Re: raid 10f2 vs 1 on 2 drives

[David Brown <david.brown@hesbynett.no>]

On 25/05/12 21:03, Bill Davidsen wrote:
> William Thompson wrote:
>> On Wed, May 23, 2012 at 12:36:12AM +0200, David Brown wrote:
>>> On 22/05/12 21:33, William Thompson wrote:
>>>> I understand that raid 10 f2 is slower on writes due to the location
>>>> of the
>>>> 2nd copy. My question is, if lots of writes are performed, could this
>>>> layout wearout the drives quicker than raid 1?
>>>
>>> No, wear is not going to be significantly different.
>>>
>>> You didn't say whether you are talking about hard disks (where
>>
>> Sorry about that (Chief). Yes, I was refering to hard drives.
>>
>>> location makes a difference, but "wear" on the drive motor is
>>> insignificant to the disk's expected lifetime), or flash disks
>>
>> I was thinking about how much more head movement there would be to
>> write the
>> 2nd copy of the data.
>>
> There _is_ no extra head motion. The location of consecutive blocks is
> different on each drive, but as I read the mapping function the distance
> between blocks on the same drive will be about the same, so amount of
> head motion (both number and distance) is the same on each drive, but
> the location of that motion is not the same.
>
> One drive may be seeking at the outer edge of the platter while another
> seeks near the spindle, but there's the same amount of seeking on each.
>
>

That's not the case for "far" layout.  When you write a block, it there 
will be two copies - one in the first half of one disk (say, disk 1), 
and the other in the second half of the other disk (disk 2).  The next 
sequential block will be written to the first half of disk 2, and the 
second half of disk 1 - exactly half a disk away from the first block. 
And no matter where the disk head ends up after the write, raid10,far 
will always read from the outer half of the disk since it is 
significantly faster.  (For SSDs it doesn't matter, but then neither 
does head positioning.)

Write merging, combining, re-ordering, etc., will minimise this effect, 
as will write caches.  But there is no doubt that raid10,far sacrifices 
write speed a little in order to get the fastest possible read speeds. 
For most use-cases, with more reading than writing, this results in the 
best overall speed.