[NTLUG:Discuss] Dell Server with PERC RAID

[Neil Aggarwal]

Chris:

OK, I see it.  They are both mirroring and striping, but
in different orders.  One is a mirror of striped drives
and the other is a stripe of mirrored drives.  I can
see why that would make a difference.

Thank you for the explanation.

Thanks,
	Neil


--
Neil Aggarwal, JAMM Consulting, (972)612-6056, www.JAMMConsulting.com
FREE! Valuable info on how your business can reduce operating costs by 
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> -----Original Message-----
> From: discuss-bounces at ntlug.org 
> [mailto:discuss-bounces at ntlug.org] On Behalf Of Chris Cox
> Sent: Friday, September 26, 2003 10:57 AM
> To: NTLUG Discussion List
> Subject: Re: [NTLUG:Discuss] Dell Server with PERC RAID
> 
> 
> Neil Aggarwal wrote:
> > Chris:
> > 
> > I looked at the referenced articles and I don't see what the 
> > difference is between RAID 0+1 and RAID 10.
> 
> Uhh... the LSI pictures do a pretty good job of
> showing what is going on (you know.. picture = 100 words).
> I'm not sure what I could say to "clarify" it more...
> 
> I'll give it my best shot though..
> 
> In RAID 10 pretend that you have 3 DRIVES.  Now in
> reality each DRIVE is really a number of drives... so
> let's say DRIVE = 2 drives
> 
> The 2 drives in each DRIVE mirror their
> contents across each other.
> 
> Now apply striping across the 3 DRIVES. When
> determining size on a 10 it's common to only
> have 2 drives in each DRIVE and you increase
> the number DRIVES to increase the size of
> the array.  You can have DRIVES that have
> more than 2 drives in them if you want (a 3
> disk mirror).. but there's a cost<->reliability
> equation to be considered.  RAID10 as mentioned
> is "expensive" (though not like it used to be..
> it's practical nowadays)... using a 3 way mirror
> set just adds to the cost (while increasing
> reliability) without increasing amount of space
> available.
> 
> I can afford to lose more than one drive in
> a RAID10.  Remember that each DRIVE consists
> of two mirrored drives... so each stripe
> can afford a disk failure.  If you use a 3-way
> mirror, obviously you could theoretically
> achieve super-duper reliability, etc.
> 
> (realize that RAID 0+1 and RAID 10 are not really
> meant to be grown.. this is something you design
> ahead of time)
> 
> In RAID 0+1 pretend that you have 2 DRIVES.  Now in
> reality each DRIVE is really a number of drives... so
> let's say DRIVE = 2 drives (same as before)
> 
> The 2 drives in each DRIVE stripe their data
> across each other.
> 
> Now apply mirroring across the 2 DRIVES.  When
> determining size on a 0+1 you usually don't see more than
> 2 DRIVES.. instead you consider the number of drives
> in each DRIVE.
> 
> Just like RAID1, I can't handle more than one
> failure in RAID0+1.  One failure in one of my
> DRIVEs kills that DRIVE (invalidates the whole stripe
> set).  This tends to invalidate more drives than
> with RAID10, even if you add more DRIVES.  Adding
> more DRIVES wastes far more disk than with RAID10
> and doesn't give you as high a reliability gain.
> 
> Clear as mud?  I like the pretty pictures better.
> 
> 
> 
> > 
> > Can you please clarify?
> > 
> > Thanks,
> > 	Neil
> > 
> > 
> > --
> > Neil Aggarwal, JAMM Consulting, (972)612-6056, 
> www.JAMMConsulting.com
> > FREE! Valuable info on how your business can reduce 
> operating costs by 
> > 17% or more in 6 months or less! => 
> http://newsletter.JAMMConsulting.com
> > 
> > 
> >>-----Original Message-----
> >>From: discuss-bounces at ntlug.org 
> >>[mailto:discuss-bounces at ntlug.org] On Behalf Of Chris Cox
> >>Sent: Friday, September 26, 2003 10:08 AM
> >>To: NTLUG Discussion List
> >>Subject: Re: [NTLUG:Discuss] Dell Server with PERC RAID
> >>
> >>
> >>Cameron, Thomas wrote:
> >>
> >>>RAID 0+1 (often called RAID 10) is two RAID 0 arrays which are
> >>
> >> > mirrored (RAID 1).  Gives you the benefit of fast striping
> >> > plus the fault tolerance of mirroring.  It's just ghastly 
> >>expensive.
> >>
> >>Actually RAID0+1 and RAID10 are radically different things.
> >>http://www.acnc.com/04_01_10.html
> >>http://www.lsilogic.com/products/stor_prod/raid/backgrounder1.html
> >>
> >>Low end RAID controllers do 0+1 (it's easier to handle).  RAID 10 is
> >>a highly reliable RAID, RAID 0+1 isn't.  You'll only find RAID 10
> >>on your high end controllers (e.g. a MegaRAID Elite 1600 can do
> >>RAID 10, perhaps a bit old, but works well with Linux).
> >>
> >>Since disk has become cheap, creation of RAID 10's is now
> >>quite practical (remember 4G drives used to cost $3000).
> >>On systems where it works right you get the performance of
> >>RAID0 with the reliability of RAID1 AND.. unlike RAID0+1,
> >>you can more than one drive failure (depends on location and
> >>number of columns though).
> >>
> >>RAID10 is considered the creme de la creme of RAID (but
> >>probably the most expensive RAID config... but practical
> >>considering today's disk prices).  You may have to pay
> >>to get a controller and setup that supports RAID10 though.
> >>
> >>If you need HIGH reliability and FAST reads/writes...
> >>RAID10 might be a viable choice for you.  Most roll
> >>the dice and use good quality SCSI and run RAID0+1
> >>though (mostly because of limitations in their choice
> >>of RAID controller though).  Or run just RAID1 or
> >>run RAID5 (slow writes and slow rebuilds).
> >>
> >>The web links above do a pretty good job showing
> >>the differences between the various RAID levels.
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
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> >>
> > 
> > 
> > 
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> 
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