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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