Yep, I should have added those requirements :-)

1) Yes I care about the data. It’s not scratch but a permanent repository of 
older, less frequently accessed data.
2) Yes, it will be backed up
3) I expect it to grow over time
4) Data integrity requirement: high

Bob Oesterlin
Sr Principal Storage Engineer, Nuance

From: <> on behalf of Stephen Ulmer 
Reply-To: gpfsug main discussion list <>
Date: Thursday, December 1, 2016 at 7:13 AM
To: gpfsug main discussion list <>
Subject: [EXTERNAL] Re: [gpfsug-discuss] Strategies - servers with local SAS 

Just because I don’t think I’ve seen you state it:  (How much) Do you care 
about the data?

Is it scratch? Is it test data that exists elsewhere? Does it ever flow from 
this storage to any other storage? Will it be dubbed business critical two 
years after they swear to you that it’s not important at all? Is it just your 
movie collection? Are you going to back it up? Is it going to grow? Is this 

That would inform us about the level of integrity required, which is one of the 
main differentiators for the options you’re considering.



On Dec 1, 2016, at 7:47 AM, Oesterlin, Robert 
<<>> wrote:

Some interesting discussion here. Perhaps I should have been a bit clearer on 
what I’m looking at here:

I have 12 servers with 70*4TB drives each – so the hardware is free. What’s the 
best strategy for using these as GPFS NSD servers, given that I don’t want to 
relay on any “bleeding edge” technologies.

1) My first choice would be GNR on commodity hardware – if IBM would give that 
to us. :-)
2) Use standard RAID groups with no replication – downside is data availability 
of you lose an NSD and RAID group rebuild time with large disks
3) RAID groups with replication – but I lose a LOT of space (20% for RAID + 50% 
of what’s left for replication)
4) No raid groups, single NSD per disk, single failure group per servers, 
replication. Downside here is I need to restripe every time a disk fails to get 
the filesystem back to a good state. Might be OK using QoS to get the IO impact 
5) FPO doesn’t seem to by me anything, as these are straight NSD servers and no 
computation is going on these servers, and I still must live with the re-stripe.

Option (4) seems the best of the “no great options” I have in front of me.

Bob Oesterlin
Sr Principal Storage Engineer, Nuance

 on behalf of Zachary Giles <<>>
Reply-To: gpfsug main discussion list 
Date: Wednesday, November 30, 2016 at 10:27 PM
To: gpfsug main discussion list 
Subject: [EXTERNAL] Re: [gpfsug-discuss] Strategies - servers with local SAS 

Aaron, Thanks for jumping onboard. It's nice to see others confirming this. 
Sometimes I feel alone on this topic.

It's should also be possible to use ZFS with ZVOLs presented as block devices 
for a backing store for NSDs. I'm not claiming it's stable, nor a good idea, 
nor performant.. but should be possible. :) There are various reports about it. 
Might be at least worth looking in to compared to Linux "md raid" if one truly 
needs an all-software solution that already exists.  Something to think about 
and test over.

On Wed, Nov 30, 2016 at 11:15 PM, Aaron Knister 
<<>> wrote:
Thanks Zach, I was about to echo similar sentiments and you saved me a ton of 
typing :)

Bob, I know this doesn't help you today since I'm pretty sure its not yet 
available, but if one scours the interwebs they can find mention of something 
called Mestor.

There's very very limited information here:

 (slide 20)

Sounds like if it were available it would fit this use case very well.

I also had preliminary success with using sheepdog 
 as a backing store for GPFS in a similar situation. It's perhaps at a very 
high conceptually level similar to Mestor. You erasure code your data across 
the nodes w/ the SAS disks and then present those block devices to your NSD 
servers. I proved it could work but never tried to to much with it because the 
requirements changed.

My money would be on your first option-- creating local RAIDs and then 
replicating to give you availability in the event a node goes offline.


On 11/30/16 10:59 PM, Zachary Giles wrote:
Just remember that replication protects against data availability, not
integrity. GPFS still requires the underlying block device to return
good data.

If you're using it on plain disks (SAS or SSD), and the drive returns
corrupt data, GPFS won't know any better and just deliver it to the
client. Further, if you do a partial read followed by a write, both
replicas could be destroyed. There's also no efficient way to force use
of a second replica if you realize the first is bad, short of taking the
first entirely offline. In that case while migrating data, there's no
good way to prevent read-rewrite of other corrupt data on your drive
that has the "good copy" while restriping off a faulty drive.

Ideally RAID would have a goal of only returning data that passed the
RAID algorithm, so shouldn't be corrupt, or made good by recreating from
parity. However, as we all know RAID controllers are definitely prone to
failures as well for many reasons, but at least a drive can go bad in
various ways (bad sectors, slow, just dead, poor SSD cell wear, etc)
without (hopefully) silent corruption..

Just something to think about while considering replication ..

On Wed, Nov 30, 2016 at 11:28 AM, Uwe Falke 
<<>>> wrote:

    I have once set up a small system with just a few SSDs in two NSD
    providin a scratch file system in a computing cluster.
    No RAID, two replica.
    works, as long the admins do not do silly things (like rebooting servers
    in sequence without checking for disks being up in between).
    Going for RAIDs without GPFS replication protects you against single
    failures, but you're lost if just one of your NSD servers goes off.

    FPO makes sense only sense IMHO if your NSD servers are also processing
    the data (and then you need to control that somehow).

    Other ideas? what else can you do with GPFS and local disks than
    what you
    considered? I suppose nothing reasonable ...

    Mit freundlichen Grüßen / Kind regards

    Dr. Uwe Falke

    IT Specialist
    High Performance Computing Services / Integrated Technology Services /
    Data Center Services
    IBM Deutschland
    Rathausstr. 7
    09111 Chemnitz
    Phone: +49 371 6978 2165<tel:%2B49%20371%206978%202165> 
    Mobile: +49 175 575 2877<tel:%2B49%20175%20575%202877> 
    IBM Deutschland Business & Technology Services GmbH / Geschäftsführung:
    Frank Hammer, Thorsten Moehring
    Sitz der Gesellschaft: Ehningen / Registergericht: Amtsgericht
    HRB 17122

    From:   "Oesterlin, Robert" 
    To:     gpfsug main discussion list
    Date:   11/30/2016 03:34 PM
    Subject:        [gpfsug-discuss] Strategies - servers with local SAS
    Sent by:<>

    Looking for feedback/strategies in setting up several GPFS servers with
    local SAS. They would all be part of the same file system. The
    systems are
    all similar in configuration - 70 4TB drives.

    Options I?m considering:

    - Create RAID arrays of the disks on each server (worried about the RAID
    rebuild time when a drive fails with 4, 6, 8TB drives)
    - No RAID with 2 replicas, single drive per NSD. When a drive fails,
    recreate the NSD ? but then I need to fix up the data replication via
    - FPO ? with multiple failure groups -  letting the system manage
    placement and then have GPFS due the restripe on disk failure

    Comments or other ideas welcome.

    Bob Oesterlin
    Sr Principal Storage Engineer, Nuance
    507-269-0413<tel:507-269-0413> <tel:507-269-0413<tel:507-269-0413>>

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Zach Giles<> 

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Aaron Knister
NASA Center for Climate Simulation (Code 606.2)
Goddard Space Flight Center
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Zach Giles<>
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