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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

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.*


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