performance bottleneck in Linux MD RAID-1

Bond Masuda bond.masuda at
Thu Jul 15 13:10:08 CDT 2010

> Call me crazy, but I'm guessing you never let it finish syncing in the
> first place.  When syncing, it caps its own throughput at some
> arbitrary amount the code deems sensible.  Sounds like it never
> un-capped, suggesting you never let it complete the sync process before
> benching it.  Further evidence that both numbers are so
> remarkably the same also suggests that.  Depending on the size of the
> dataset, it could take a while to finish.  I could swear that I recall
> there being some parameter which allows you to create a mirror but skip
> the sync step.  If you do that, then basically you're swearing on the
> storage bible that both volumes were zeroed before the mirror creation,
> and the first thing you did after creating the volume was mkfs.
> Personally I've never tried it, and searching the documentation, I
> can't find it now, so perhaps that was just a bad dream I had.

Hi Andrew,

Thanks for the response. Indeed, we did let it finish syncing.. we had to
let it run overnight to finish syncing. But, we also noticed that while it
was syncing /dev/sda to /dev/sdc, the sync rate as observed in /proc/mdstat
and iostat was limited to 200MB/sec.


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