor Windows 10
>
>
>
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>
> You just set up slices (with format utility on one disk, replicate setup
> with prtvtoc to others if they are identical). Then you use slice numbers as
> vdevs to zpool command. Note that slices s0 and s2 are reserved, so you have
> s1 and s3-s7 to play with. (X86 may expose s8 and s9 - also reserved).
>
>
> This is not correct. For SMI labels, s2, by convention, defaults to the
> whole disk — aka the “backup” partition (slice).
> Since around 1980, I know of no backup software that expects s2 to be the
> whole disk. This was also an overlapping
> slice. For modern disks and EFI labels, overlapping slices are not permitted
> — thus preventing many common mistakes
> that led to corrupted data on the old SMI-labeled drives.
>  — richard
>
>
>
> Also note that dedup may require some RAM and maybe a separate fast SSD to
> cache reads and writes, to be efficient. Otherwise it can bog you down below
> single mb/s of i/o.
>
> Jim
> --
> Typos courtesy of K-9 Mail on my Samsung Android
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> --
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> Richard.Elling at RichardElling.com
> +1-760-896-4422
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>
>
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