Re: RAID5 / 6 Growth

[Goswin von Brederlow <goswin-v-b@web.de>]

"Leslie Rhorer" <lrhorer@satx.rr.com> writes:

>> > High enough?  Wouldn't a higher speed limit mean more stress on the
>> systems?
>> > Its value is 1000.
>> 
>> A higher min value will block more normal IO (if there is
>> any).
>
> 	No, not really.  All the system that is currently being grown does
> is incrementally rsync the data from the main server every morning.  The
> main server, which will be grown after this server is done, is another
> matter.
>
>> Raising min is usefull to ensure the job gets done in a certain
>> time, to not let normal IO slow down a rebuild too much.
>
> 	OK, but like I said, for the most part there isn't any other I/O.
>  
>> >> MB/s) along with /proc/sys/dev/raid/speed_limit_max?
>> >
>> > It's 200,000
>> 
>> I only ever had to tune this once when too much IO would deadlock an
>> external enclosure. Otherwise keep this really high so it uses all
>> idle IO there is.
>
> 	200MBps is far more than the system can handle.  These are consumer
> class drives on a relatively inexpensive 4 port controller feeding a Port
> Multiplier chassis.
>
>> As to your initial question: Being able to keep the filesystem mounted
>> and used is the whole point of having online growing of the raid
>> system. If that weren't save then there would be no point to it as you
>> could just as well stop the raid if you already umounted it and grow
>> it offline.
>
> 	I know that is the point of the utility.  My question boils down to,
> "How safe is it to avail one's self of the capability if it is not essential
> to have the array mounted for the duration?"  I don't particularly like
> having the array unavailable (especially not for nearly 5 days), but I
> prefer that to risking data loss, or especially risking irretrievably losing
> the entire array.  The question is particularly pertinent given the fact the
> growth is going to take nearly 5 days (a lot can happen in 5 days), and the
> fact the system was having the rather squirrelly issue a few days back which
> seems - emphasis on SEEMS - to have been resolved by disabling NCQ.  What
> happens if the system kicks a couple of drives, especially if one drive gets
> kicked, a bunch of data gets written and then a few minutes later another
> drive gets kicked?  In particular, what if neither of the two drives that
> get kicked are the new drive?

It should just keep going. I don't see how having the filesystem in
use will have any influence there other than adding a little more load
on the disks. Reshaping is heavy on the drive. Lots of reads, writes
and seeks. A daily rsync probably doesn't even register as extra load.

MfG
        Goswin