Theoretical Question...RAID 10 vs RAID 5

Theoretical Question...RAID 10 vs RAID 5

Post by Don Whitlo » Sat, 21 Sep 2002 23:10:42



Quick question.

If I have a 6 member RAID 5 array, and a 10 member RAID 10 array (5 mirrors,
then striped), which is technically faster for both reads & writes? I know
that in general, more spindles is better for performance. I also know that
in general, RAID 5 writes take a penalty for computing and writing parity
across all the drives, at least compared to pure striping (RAID 0). However,
don't I suffer a write penalty by having to commit 10 writes in my 10 member
RAID 10 array? I know reading should be faster on the RAID 10 array because
for each of the stripe members, I have 2 options from which to read my data
from, and will usually come from the least busy spindle.

Basically, I'm just looking to settle a bet with myself :). I know there are
some other factors that go into the above, but assuming the arrays are using
the same hardware, same disks, and so on, I'm just looking for which should
give me more 'pure' IO capability.

Thanks in advance,
Don

 
 
 

Theoretical Question...RAID 10 vs RAID 5

Post by Rob You » Sun, 22 Sep 2002 01:06:46



> Quick question.

> If I have a 6 member RAID 5 array, and a 10 member RAID 10 array (5 mirrors,
> then striped), which is technically faster for both reads & writes? I know
> that in general, more spindles is better for performance. I also know that
> in general, RAID 5 writes take a penalty for computing and writing parity
> across all the drives, at least compared to pure striping (RAID 0). However,
> don't I suffer a write penalty by having to commit 10 writes in my 10 member
> RAID 10 array? I know reading should be faster on the RAID 10 array because
> for each of the stripe members, I have 2 options from which to read my data
> from, and will usually come from the least busy spindle.

> Basically, I'm just looking to settle a bet with myself :). I know there are
> some other factors that go into the above, but assuming the arrays are using
> the same hardware, same disks, and so on, I'm just looking for which should
> give me more 'pure' IO capability.

        Well... Bill Todd could probably answer this as good as any
        in this forum .. but let me take a low-key stab.

        RAID10 will be faster for a number of factors.  First, on writes
        parity does not have to be calculated , therefore you don't
        have to do a read just to write (read in parity information).
        Secondly,  on reads you will actually have 10 members to read
        from versus 6 members of the RAID5.

        You could take the time to learn a whole bunch more by going
        out to:  www.google.com and typing in some relevant keywords,
        i.e.

                raid 5 performance comparison

        and looking at some good hits, for example:

http://www.exadrive.com/pdf/RAID10vs5.pdf

                                Rob

"Even if the biblical assertion is incorrect that where there is no vision,
 the people perish, it is difficult to think what could be the engine or
 stimulus for social behavior in a nihilistic system committed only to the
 certainty of the passage of time, without any energetic relationship to
 another principle or purpose."  --Lionel Tiger

 
 
 

Theoretical Question...RAID 10 vs RAID 5

Post by Bill Tod » Sun, 22 Sep 2002 02:54:22



Quote:> Quick question.

> If I have a 6 member RAID 5 array, and a 10 member RAID 10 array (5
mirrors,
> then striped), which is technically faster for both reads & writes? I know
> that in general, more spindles is better for performance. I also know that
> in general, RAID 5 writes take a penalty for computing and writing parity
> across all the drives, at least compared to pure striping (RAID 0).
However,
> don't I suffer a write penalty by having to commit 10 writes in my 10
member
> RAID 10 array?

No, because  a) writes often will be small enough to target only one pair of
disks in the array and  b) even when they're larger, they still only have to
wait for the least-well-positioned disk-head to complete the write rather
than perform read/wait-full-revolution/write operations to one or more
disks.

The best case for the RAID-5 array is when the write modifies exactly one or
more full stripes, in which case no reads are required and every disk can
simply be written.  In this one case, you write to 6 disks rather than 10,
and there may be some measurable difference in average speed but even then
likely not a noticeable one.  However, if the array is supported by some
modest amount of battery-backed write-back cache, and most of your activity
involves sequential writing (to largish files that are logically contiguous
on the array), then most such writes can be converted to full-stripe writes
at the disks and RAID-5 can start looking write-performance-competitive.

So even with a pure write load, the RAID-10 approach will *almost* always be
faster and will never be noticeably slower.  Mix in any read activity, and
its advantage improves even more.

- bill

 
 
 

Theoretical Question...RAID 10 vs RAID 5

Post by Peter da Sil » Mon, 23 Sep 2002 01:09:39


What's RAID 10? I know RAID 1+0 and RAID 0+1, and it sounds like you're
talking about RAID 1+0... but I've never heard that called RAID 10.

[insert flame about the marketing beggars who came up with the idea of
 calling a collection of related techniques "levels"...]

In any case, RAID 1+0 is faster on writes because it only has to write
one block to two disks, rather than (possibly) reading two blocks, doing
the parity computations, and writing two different blocks.

And of course on reads it has a choice of two disks to read the block
from, so reads will on average be faster.

--
I've seen things you people can't imagine. Chimneysweeps on fire over the roofs
of London. I've watched kite-strings glitter in the sun at Hyde Park Gate.  All
these things will be lost in time, like chalk-paintings in the rain.   `-_-'
Time for your nap.  | Peter da Silva | Har du kramat din varg, idag?    'U`

 
 
 

Theoretical Question...RAID 10 vs RAID 5

Post by Will Dorman » Wed, 25 Sep 2002 12:29:54



> What's RAID 10? I know RAID 1+0 and RAID 0+1, and it sounds like you're
> talking about RAID 1+0... but I've never heard that called RAID 10.

Often the "+" is omitted, for example notating RAID 1+0 and 0+1 as "10"
and "01", respectively.   (I would assume the former pronounced as "One
Zero", as opposed to "Ten")

-WD

 
 
 

Theoretical Question...RAID 10 vs RAID 5

Post by Peter da Sil » Wed, 25 Sep 2002 21:16:44





> > What's RAID 10? I know RAID 1+0 and RAID 0+1, and it sounds like you're
> > talking about RAID 1+0... but I've never heard that called RAID 10.
> Often the "+" is omitted, for example notating RAID 1+0 and 0+1 as "10"
> and "01", respectively.   (I would assume the former pronounced as "One
> Zero", as opposed to "Ten")

Cor, guvner, fair makes me skin crawl just thinkin' about it.

--
I've seen things you people can't imagine. Chimneysweeps on fire over the roofs
of London. I've watched kite-strings glitter in the sun at Hyde Park Gate.  All
these things will be lost in time, like chalk-paintings in the rain.   `-_-'
Time for your nap.  | Peter da Silva | Har du kramat din varg, idag?    'U`

 
 
 

1. RAID-10 vs. RAID-01 ?

Is there any paper that compares the advantages of either RAID-10 or Raid-01 ?

Martin.
--
        martin "at" quickstep "punkt" dirnet "punkt" com
--
 Unix _IS_ user friendly - it's just selective about who its friends are !
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