[K12OSN] Linux "Software RAID"

[David Hopkins]

I"ve enjoyed reading this thread, but have a question: I thought one
of the reasons to continue using RAID 5 vs RAID 1 was speed.  Tests on
my systems show that the RAID 5 I/O is at least 3x the speed of RAID
1.  I have hardware based RAID (either 3ware SATA controllers (large
disks) or Adaptec SCSI).  I put the OS on a RAID 1, and all data (e.g.
/home) is on a RAID 5.  I know I could get very cheap SATA drives and
my controllers support  mirroring so perhaps that is the next step.

Sincerely,
Dave Hopkins

On Fri, Aug 8, 2008 at 3:24 PM, Les Mikesell <lesmikesell at gmail.com> wrote:
> Rob Owens wrote:
>>
>>> raid5: automatically using best checksumming function: pIII_sse
>>>   pIII_sse  :  2604.000 MB/sec
>>> raid5: using function: pIII_sse (2604.000 MB/sec)
>
>> I don't see any of that info on my systems with software RAID 1.
>
> Raid1 doesn't do any calculations - it just tells the hardware to write
> twice and reads from whichever copy is faster.  Any performance hit comes
> from CPU involvement during writes (not much unless you are on old IDE or
> USB connections) or overloading the PCI bus.
>
>> What
>>
>> do those values mean?  Are they read speads?  Write speeds?
>
> That's the math to compute the parity that needs to be written along with
> the data.  And if you have a failed drive in the set it will do a similar
> computation to reconstruct the data in degraded mode (where raid1 just reads
> the other copy at full speed).
>
> --
>  Les Mikesell
>   lesmikesell at gmail.com
>
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