Question about RAID5

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yegor

Limp Gawd
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Jan 31, 2007
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I currently run a 0+1 RAID array, and I wanted to convert to RAID5. I have 6 drives which I wanna use. I know to use RAID5 you need at least 3 drives (2 will be for normal storage, and 3rd for the RAID stuff). Does it work the same way with 6 drives? My total space will be the total of all my drives, minus one? One drive stores all the necessary data to overcome the failure of any other drive int he array regardless if we have 3 or 6 drives?

Help would be appreciated.
 
Yes, it works the same for 6 drives as three, but, it's not simply using one drive for parity with the others as normal. RAID-5 is distributed parity, so each of the drives has 1/n of the parity.
 
What exactly is parity?

So a RAID5 array with 6 drives will be able to recover from a single HD failure. What about read/write speed, is it increased?
 
yegor said:
What exactly is parity?
Click to expand...
Literally it means `equality'.
In a RAID context, there is some info here on how it works:
http://en.wikipedia.org/wiki/Standard_RAID_levels
So a RAID5 array with 6 drives will be able to recover from a single HD failure.
Click to expand...
yes
What about read/write speed, is it increased?
Click to expand...
Sequential read speed is increased. It is usually comparable for a RAID-0 array of n-1 drives. Write speeds are very controller dependent.

Most software RAID-5 that I have used has write speeds in the order of a single drive. Sequential writing is usually faster than random writing. Writing less than $STRIPE_SIZE * ($NUMBER_OF_STRIPES-1) bytes is `slow' because the data needs to be read back for parity calculation.
 
drizzt81 said:
Writing less than $STRIPE_SIZE * $NUMBER_OF_STRIPES bytes is `slow' because the data needs to be read back for parity calculation.
Click to expand...

Nitpick: $STRIPE_SIZE * ($NUMBER_OF_STRIPES - 1). The last stripe is made up of the parity generated by the other blocks.

Parity for raid 5 is generated (almost always) by the exclusive-or function. The Wikipedia article explains it well. For raid 6 more complex codes are used, but the basic idea is the same.
 
Ok! Thanks! Looks like Im going to convert my raid 0+1 to raid 5.
 
RAID 0+1 should be faster and safer than RAID 5.

RAID 5, however, is easier to maintain because of expandability and not requiring a certain multiple of hard drives.
 
Im only going to use it to store large quantities of video content for my home theater PC, so raid 5 should be enough
 
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