Re: [gentoo-user] External hard drive and idle activity

[Rich Freeman]

On Mon, Aug 3, 2020 at 1:30 AM Dale <rdalek1967@×××××.com> wrote:
>
> Little update here.  Rich, I think you mentioned it would slow down when it ran out of PMR space while trying to redo the shingled part.  Up until now, I hadn't ran into that issue.  It seems the PMR section for this drive is somewhere around 40 or 50GBs, maybe 60GBs.  I hadn't had time for backups in over a week so it was a good bit larger than usual.  It was around 70GBs, maybe 75.  When it got close to the end of the rsync process, I noticed it slowed down quite a bit.  I'd guess about half or so.  Usually it runs at around 180 to 190MBs/sec for larger files.  Pretty close to the end, rsync was showing around 100MBs/sec at best.  It was a little over on some but mostly a little below that.  Earlier in the process, it was the normal speed.

I doubt this particular drop is the result of SMR, assuming 100MB/s is
the instantaneous speed.  100MB/s is still reasonable for a hard drive
- on newer CMR drives I've seen the speed of dd drop from 200MB/s to
100MB/s for sequential writes as the heads move from one end of the
drive to the other, and then it goes back up to 200MB/s if you start
over at the beginning (badblocks testing and so on).

That level of drop is probably more likely to be due to filesystem
overhead and so on - fragmentation/etc.  When SMR buffer overrun
occurs you REALLY hit a wall and the rates drop quite a bit more than
that.  If it were a difference of only 50% most would probably
tolerate it.

Now, if 100MB/s were an updating average across the entire run then
that would be a different matter, because that would mean that it was
running at 200MB/s for most of the run, and then probably dropping
much closer to 0 for a while so as to drive the overall average down
to 100MB/s.  I'm not sure where you're getting those numbers from so I
don't know what period that 100MB/s reflects.  For an instantaneous
speed I'd consider it a completely reasonable performance for a
typical hard drive when you're writing to a filesystem.  If you were
using dd or maybe copies of very large files on an efficient
filesystem you would get better results.

-- 
Rich