Re: AWFUL reshape speed with raid5.

Jon Nelson wrote:

Some more data points, observations, and questions.

For each test, I'd --create the array, drop the caches, --grow, and
then watch vmstat and also record the time between

kernel: md: resuming resync of md99 from checkpoint.
and
kernel: md: md99: resync done.

I found two things:

1. metadata version matters. Why?
2. VERY LITTLE I/O takes place (between 0 and 100KB/s, typically no
I/O at all) according to vmstat. Why? If it takes 1m34s to "grow" the
array, but no I/O is taking place, then what is actually taking so
long?

I *think* that internal md io is not being shown.

I know I can tell an array to check itself, and have mdstat indicate a speed of 35MB/second and vmstat indicates no IO  was happening.   The same happens when an array is rebuilding, vmstat indicates no IO.   If you do IO to the md device from outside it does show that.   And in both cases visually checking the confirms that quite a lot appears to be going on.

                              Roger