[TUHS] /dev/drum

Warner Losh imp at bsdimp.com
Tue Apr 24 14:59:19 AEST 2018

On Mon, Apr 23, 2018 at 10:49 PM, Bakul Shah <bakul at bitblocks.com> wrote:

> On Mon, 23 Apr 2018 22:32:26 -0600 Grant Taylor via TUHS <
> tuhs at minnie.tuhs.org> wrote:
> >
> > I had always assumed that the outer edge (what I thought was the end of
> > the disk) was faster than the inner edge (what I thought was the
> > beginning of the disk) because of geometry.  However, as Ronald stated,
> > hard drives were constant angular density.  Thus negating what I
> > originally thought about speed.
> Constant angular velocity means faster "linear" velocity for
> tracks further away from the center.  Since 1990 or so disk
> tracks are divided up in 16 or so "zones", where outer zones
> have more blocks per track.  This translates to higher
> throughput.
> A modern Seagate Exos SAS disk may have a range of 279MB/s
> (outermost) to 136MB/s (innermost) or 300MB/s to 210MB/s for
> faster disks (15Krpm).  Disk vendors don't seem to break this
> range out for consumer drives. But you can measure it using
> tools like diskinfo on FreeBSD. For example:
> # diskinfo -t /dev/ada4 # this is an 5 year old 1TB WD "Black" disk.
> /dev/ada4
>         ...
>         Not_Zoned       # Zone Mode <<== this seems wrong.

That's right. This is for BIO_ZONE stuff, which has to do with host managed
and host aware SMR drive zones. That's different than the zones you are
talking about.

>         ...
> Transfer rates:
>         outside:       102400 kbytes in   0.972176 sec =   105331
> kbytes/sec
>         middle:        102400 kbytes in   1.088977 sec =    94033
> kbytes/sec
>         inside:        102400 kbytes in   1.804460 sec =    56748
> kbytes/sec

Yes. This matches our experience where we get 1.5x better on the low LBAs
than the high LBAs. We're looking to 'short stroke' the drive to the first
part of it to get better performance... Toss a filesystem on top of it, and
have a more random workload and it's down to about 30% better than using
the whole drive....

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