[TUHS] /dev/drum

Michael Kjörling michael at kjorling.se
Tue Apr 24 19:37:09 AEST 2018

On 23 Apr 2018 17:30 -0600, from tuhs at minnie.tuhs.org (Grant Taylor via TUHS):
> On 04/23/2018 04:15 PM, Warner Losh wrote:
>> It's weird. These days lower LBAs perform better on spinning
>> drives. We're seeing about 1.5x better performance on the first
>> 30% of a drive than on the last 30%, at least for read speeds for
>> video streaming....
> I think manufacturers have switched things around on us.  I'm used
> to higher LBA numbers being on the outside of the disk.  But I've
> seen anecdotal indicators that the opposite is now true.

I couldn't quite resist, so tried it out. Take this for what it is, an

Reading 10 GB in direct mode using dd with no skip at the beginning,
in 1 MiB blocks, gives me about 190 MB/s on one of the Seagate SAS
disks in my PC, and some 165 MB/s on one of the HGST SATA disks in the
same PC. Obviously, that's for purely sequential I/O, with very little
other I/O load.

Doing the same with an initial skip of 3,500,000 blocks (these are 4
TB drives, so this puts the read toward the outer limit), I get 105
MB/s on the Seagate SAS and 100 MB/s on the HGST SATA.

I did the same thing twice to make sure caching wasn't somehow
interfering with the values. The differences for all reported transfer
rates were marginal, and well within a reasonable margin of error.

That's definitely statistically significantly slower toward the outer
edge of the disk as presented by the OS. That _should_ translate to
slower for higher LBAs, but with all the magic happening in modern
systems, you might never know...

Of course, back in ye olden days, even 100 MB/s would have been
blazingly fast. Are we spoiled these days to think of throughputs on
the order of a gigabit per second as slow?

Michael Kjörling • https://michael.kjorling.semichael at kjorling.se
  “The most dangerous thought that you can have as a creative person
              is to think you know what you’re doing.” (Bret Victor)

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