On Tue, 21 Oct 2003, Peter Childs wrote:
>
>
> On Tue, 21 Oct 2003, Ben-Nes Michael wrote:
>
> > what about mirroring only ? raid 1 ?
> >
> > I always thought that raid 1 is the fastest, am I true ?
> >
> > I don't really need more then 3GB data and I have two 36GB HD. so I don't
> > need lvl 0 nor lvl 5 unless raid 1 is slower.
>
> Raid 1 should not be slower than raid 5. hence
>
> Raid 0
> Write = Deciede which disk, Write
> Read = Deciede Which disk, Read
>
> Raid 1
> Write = Write Disk 1, Write Dist 2
> Read = Read (Don't matter which one)
>
> Raid 5
> Write = Write Disk 1, Write Disk 2, Calc Check Sum, Write Disk 3
> Read = Read Disk 1, Read Disk 2, Regenate Data.
That's not quite right.
Raid 5
Write:
Read Old Checksum Disk 1
XOR old Checksum with new data
Write Checksum to Disk 1
Write Data to Disk 2
Read = Read Data from Disk 1. That is all.
Raid 5 is lightening fast in a mostly read database (report databases) but
a little slower at writes, especially when there are only a few disks.
When the number of disks gets high enough to allow multiple reads and
writes to mostly hit different disks, you can get good parallel
performance until you saturate your bandwidth.