Re: very slow RAID-1 resync

[Neil Brown]

On Monday October 15, [e-mail] wrote:
> I just plugged in a new RAID-1(+0, 2 2-disk stripe sets mirrored) to a
> 2.4.12-ac3 machine.  The md code decided it was going to resync the mirror
> at between 100KB/sec and 100000KB/sec.  The actual rate was 100KB/sec,
> while the device was otherwise idle.  By increasing
> /proc/.../speed_limit_min, I was able to crank the resync rate up to
> 20MB/sec, which is slightly more reasonable but still short of the
> ~60MB/sec this RAID is capable of.
> 
> So, two things: there is something wrong with the resync code that makes
> it run at the minimum rate even when the device is idle, and why is the
> resync proceeding so slowly?

The way that it works out where there is other activity on the drives
is a bit fragile.  It works particularly badly when the underlying
devices are md devices.

I would recommend that instead of mirroring 2 stipe sets, you stripe
two mirrored pairs.  The resync should be faster and the resilience to
failure is much better.

NeilBrown