Re: "Missing" RAID devices

[Stan Hoeppner <stan@hardwarefreak.com>]

On 5/24/2013 1:32 AM, keld@keldix.com wrote:
> On Thu, May 23, 2013 at 10:45:56PM -0500, Stan Hoeppner wrote:
>> On 5/23/2013 3:30 AM, keld@keldix.com wrote:
>>> On Thu, May 23, 2013 at 12:59:39AM -0500, Stan Hoeppner wrote:
>>
>>>> You may be tempted to use md/RAID10 of some layout
>>>> to optimize for writes, but you'd gain nothing, and you'd lose some
>>>> performance due to overhead.  The partitions you'll be using in this
>>>> case are so small that they easily fit in a single physical disk track,
>>>> thus no head movement is required to seek between sectors, only rotation
>>>> of the platter.
>> ...
>>> I think a raid10,far3 is a good choice for swap, then you will enjoy
>>> RAID0-like reading speed. and good write speed (compared to raid6),
>>> and a chance of live surviving if just one drive keeps functioning.
>>
>> As I mention above, none of the md/RAID10 layouts will yield any added
>> performance benefit for swap partitions.  And I state the reason why.
>> If you think about this for a moment you should reach the same conclusion.
> 
> I think it is you who are not fully aquainted with Linux MD. Linux 
> MD RAID10,far3 offers improved performance in single read, 

On most of today's systems, read performance is largely irrelevant WRT
swap performance.  However write performance is critical.  None of the
md/RAID10 layouts are going to increase write throughput over RAID1
pairs.  And all the mirrored RAIDs will be 2x slower than interleaved
swap across direct disk partitions.

-- 
Stan