Re: RAID5 with two drive sizes question

["Joachim Otahal (privat)" <Jou@gmx.net>]

Stan Hoeppner schrieb:
> On 6/5/2012 2:59 PM, Roman Mamedov wrote:
>> On Tue, 05 Jun 2012 21:41:39 +0200
>> "Joachim Otahal (privat)"<Jou@gmx.net>  wrote:
>>
>>> Use only 750GB partitions, use the 3*250 GB loss at the end of each 1 TB
>>> drive for the fourth 750 GB, and RAID6 those 8*750. Result is 4.5 TB
>>> with a one-drive-loss tolerance and really bad performance.
>>> I spare you the 500 GB partitions example which result in 4.5 TB with a
>>> one-drive-loss tolerance and really bad performance.
>> Except this would not make any sense even as a thought experiment. You don't
>> want a configuration where two or more areas of the same physical disk need to
>> be accessed in parallel for any read or write to the volume. And it's pretty
>> easy to avoid that.
> You make a good point but your backing argument is incorrect:  XFS by
> design, by default, writes to 4 equal sized regions of a disk in parallel.
>
> The problem here is running multiple RAID arrays, especially of
> different RAID levels, on the same physical disk.  Under high IO load
> you end up thrashing the heads due to excessive seeking as the access
> patterns are very different between the arrays.  In some situations it
> may not cause problems.  In others it can.
>
> For a home type server with light IO load you probably won't have any
> problems.  For anything with a high IO load, you don't want to do this
> type of RAID setup.  Anyone with such an IO load already knows this,
> which is why it's typically only hobbyists who would consider using such
> a configuration.
>

Please stop. Next time I use <irony></irony> tags. RAID5 with 1 TB 
packages and appending the remaining 2*500 GB as RAID1 (as suggested by 
Roman Mamedov) is indeed the only sensible way, everything else is nonsense.