[K12OSN] RAID5 so slow?

[Chris Thomas]

Raid5 not only stripes the data across multiple drives
like Raid0 but, Raid5 also calculates a checksum on
the data and puts that on the drive too. This is so
that when a drive is lost, the data is safe and with
the checksum data, you can put in a new drive and
restore the lost info. With Raid0, when you loose a
drive, you loose the data for good.

What makes Raid 5 so slow is that it takes a lot of
cpu power to calculate the checksum on the fly. I
assume you are doing software raid 5, which makes it
even slower. I would get a pci raid controler that has
hardware raid support (not all pci raid cards are
hardware based raid controllers). This will make Raid
5 much faster. But it will never be as fast as Raid0.

Chris 

--- Mella <mellax at pm.ee> wrote:
> Hi!
> 
> I test 2 similar computers today. Almost similar
> except SCSI controllers 
> and kernels.
> I found RAID5 2x slower than disks separately in
> other computer. Is it 
> normal or I have some misconfigutations in Intel
> controller or default 
> FC3 kernel?
> 
> Computer 1- Intel HG2 MB, DualXeon 2,8GHz, Intel
> RAID U320 add-on 
> controller with 3 disks RAID5 array.
> 
> array (sda) 3x73GB Atlas 10K3 Kernel
> 2.6.11-1.14_FC3smp #1 SMP
> 
> [root at ts ~]# hdparm -t /dev/sda
> /dev/sda: Timing buffered disk reads:   82 MB in 
> 3.02 seconds =  27.18 
> MB/sec
> 
> Computer 2 - Intel VB2 DualXeon 2,4GHz, Adaptec
> U160.
> Disk (sda) 36GB Atlas 10K4 Kernel 2.4.30-1.smp.dma
> #1 SMP (self compiled)
> 
> [root at mail root]# hdparm -t /dev/sda
> /dev/sda: Timing buffered disk reads:  64 MB in 
> 0.95 seconds = 67.37 
> MB/sec
> ====
> Why faster disk and controller give slower
> performance data with RAID?
> 
> _______________________________________________
> K12OSN mailing list
> K12OSN at redhat.com
> https://www.redhat.com/mailman/listinfo/k12osn
> For more info see <http://www.k12os.org>
>