Re: limits on raid

On Sat, 16 Jun 2007, Neil Brown wrote:

It would be possible to have a 'this is not initialised' flag on the
array, and if that is not set, always do a reconstruct-write rather
than a read-modify-write.  But the first time you have an unclean
shutdown you are going to resync all the parity anyway (unless you
have a bitmap....) so you may as well resync at the start.

And why is it such a big deal anyway?  The initial resync doesn't stop
you from using the array.  I guess if you wanted to put an array into
production instantly and couldn't afford any slowdown due to resync,
then you might want to skip the initial resync.... but is that really
likely?

in my case it takes 2+ days to resync the array before I can do any 
performance testing with it. for some reason it's only doing the rebuild 
at ~5M/sec (even though I've increased the min and max rebuild speeds and 
a dd to the array seems to be ~44M/sec, even during the rebuild)

I want to test several configurations, from a 45 disk raid6 to a 45 disk 
raid0. at 2-3 days per test (or longer, depending on the tests) this 
becomes a very slow process.

also, when a rebuild is slow enough (and has enough of a performance 
impact) it's not uncommon to want to operate in degraded mode just long 
enought oget to a maintinance window and then recreate the array and 
reload from backup.

David Lang