Re: "Missing" RAID devices

[Stan Hoeppner <stan@hardwarefreak.com>]

On 5/24/2013 12:15 PM, keld@keldix.com wrote:
> On Fri, May 24, 2013 at 02:37:01AM -0500, Stan Hoeppner wrote:
>> 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.
> 
> In my experience read performance from swap is critical, at least 
> on single user systems. Eg swapping in firefox  or libreoffice 
> may take quite some time and there raid10,far helps by almost halfing
> the time for the swapping in. writes are not important, as long as you are not trashing.

If a single user system has multiple drives configured in RAID10 and
productivity applications are being swapped, then the user should be
smacked in the head.  2GB DIMMs are $10.  Any hard drive is $50+ but
usually much more.

This is not a valid argument.

> In general halfing the swapping in with raid10,far is nice for a process, but 
> for small processes it is not noticable for a laptop user or a 
> server user, say http or ftp.

Neither is this.  Laptop users don't run RAID10.  And server swap
performance is all about page write, not read, as I previously stated.

-- 
Stan