[gpfsug-discuss] IO performance of replicated GPFS filesystem

[Marc A Kaplan]

Generally yes.  When reading, more disks is always better then fewer 
disks, both for replication and with striping over several or many disks.
When writing, more disks is good with striping.  But yes, replication 
costs you extra writes.  Those writes don't necessarily cost you loss of 
time, provided they can be done concurrently.

When I write "disks" I mean storage devices that can be accessed 
concurrently.   Watch out for virtual LUNs. 
With conventional controllers and drives, it does GPFS little or no good 
when multiple LUNs map to the same real disk device, since multiple 
operations to different LUNs will ultimately be serialized at 
one real disk arm/head!

For high performance, you should not be thinking about "two NSDs" ... you 
should be thinking about many NSD, so data and metadata can be striped, 
and written and read concurrently.
But yes, for replication purposes you have to consider defining and 
properly configuring at least two "failure groups".



From:   "Tomasz.Wolski at ts.fujitsu.com" <Tomasz.Wolski at ts.fujitsu.com>
To:     "gpfsug-discuss at spectrumscale.org" 
<gpfsug-discuss at spectrumscale.org>
Date:   11/30/2015 05:46 AM
Subject:        [gpfsug-discuss] IO performance of replicated GPFS 
filesystem
Sent by:        gpfsug-discuss-bounces at spectrumscale.org



Hi All,
 
I could use some help of the experts here J Please correct me if I’m 
wrong: I suspect that GPFS filesystem READ performance is better when 
filesystem is replicated to i.e. two failure groups, where these failure 
groups are placed on separate RAID controllers. In this case WRITE 
performance should be worse, since the same data must go to two locations. 
What about situation where GPFS filesystem has two metadataOnly NSDs which 
are also replicated? Does metadata READ performance increase in this way 
as well (and WRITE decreases)?
 
Best regards,
Tomasz Wolski_______________________________________________
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