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Re: Very slow LVM performance



Aaron Toponce put forth on 7/12/2010 5:16 PM:
> On 7/12/2010 4:13 PM, Stan Hoeppner wrote:
>> Is that a typo, or are you turning those 3 disk mdadm sets into RAID10 as
>> shown above, instead of the 3-way mirror sets you stated previously?  RAID 10
>> requires a minimum of 4 disks, you have 3.  Something isn't right here...
> 
> Incorrect. The Linux RAID implementation can do level 10 across 3 disks.
> In fact, it can even do it across 2 disks.

Only throw the bold "incorrect" or "correct" statements around when you really
know the subject material.  You don't.  Linux md RAID 10 is not standard RAID
10 when used on 2 and 3 drives.  When used on 3 drives it's actually RAID 1E,
and on two drives it's the same as RAID1.  Another Wikipedia article linked
within the one you quoted demonstrates this.  Note the page title
"Non-standard_RAID_levels".

http://en.wikipedia.org/wiki/Non-standard_RAID_levels
Linux MD RAID 10

The Linux kernel software RAID driver (called md, for "multiple device") can
be used to build a classic RAID 1+0 array, but also (since version 2.6.9) as a
single level[4] with some interesting extensions[5].

The standard "near" layout, where each chunk is repeated n times in a k-way
stripe array, is equivalent to the standard RAID-10 arrangement, but it does
not require that n divide k. For example an n2 layout on 2, 3 and 4 drives
would look like:

2 drives         3 drives            4 drives
--------         ----------        --------------
A1  A1           A1  A1  A2        A1  A1  A2  A2
A2  A2           A2  A3  A3        A3  A3  A4  A4
A3  A3           A4  A4  A5        A5  A5  A6  A6
A4  A4           A5  A6  A6        A7  A7  A8  A8
..  ..           ..  ..  ..        ..  ..  ..  ..

*The 4-drive example is identical to a standard RAID-1+0 array, while the
3-drive example is a software implementation of RAID-1E. The 2-drive example
is equivalent RAID 1.*

-- 
Stan





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