Re: Hardware RAID vs. Software RAID

From: Anna C. Dent <anacdent_at_hotmail.com>
Date: Tue, 21 Oct 2003 17:08:07 -0700
Message-ID: <HTjlb.79755$Ms2.40052@fed1read03>

Chuck Lucas wrote:

> "Anna C. Dent" <anacdent_at_hotmail.com> wrote in message
> news:Ye%kb.79328$Ms2.67542_at_fed1read03...
> 

>>Chuck Lucas wrote:
>>
>>>"Daniel Morgan" <damorgan_at_x.washington.edu> wrote in message
>>>news:1066587431.204277_at_yasure...
>>>
>>>
>>>>You are correct on all points.
>>>>
>>>>But what version you implement can make a big difference. I'd stay away
>>>
>>>>from RAID 5 and vendor RAID
>>>
>>>>implementations with numbers like 2 and 4.
>>>
>>>
>>>What's wrong with RAID 5?
>>
>>With Raid-5 the redundancy information is stripped across all disks.
>>For EVERY data block which is written to a RAID-5 volume the parity
>>block must be recalcuated and also written to disk. For all intents
>>and purposes RAID-5 doubles the number of write operations to disk.
>>As the percentage of WRITE to READ operations increases available
>>useful/application disk throughput decreases.
>>

> 
> Wow!  I didn't expect this much of a response.  Thanx, folks, for the input.
> I, OBVIOUSLY, have some more research to do, huh?
> 
> But I did notice throughout this that it seemed as though you guys were
> talking about software RAID5.  I'm using hardware RAID5...does that make any
> difference in your opinions?

The major difference between hardware based RAID-5 & software based RAID-5 is performance. With S/W RAID-5, such as NT used to implement, the actual PC computed the parity block; which is somewhat CPU intensive. This drains even more resources from the application. With H/W RAID-5 the parity calulated on the controller thereby "giving" those CPU cycles back to the application.

In round numbers and folks will argue about the ROM SWAG to follow; accessing RAM is three orders of magnitude (1000 times) faster than transferring the data blocks from the platters into memory which is 1000 times faster than moving the heads of the disk drive to the desired cylinder and then wait for the platter to spin the data under the heads. The bottom line is that I/O operations are about 1,000,000 time slower than accessing data in memory. Therefore it is best to minimize the number of READS or WRITES which are done to & from disk. RAID-5 trades disk space (gives you more) but you pay a perfromance penalty. Some folks can live with this trade-off, while others need PERFORMANCE and are willing to pay for it (by buying more/better/faster disks & controllers). Received on Tue Oct 21 2003 - 19:08:07 CDT