[Info-vax] SET VOLUME/SIZE and DVE

[John Santos]

In article <kb7aso$kq3$1 at online.de>, helbig at astro.multiCLOTHESvax.de 
says...> 
> In article <kb79vk$a37$1 at dont-email.me>, Stephen Hoffman
> <seaohveh at hoffmanlabs.invalid> writes: 
> 
> > On 2012-12-23 15:55:47 +0000, Phillip Helbig---undress to reply said:
> > 
> > > When using dynamic volume expansion, at some point I will have a shadow
> > > set where all disks have a logical size less than the physical size and
> > > will a) want to add another disk of the same (or larger) physical size
> > > to it and b) increase the logical size to the physical size of the
> > > smallest member...
> > 
> > Enabling Dynamic Volume Expansion (DVE) <http://labs.hoffmanlabs.com/node/207>
> 
> Thanks (and it works on VMS), and interesting info about bugs 
> (fortunately there are workarounds).  However, I still have the question 
> if shadow copy before expansion is better or if it doesn't matter.

Don't really know the answer to your question (I suspect in the broader
picture, it doesn't really matter much) but I do have a couple of
observations...

1) You mentioned some place you want to add a 36GB disk to a shadow set
currently with a 9GB size, but consisting of 36GB (or larger) disks.

If the smallest disk in the existing shadow set is actually 36.01GB and
your new disk is 36.00GB, then if you expand first, you won't be able to
add the new disk!  (Unless you $ set volume/size=<exact size of new 
disk> first.)  So adding first is slightly safer.

2) Not relevant to your case, but a gotcha I've experienced in expanding
units on an MSA1000.  (Not sure if newer or better SAN controllers are
smarter or better behaved.)  I was expanding both members of a shadow
set at the same time.  While unit expansion is happening, the hardware
caching in the MSA is pretty much disabled, at least for the unit being
expanded.  For a mostly-reads environment, if a single member is 
expanding, the reads tend to get done on the other member and there is
only a slight degradation (10-20%).  Expanding both members at the same
time is a disaster!  Access to the shadow set gets 4 to 10 times slower,
400% to 1000% degradation.

-- 
John Santos
Evans Griffiths & Hart, Inc.