Discussion:
SSDs - RAID or Mirror ?
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Evan Harris
2014-10-15 23:01:24 UTC
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Hi all

I have a customer who has a ordered a system with 4 x 387Gb SSDs in the CEC
and 24 x 283Gb disks housed in a 5887.

With respect to the SSDs, what is the better approach - Mirroring or RAID
? The system is not pushed for space so mirroring is an option if there is
a reason for doing that, otherwise either approach works fine for this
install.

I normally prefer to hot spare RAID sets these days and I guess I could do
a 3 disk RAID set and hot spare it (not my preference...) but in that case
it seems to me MIrroring would probably be a better approach.

Any opinions as to which approach is better ?
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Evan Harris
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DrFranken
2014-10-15 23:29:24 UTC
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Mirror. Done.


- Larry "DrFranken" Bolhuis

www.frankeni.com
www.iDevCloud.com
www.iInTheCloud.com
Post by Evan Harris
Hi all
I have a customer who has a ordered a system with 4 x 387Gb SSDs in the CEC
and 24 x 283Gb disks housed in a 5887.
With respect to the SSDs, what is the better approach - Mirroring or RAID
? The system is not pushed for space so mirroring is an option if there is
a reason for doing that, otherwise either approach works fine for this
install.
I normally prefer to hot spare RAID sets these days and I guess I could do
a 3 disk RAID set and hot spare it (not my preference...) but in that case
it seems to me MIrroring would probably be a better approach.
Any opinions as to which approach is better ?
--
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Evan Harris
2014-10-15 23:53:06 UTC
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Hi Larry

that's what I have done in the past and that is my preference. Is there an
IBM best practise recommendation I can refer to or do that you know of or
do you have some logic as to why (like, because they are SSD's)

I'm guessing you have something due to the quick answer.. :)
Post by DrFranken
Mirror. Done.
- Larry "DrFranken" Bolhuis
www.frankeni.com
www.iDevCloud.com
www.iInTheCloud.com
Hi all
Post by Evan Harris
I have a customer who has a ordered a system with 4 x 387Gb SSDs in the CEC
and 24 x 283Gb disks housed in a 5887.
With respect to the SSDs, what is the better approach - Mirroring or RAID
? The system is not pushed for space so mirroring is an option if there is
a reason for doing that, otherwise either approach works fine for this
install.
I normally prefer to hot spare RAID sets these days and I guess I could do
a 3 disk RAID set and hot spare it (not my preference...) but in that case
it seems to me MIrroring would probably be a better approach.
Any opinions as to which approach is better ?
--
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--
Regards
Evan Harris
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DrFranken
2014-10-16 00:32:34 UTC
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Mirror will nearly always outperform RAID.
Mirror is always two writes for a write, one read (with two choices) for
a read.
RAID 5 is two reads and two writes for a write, one read (with one
choice) for a read.

SSDs are so fast you can do mirroring on some of the lower cost
controllers with NO Cache. In fact some are SSD only.

I can't put my finger on it just now but Sue Baker has frequently
indicated Mirror over RAID as well for SSDs.

- Larry "DrFranken" Bolhuis

www.frankeni.com
www.iDevCloud.com
www.iInTheCloud.com
Post by Evan Harris
Hi Larry
that's what I have done in the past and that is my preference. Is there an
IBM best practise recommendation I can refer to or do that you know of or
do you have some logic as to why (like, because they are SSD's)
I'm guessing you have something due to the quick answer.. :)
Post by DrFranken
Mirror. Done.
- Larry "DrFranken" Bolhuis
www.frankeni.com
www.iDevCloud.com
www.iInTheCloud.com
Hi all
Post by Evan Harris
I have a customer who has a ordered a system with 4 x 387Gb SSDs in the CEC
and 24 x 283Gb disks housed in a 5887.
With respect to the SSDs, what is the better approach - Mirroring or RAID
? The system is not pushed for space so mirroring is an option if there is
a reason for doing that, otherwise either approach works fine for this
install.
I normally prefer to hot spare RAID sets these days and I guess I could do
a 3 disk RAID set and hot spare it (not my preference...) but in that case
it seems to me MIrroring would probably be a better approach.
Any opinions as to which approach is better ?
--
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Evan Harris
2014-10-16 01:39:55 UTC
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Hi Larry

Thanks for taking the trouble to reply.

