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It seems to me that while raid 1 should double the read performance (supposing to have 2 disk) this, at least with mdadm, don't work. I have centos 5.

Will a megaraid 9280 controller change something?

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mdam doesn't have any info on whereabouts of the heads. Get yourself a real raid controller with a big cache. I would recommend Areca, it has proven itself to us. – anttiR Nov 8 '11 at 12:20
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Probably very little change in performance with the addition of a hardware Raid card, if still only using two mirrored disks.

Raid 1 mirroring, doesn't stripe the data so the increase in throughput for a single request shouldn't be much faster than a single drive. Writes should be slower than a single disk.

Raid 0 striping, should give you the fastest performance boost with NO data redundancy. Many people use Raid 10 - 4 drives to get the best of both, redundancy and speed. Sometimes Raid 5 with 4 drives is faster than Raid 10, depends on usage patterns.

Good raid hardware will be faster for more money. If you don't see any improvements with software raid you may be I/O bound, have software issues or the data access patterns might not be optimal for raid. I'd bet that for some usage patterns, spending money on more disks for software Raid would outperform a hardware Raid card with only two mirrored disks.

What are your benchmark numbers for a single disk and the software raid?

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Raid 1 with two drives should absolutely not be slower than a single disk. Writes should be the same speed, and reads should be twice as fast. If this isn't the case, then I'd start by looking at the raid software. – Basil Nov 8 '11 at 21:17

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