RAID 5, Read Speed and Caching

I currently have two WDC 4TB in Raid 1. I have a lot of large files on drives (ISOs, MSDN, etc). I was thinking about adding another drive for a RAID 5. Will increase sequential speed by noticeable amount? Also, I have a SSD drive. If I use part of the SSD as cache partition will it help the writes speed issue of a RAID 5 configuration? Thanks.

David
davidst98Asked:
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epichero22Commented:
Your SSD will be much faster than conventional hard drives in RAID.  I would put whatever you want to optimize performance for on the SSD and keep your data on the RAID.  

Adding another hard drive would be a good idea as you will gain redundancy but it also depends on your RAID controller.  I've seen a three hard drive RAID 5, for example, be way slower than RAID 1 with two.  

If you really want to improve performance and data safety, add another two hard drives to go with RAID 10 for your data, and place all your programs and operating system on the SSD as that should give you the best configuration.
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davidst98Author Commented:
Thanks for the info.   I guess I'll do more research.
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andyalderCommented:
3 drives in RAID 5 will be about twice as fast as two in RAID 1 for large sequential reads, at least on a HP controller and probably on other controllers as well.

The reason is that the controller does not know it is a sequential and so the load balancing algorithm for RAID 1/10 is optimised for random read; when it gets asked for sequential data it ends up sending both drives after the same block of data so it can reply from whichever drive provides the data first. This is excellent for random data but pretty useless for sequential. With RAID 5 on the other hand it doesn't send one drive for the data plus all the rest including parity disk for a second copy of the data since the chances of all the rest getting to the data faster than the single disk is vanishingly small, the other drives are therefore available to read other tracks. It is after all the seek time that matters for sequential operations and read-ahead sorts that out anyway.

If it's large sequential writes I wouldn't bother using the SSD as cache although normal controller cache is needed. Your large sequential writes will be written as full stripes so there is no need for the controller to do the normal 4 logical I/Os per physical I/O so long as it has just a little bit of cache available.
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andyalderCommented:
Guess I should type quicker :(
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davidst98Author Commented:
Thanks for the response.   I have a Gigabyte GT Z97.   It has a Intel RAID 0/1/5/10 controller on it.    Is it good for a RAID 5 or should I buy a separate PCIe controller?

David
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Raivo RessarCommented:
Some RAID 5 controllers with some firmware version are not reliable and loose hard drives sometimes. But most controllers are reliable. You can check available information about controller and firmware. But RAID 5 is faster than RAID 1.  You can test speed difference if you copy a folder containing files. Probably speed difference is not same if you copy small or big files.
Exchange messages are saved in a database. Controller reads small files or small pieces of database. So internal data bus speed is not important always. More important are hard drive speed and drive redundancy.
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