Re: AWFUL reshape speed with raid5.

On Wed, Jul 30, 2008 at 12:08 PM, Justin Piszcz [off-list ref] wrote:

In dstat, the speed is doubled for the total for some reason, divide by 2
(if you compare with iostat -x -k 1) you should see the difference.

Ah, nice catch. It still doesn't change my question.
Upon further reflection, I believe I know what has caused the
read/write disparity:

1. I've done so much testing with these devices that the contents have
been zeroed many times.
2. I am GUESSING that if the raid recovery code reads from drives A,
B, and C, builds the appropriate checksum and verifies it, if the
checksum matches it skips the write. I believe that this is what is
happening. To confirm, I wrote a few gigs of /dev/urandom to one of
the devices and re-tested. Indeed, this time around I saw plenty of
writing. One mystery solved.

Remaining questions:

1. Why does the version of metadata matter so much in a --grow --size operation?
2. There appear to be bugs when a bitmap is used. Can somebody else confirm?
3. I'll look into the awful speed thing later as that doesn't seem to
be an issue.

-- 
Jon