"md/raid10:md5: sdf: redirecting sector 2979126480 to another mirror"

[Andy Smith <andy@strugglers.net>]

Hi,

"md/raid10:md5: sdf: redirecting sector 2979126480 to another
mirror"

I've actually been seeing these messages very occasionally across
many machines for years but have never found anything wrong so kept
putting investigation of it to the bottom of my TODO list. I have
even in the past upon seeing this done a full scrub and check and
found no issue.

Having just seen one of them again now, and having some spare time I
tried to look into it.

So, this messages comes from here:

    https://github.com/torvalds/linux/blob/v5.8/drivers/md/raid10.c#L1188

but under what circumstances does it actually happen?

This time, as with the other times, I cannot see any indication of
read error (i.e. no logs of that) and no problems apparent in SMART
data.

err_rdev there can only be set inside the block above that starts
with:


    if (r10_bio->devs[slot].rdev) {
        /*
         * This is an error retry, but we cannot
         * safely dereference the rdev in the r10_bio,
         * we must use the one in conf.

…but why is this an error retry? Nothing was logged so how do I find
out what the error was?

Cheers,
Andy