Re: AWFUL reshape speed with raid5.

["Jon Nelson" <jnelson-linux-raid@jamponi.net>]

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?
3. I removed the bitmap for these tests. Having a bitmap meant that
the overall speed was REALLY HORRIBLE.

The results:

metadata: time taken

0.9: 27s
1.0: 27s
1.1: 37s
1.2: 1m34s

Questions (repeated):

1. Why does the metadata version matter so much?
2. If no I/O is taking place, why does it take so long? [ NOTE: I/O
must be taking place but why doesn't vmstat show it? ]

-- 
Jon