I'd be interested to see what Sue comes up with but you've confirmed what
I'm thinking so I'll just go with that.
Post by DrFranken
Mirror will nearly always outperform RAID.
Mirror is always two writes for a write, one read (with two choices) for a
read.
RAID 5 is two reads and two writes for a write, one read (with one choice)
for a read.
SSDs are so fast you can do mirroring on some of the lower cost
controllers with NO Cache. In fact some are SSD only.
I can't put my finger on it just now but Sue Baker has frequently
indicated Mirror over RAID as well for SSDs.
- Larry "DrFranken" Bolhuis
www.frankeni.com
www.iDevCloud.com
www.iInTheCloud.com
Hi Larry
Post by Evan Harris
that's what I have done in the past and that is my preference. Is there an
IBM best practise recommendation I can refer to or do that you know of or
do you have some logic as to why (like, because they are SSD's)
I'm guessing you have something due to the quick answer.. :)
Mirror. Done.
Post by DrFranken
- Larry "DrFranken" Bolhuis
www.frankeni.com
www.iDevCloud.com
www.iInTheCloud.com
Hi all
Post by Evan Harris
I have a customer who has a ordered a system with 4 x 387Gb SSDs in the CEC
and 24 x 283Gb disks housed in a 5887.
With respect to the SSDs, what is the better approach - Mirroring or RAID
? The system is not pushed for space so mirroring is an option if there is
a reason for doing that, otherwise either approach works fine for this
install.
I normally prefer to hot spare RAID sets these days and I guess I could do
a 3 disk RAID set and hot spare it (not my preference...) but in that case
it seems to me MIrroring would probably be a better approach.
Any opinions as to which approach is better ?
--
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Regards
Evan Harris
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r***@public.gmane.org
2014-10-16 11:56:53 UTC
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We have similar systems, although the size of the drives in my expansion
unit dwarf yours.
Of those four drives in the CEC two are mirrored to one VIOS partition.
Two are mirrored to another VIOS partition.
All the drives in the expansion unit are raid 5 with hot spare to one
partition of IBM i which then hosts disk for multiple partitions of IBM i
and one partition of AIX.
I have three Power 8's set up this way.


Rob Berendt
--
IBM Certified System Administrator - IBM i 6.1
Group Dekko
Dept 1600
Mail to: 2505 Dekko Drive
Garrett, IN 46738
Ship to: Dock 108
6928N 400E
Kendallville, IN 46755
http://www.dekko.com





From: Evan Harris <auctionitis-***@public.gmane.org>
To: Midrange Systems Technical Discussion <midrange-l-Zwy7GipZuJhWk0Htik3J/***@public.gmane.org>
Date: 10/15/2014 07:01 PM
Subject: SSDs - RAID or Mirror ?
Sent by: "MIDRANGE-L" <midrange-l-bounces-Zwy7GipZuJhWk0Htik3J/***@public.gmane.org>



Hi all

I have a customer who has a ordered a system with 4 x 387Gb SSDs in the
CEC
and 24 x 283Gb disks housed in a 5887.

With respect to the SSDs, what is the better approach - Mirroring or RAID
? The system is not pushed for space so mirroring is an option if there is
a reason for doing that, otherwise either approach works fine for this
install.

I normally prefer to hot spare RAID sets these days and I guess I could do
a 3 disk RAID set and hot spare it (not my preference...) but in that case
it seems to me MIrroring would probably be a better approach.

