Xen users - Oct 2010 - SW RAID powered Xen node - knowledgeable feedback much needed

I would like to know your opinion on the viability of a SW RAID 1 powered
Xen node. The CPU is a i7-920 Quad core, 2.66Ghz, 8MB cache and 8GB of RAM.
The 2 HDDs are absolutely normal (no raptor, 10k-15k etc - just normal
SATA) and there''s no HW RAID. I was thinking to allocate some 70% of
the
resources to VEs and the rest to the node itself.

Can this be done reasonably?

Adding HW raid would mean an 60% price increase which would lead to 60%
more VEs on the node. Would the 60% increase in VEs count but with HW RAID
be preferable over the SW RAID with less VEs?

Thanks a lot in advance for any knowledgeable feedback.


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> Adding HW raid would mean an 60% price increase which would lead to 60%
> more VEs on the node. Would the 60% increase in VEs count but with HW RAID
> be preferable over the SW RAID with less VEs?
Why would adding hardware raid result in a 60% increase in guests?  HW 
RAID won''t necessarily perform any better than SW RAID -- particularly 
depending on your workload -- so I wouldn''t count on being able to load
up more i/o.  I think Linux''s software RAID is great, I''ve
used it for
years, I''ve done things like mirror a local SATA disk to a remote iSCSI
disk for better cross-box redundancy, I''ve booted from it, run VMs from
it, etc., but I would still almost always prefer hardware raid as long 
as we''re talking about a decent (LSI, Mylex, Intel, Adaptec)
controller.

John




-- 
John Madden
Sr UNIX Systems Engineer / Office of Technology
Ivy Tech Community College of Indiana
jmadden@ivytech.edu

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John Madden wrote:
>>Adding HW raid would mean an 60% price increase which would lead to 60%
>>more VEs on the node. Would the 60% increase in VEs count but with HW
RAID
>>be preferable over the SW RAID with less VEs?
>
>Why would adding hardware raid result in a 60% increase in guests?
I suspect he meant that if the price goes up 60% to pay for hardware 
raid, then they need to put 60% more guests on it to pay for it.

So I suppose the question would be, if you were having to do that, 
would the extra performance of a hardware controller support that - 
and I think the answer is no.

As you say, a hardware raid controller isn''t necessarily faster than 
software raid. They "tend" to be simply because a good one will have 
a good processor and hardware to accelerate certain operations (such 
as parity computation for RAID 5). The processor may not be better 
than or even as good as, the one in the system - but they tend to win 
out by being dedicated and so available for full performance even 
when the system processor(s) is heavily loaded.

The other way that hardware raid can gain is by having battery backed 
cache and using write-back to cache (and delay) disk writes. That can 
have a significant effect on systems with a heavy disk I/O with 
significant writes. Such a cache in software raid wouldn''t be safe as 
I''m not aware of any battery backed memory being available and 
supported for such use - and losing the wrote cache would almost 
certainly cause corruption in the event of a power failure or unclean 
shutdown.

-- 
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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On 26/10/10 20:25, office@webtouch.eu wrote:
> I would like to know your opinion on the viability of a SW RAID 1 powered
> Xen node. The CPU is a i7-920 Quad core, 2.66Ghz, 8MB cache and 8GB of RAM.
> The 2 HDDs are absolutely normal (no raptor, 10k-15k etc - just normal
> SATA) and there''s no HW RAID. I was thinking to allocate some 70%
of the
> resources to VEs and the rest to the node itself.
>
> Can this be done reasonably?
>
> Adding HW raid would mean an 60% price increase which would lead to 60%
> more VEs on the node. Would the 60% increase in VEs count but with HW RAID
> be preferable over the SW RAID with less VEs?
>
> Thanks a lot in advance for any knowledgeable feedback.
>
>    
 From personal experience, I can say that unless you are going to spend 
a lot of money on an expensive RAID controller, just go with software 
RAID. We have servers that use Linux''s software RAID (mdadm) and it is 
fantastic!

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On Tue, Oct 26, 2010 at 4:06 PM, Jonathan Tripathy <jonnyt@abpni.co.uk>
wrote:
> We have servers that use Linux''s software RAID (mdadm) and it is
fantastic!
yes it is!

just be sure to have good hardware (a bad chasis meant a nightmare for
me once), and cards with good dirvers.

i once had a multiport SATA card with proprietary drivers.  very fast,
very CPU efficient, very stable.... but didn''t detect hot-swapped
drives.  IOW: i could change a drive while running, but to detect it,
i had to rmmod/insmod the driver.  aarrgghh!!!

-- 
Javier

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On Tuesday 26 October 2010 23:06:20 Jonathan Tripathy
wrote:
> On 26/10/10 20:25, office@webtouch.eu wrote:
> > I would like to know your opinion on the viability of a SW RAID 1
powered
> > Xen node. The CPU is a i7-920 Quad core, 2.66Ghz, 8MB cache and 8GB of
> > RAM. The 2 HDDs are absolutely normal (no raptor, 10k-15k etc - just
> > normal SATA) and there''s no HW RAID. I was thinking to
allocate some 70%
> > of the resources to VEs and the rest to the node itself.
> > 
> > Can this be done reasonably?
> > 
> > Adding HW raid would mean an 60% price increase which would lead to
60%
> > more VEs on the node. Would the 60% increase in VEs count but with HW
> > RAID be preferable over the SW RAID with less VEs?
> > 
> > Thanks a lot in advance for any knowledgeable feedback.
> 
>  From personal experience, I can say that unless you are going to spend
> a lot of money on an expensive RAID controller, just go with software
> RAID. We have servers that use Linux''s software RAID (mdadm) and
it is
> fantastic!
> 
> _______________________________________________
> Xen-users mailing list
> Xen-users@lists.xensource.com
> http://lists.xensource.com/xen-users

You have of course no goodies like battery backed up cache.

B.

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<office@webtouch.eu> writes:
> I would like to know your opinion on the viability of a SW RAID 1 powered
> Xen node. The CPU is a i7-920 Quad core, 2.66Ghz, 8MB cache and 8GB of RAM.
> The 2 HDDs are absolutely normal (no raptor, 10k-15k etc - just normal
> SATA) and there''s no HW RAID. I was thinking to allocate some 70%
of the
> resources to VEs and the rest to the node itself.

Yes. I did fairly extensive tests on hw vs software raid 1 and 1+0
and even with a battery backed cache (only 64MiB at the time, this
was a few years back)  the hw raid was slightly slower than the 
linux MD raid.  It was about the same.   

Now, with a larger (and battery-backed) cache and raid5 or 6 rather
than raid 1 or 1+0, the story may be different, but if you only
have two spindles, in my opinion, it''s better to spend the money
on more spindles.

Me, all my servers have 8 cores (a single 2.0ghz G34 opteron) 
32GiB of registered ecc ram and 4 "enterprise" sata disks in a 1+0 
linux MD raid.

The big thing you''ve gotta watch for are people who use a bunch 
of disk IO.  even one guest can make I/O suck for a lot of people
(and when I have better tools for doing that, I''ll post them here. 
Ideas are appreciated.)  

but overall, my system seems to work fairly well, even when I put 31GiB
worth of 512MiB, 256MiB, 128MiB and 64MiB guests on the same host.  
I/O does suck, but I don''t think we''re going to get around
that
without spending a whole lot on a raid card.

I have been looking at bcache[1]... when it''s done, it will be 
pretty awesome, but I don''t know how "done" it is just yet. 

[1]http://bcache.evilpiepirate.org/

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