different drives in raid 5 array

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I have a raid configuration in one of my file servers consisiting of a perc 5i with 256 megs of cache the arrary is made up of 5 Samsung 2TB drives 5400RPM and 32 megs of cache and these drives have 3 platters. I just got 3 Western digital red series drives they are 2TB 5400RPM but they have 64 megs of cache and they use 2 platters. So my question is will it be a problem adding the WD's into the raid 5 array since they have different cache sizes I always thought the most important was having the same drive size and speed? Also can you have to many drives in a raid 5 array for example if I add the drives I would have 8 drives total in one raid 5 array another option would be to create a separate array for the WD drives but than I lose usable space.

 

[Doward]

No, you can use all kinds of different drives together in RAID - even different sizes. BUT - you will be limited by the smallest and slowest drive.

 

[staticlag]

^ Indeed, just make sure they are exactly the same size or bigger than what you already have. Some drives are just a few blocks smaller and will not work. Cache makes no difference.

You can also have as many drives in RAID 5 as you want. You could have 100 if you desired. However, past 4 drives RAID 5 looses its effectiveness. This is because if a drive were to fail, and you were to replace it and have the array rebuild with a large amount of drives (8), it may take a few days to a week of straight full disk usage. In this time it is very likely that another drive may fail, bombing your data.

So typical recommendations are 2-5 Disks (RAID5, one disk lost), 5-10 Disks (RAID6, two disks lost), 10+ disks (3 Parity RAID, Z3, three disks lost)

 

[staticlag]

On that note, a single 8 disk RAID5 would be considered quite reckless by most people in the know. Unless of course you had a full backup of everything somewhere, in which case I would just run RAID 0 on 8 disks.

Typically when you get big arrays of disks you get diminishing returns. If you get into 20+ disk arrays you might loose 6+ disks of space just to get things stable and safe.