vk101

RAID 10 slower than RAID 0?

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How much slower is RAID 10 than RAID 0? For the extra mirroring that takes place in RAID 10, does it make any difference AT ALL in terms of performance over RAID 0?

Also, is it possible to have a RAID 10 with each stripe mirrored, say, three times instead of twice? Leading to 6 drives instead of 4? Would this be a lot slower than RAID 0? How about than regular RAID 10 with double- instead of triple-mirroring?

Edited by vk101

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Normally, a 6 drive RAID 0 would be a 3 drive RAID 0 mirrored to another 3 drive RAID 0. I guess you might be able to make it so that you had 2 mirrors of a 2 drive RAID 0, but most controllers wouldn't support this - you'd use some form of RAID with parity and maybe hot spares instead.

A RAID 10 array of 4 drives would be no slower than a RAID 0 array of 2 drives. It might have a similar read speed to a RAID 0 array of 4 drives, but the write speed of a 4 drive RAID 0 array would be up to twice as fast as a RAID 10, depending on interface and controller bottlenecks.

Mirroring doesn't incur a performance penalty, but calculating parity for RAID 5, etc. does.

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Big SNIP

Mirroring doesn't incur a performance penalty, but calculating parity for RAID 5, etc. does.

I would think that the MB/s traffic of data to the mirror drives would have some sort of penalty on the bus they're on, the cache allocation( caching controller) and some management of this traffic and SCSI commands by the RAID controllers' processor.

Of course I agree that calculating parity is a task that occupies the processor ( wether software RAID or dedicated RAID processor)

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It would depend on the controller, but most SATA controllers have a dedicated 150 MB/s link to each drive, and since the additional data for mirroring is generated by the controller itself, it doesn't matter how the controller is connected to the rest of the system... at least as far as mirroring performance impact is concerned.

With SCSI, you're likely to have more devices on each channel, so mirroring to several drives on the same channel could have an impact - especially with more than 4 drives on one channel (assuming U320 drives & controller).

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There's no generally-valid answer to such a question. Everything depends on the particular storage controller's implementation and the system, etc.

But, as a specific example, I did simple HD Tach measurements of a single drive vs. a mirror of two drives, and the mirror had around 7% less average performance.

To get back to my point about implementation, if this was a smart RAID 1 implementation, it would have "striped" the reads, taking advantage of data duplication on the two drives, and RAID 1 should have been faster for reads than the single drive. But it ain't so in this system. Some others...

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Madwand,

I did find the same lack of "smartness" in both NVidia (NForce4Ultra) and Intels (ICH8R) integrated RAID controllers.

I was wondering if the same applies to the low end server controllers like the HP Smart Array E200i?

Does anybody know where or how can I find which controllers support "smart" RAID1 reads?

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You could get a U160 enterprise class caching RAID controller for not much on eBay. Some of them work in a 32bit PCI slot.

The ones that work are eXtremeRAID 2000, ServeRAID-4H and MegaRAID Enterprise 1600.

They are 4 channels U160 controllers, so you could put 2 drives per channel and not feel much bottleneck except for the burst to U320.

Maybe it would be an inexpensive try. They also have all the high-end cabalities like hot-sparing, capacity expansion, some RAID migration and rebuild in background.

Some ideas for you

JB

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well, 10x for the info but I think for the desktop I prefer some kind of inexpensive SATA RAID solution for PCI or PCI-E 1x/4x ... if such a think exists.

For the server at work I need a SAS controller. The currnet HP controller is expanded with 128MB DDR2 RAM for RAID5 support. At the moment I don't think we will invest in extarnal RAID controller as the SAS version tend to be very expensive :(

So my question is does anybody know if the HP smart array E200i supports "smart" RAID1 reading. I'll try to check it when I'm back at work but I just wanted to know before

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