On Sun, Mar 23, 2008 at 8:11 AM, Kieran Clancy clancy.kieran+centos@gmail.com wrote:
Hi,
I have two 320 GB SATA disks (/dev/sda, /dev/sdb) in a server running CentOS release 5.
They both have three partitions setup as RAID1 using md (boot, swap, and an LVM data partition).
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When I do tests though, I find that the md raid1 read performance is no better than either of the two disks on their own
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Thanks, Kieran Clancy.
As a few already pointed out, RAID 1 is mirroring. So whatever gets written to the first drive also gets written to the second drive as well. It provides redundancy, not performance. I've used RAID 5 in the past to achieve performance with redundancy. But of course it has to be stripped across drives and not partitions. I saw a case where someone implemented RAID 1 by partitioning the drive into two and setting up the two partitions as RAID 1. So drive performance took a drastic nose dive and redundancy was practically worthless as the mirror resided on the same drive.
With only two drives, your options are RAID 0 or RAID 1 (see http://en.wikipedia.org/wiki/RAID#Standard_levels). RAID 0 will cause the two drives to be treated as one big drive (or each combination of partitions to be treated as larger partitions). The data would be stripped across the two drives (for each respective partition) which would give you a performance increase. However there would be no fault tolerance. If something crashed, your data is gone. And data recovery on a RAID where the data is stripped across drives is no easy task vs a standalone drive. So if redundancy is an important factor you will want a good backup system in lieu of another RAID level which offers fault tolerance.
Jacques B.