Any opinions as to which approach is better ?
--
Regards
Evan Harris
--
This is the Midrange Systems Technical Discussion (MIDRANGE-L) mailing
list
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Sue Baker
2014-10-17 19:45:04 UTC
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Post by Evan Harris
I have a customer who has a ordered a system with 4 x 387Gb
SSDs in the CEC and 24 x 283Gb disks housed in a 5887.
With respect to the SSDs, what is the better approach -
Mirroring or RAID
In this case, I would mirror because you cannot create 2x parity
sets with 4 drives. Latency introduced by a single parity set
would cause your customer to question the investment in SSDs.
--
Sue
IBM Americas Advanced Technical Sales Support (ATS) Power
Systems
Rochester, MN
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Evan Harris
2014-10-17 20:36:28 UTC
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Thanks Sue, I appreciate the feedback.
Post by Sue Baker
Post by Evan Harris
I have a customer who has a ordered a system with 4 x 387Gb
SSDs in the CEC and 24 x 283Gb disks housed in a 5887.
With respect to the SSDs, what is the better approach -
Mirroring or RAID
In this case, I would mirror because you cannot create 2x parity
sets with 4 drives. Latency introduced by a single parity set
would cause your customer to question the investment in SSDs.
--
Sue
IBM Americas Advanced Technical Sales Support (ATS) Power
Systems
Rochester, MN
--
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--
Regards
Evan Harris
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James H. H. Lampert
2014-10-17 21:05:28 UTC
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Allow me to point out that mirroring IS a type of RAID (specifically,
RAID 1).

;-p

--
JHHL
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DrFranken
2014-10-18 15:09:57 UTC
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And a Yugo was classified as an automobile....

- Larry "DrFranken" Bolhuis

www.frankeni.com
www.iDevCloud.com
www.iInTheCloud.com
Post by James H. H. Lampert
Allow me to point out that mirroring IS a type of RAID (specifically,
RAID 1).
;-p
--
JHHL
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James H. H. Lampert
2014-10-20 15:29:30 UTC
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Post by DrFranken
And a Yugo was classified as an automobile....
Uh, yes, but a Yugo was a cheap, basic, lowest-bidder version of an
automobile (although I can't imagine that they could possibly be any
worse than the ever-popular Citroen 2CV), whereas full mirroring, though
simple and basic, is the most extravagant form of RAID, such that
failure of the whole array requires not only the failure of at least two
drives, but of two drives in the same mirrored pair.

--
JHHL
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Sue Baker
2014-10-20 18:58:50 UTC
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Post by James H. H. Lampert
is the most extravagant form of RAID, such that
failure of the whole array requires not only the failure of at
least two drives, but of two drives in the same mirrored pair
Mirroring protects from more than disk drive failures.

I have customers running with multiple CEC POWER6 boxes who have
processor node level mirroring. Meaning they can lose a whole
processor enclosure and continue to operate. Before getting
paired or high availability RAID adapters this was not possible
.... for any operating system.

RAID 10 with the EJ0L adapters properly placed in I/O enclosures
can create similar level or redundancy except when an adapter
fails, write cache will be disabled and performance will suffer.
A properly validated mirrored config can even keep the write
cache operational with an adapter failure in some situations.
--
Sue
IBM Americas Advanced Technical Sales Support (ATS) Power
Systems
Rochester, MN
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DrFranken
2014-10-20 19:56:59 UTC
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One needs to have progressed beyond "AS/400 Technician" to build this. :-)

- Larry "DrFranken" Bolhuis

www.frankeni.com
www.iDevCloud.com
www.iInTheCloud.com
Post by Sue Baker
I have customers running with multiple CEC POWER6 boxes who have
processor node level mirroring. Meaning they can lose a whole
processor enclosure and continue to operate. Before getting
paired or high availability RAID adapters this was not possible
.... for any operating system.
RAID 10 with the EJ0L adapters properly placed in I/O enclosures
can create similar level or redundancy except when an adapter
fails, write cache will be disabled and performance will suffer.
A properly validated mirrored config can even keep the write
cache operational with an adapter failure in some situations.
--
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