Re: raid5 to raid6 - reshape very slow

[NeilBrown <neilb@suse.de>]

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On Tue, 29 May 2012 10:33:52 +1000 Jonathan Molyneux
<jonathan@infinitedepth.com.au> wrote:

> Hi Everyone,
> 
> I'm currently reshaping my raid5 array into a raid6 array (by adding a 
> spare and growing).
> 
> Unfortunately I didn't research the nuances before doing so and missed 
> the opportunity to --layout=preserve.
> 
> As a result the array is rebuilding by use of a backup-file, at a rather 
> slow pace.
> 
> md1 : active raid6 sde1[0] sdb1[6] sdh1[5] sdc1[4] sdg1[3] sdd1[2] sdf1[1]
>        7325679680 blocks super 0.91 level 6, 64k chunk, algorithm 18 
> [7/6] [UUUUUU_]
>        [=========>...........]  reshape = 47.0% (689413888/1465135936) 
> finish=4126.9min speed=3132K/sec
> 
> I have tried the following so far:
> 
> * read ahead tweaking
> * stripe_cache_size tweaking (not a limiting factor - mdadm seems to 
> tune this appropriately)
> * running the backup file from dev/shm (only for testing purposes)
> 
> The reshape does not seem to be IO or CPU bound, it just seems to be 
> limited by the sync's and meta data updates to each disk.
> 
> Is it possible to increase the buffer/stripes that mdadm process in a 
> batch to speed up the process (~41m/2 -> 1G) ?
> 
> Any thoughts would be appreciated (even to confirm there is nothing to 
> be done).

The only thing to do is to wait.  This is very much a seek-bound operation
and there is little room for making it go faster.

In a few months we might see the need for the backup file disappear, which
will improve things somewhat, but it will still be slowish.

NeilBrown

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