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Question About Using RAID 5

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David40
Question About Using RAID 5
on Jun 2, 2008 at 2:49:35 pm

I wanted to use RAID 5 for my machine but something occured to me. RAID 5 combines mirror for security a and stripe for speed. How can both exist at the same time? If the striped drives are moving data faster than the mirrored drives how can the mirrored drives keep up with it? It would seem to me that RAID 5 can only be as fast as the mirrored drives which would be the slowest, and that would make using the striping useless. The only way around that I can see is a system where there is a ram cache where the data moving to the striped drives is stored and then written to the mirrored drives as they can catch up. Anyone have any insight into how this works? Confused
Thanks,
David


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jeffcss
Re: Question About Using RAID 5
on Jun 3, 2008 at 8:40:09 am

RAID 5 does NOT do a mirror. It is only a stripe set. All of the data, along with extra "recovery" data is spread across all drives. You must have at least 3 drives. They should be all the same size. With 3 drives you get the storage capacity of 2 drives.

RAID 5 can be fairly fast since the data is spread across all the drives. That means that a read request can be handled by multiple drives which, in theory, should be fast.

Writing is a little slower because the "recovery" data must be calculated and written along with the data.

The best part is when one drive fails, the system keeps working and you replace the drive and system recovers without any downtime or loss of data.

A good 3rd party RAID controller ($500 and up) can make RAID 5 be very fast.

RAID 10 or RAID 0+1 combines a stripe and a mirror. When reading, it is very fast because it just reads from the RAID 0 stripe which has 2 or more drives. Writing is slower for the same reason you mentioned.

You need at least 4 drives and only get the storage capacity of 2.

Again, having a good RAID controller will help. In some cases you can use two RAID controllers. Also a write cache can let the computer continue even before the data is actually written to the HD. Like the RAM cache you mentioned. Dangerous if there is a system failure/power outage.

RAID 0 only does striping with NO recovery data so it is very fast and very dangerous. 1 drive fails and you lose EVERYTHING.

The bottom line is that RAID (except 0) usually has fast reads and slower writes. You trade security for speed.


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