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What is the minimum number of drives for RAID 5?

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  • NAS / RAID
  • Storage
Last response: in Storage
August 5, 2011 1:49:23 PM

I've heard you need to have 4 drives for RAID 5, but I've also read you only need 3 drives. Which is true? Thanks.

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a c 415 G Storage
August 5, 2011 2:57:56 PM

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a c 394 G Storage
August 5, 2011 4:17:36 PM

Another question: How many drives do you need in RAID5 before you get back to the performance level of a single drive? I have seem some reports here that 3-disk RAID5 is unexpectedly slow.

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August 5, 2011 4:49:34 PM

I thought 3 was correct. Just wanted to make certain. Thanks.
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August 5, 2011 4:50:01 PM

Best answer selected by ahthurungnone.
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a c 415 G Storage
August 6, 2011 1:48:38 AM

WyomingKnott said:
Another question: How many drives do you need in RAID5 before you get back to the performance level of a single drive? I have seem some reports here that 3-disk RAID5 is unexpectedly slow.
For reads, even 3 disks RAID5 can have faster transfer rates than an individual drive, although access time is no better. But for writes, no number of disks will ever be as fast as a single drive because writing a sector always means reading the old sector and the old parity block, recomputing parity, and then writing the new sector and parity block.
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a b G Storage
August 6, 2011 2:53:48 AM

WyomingKnott said:
Another question: How many drives do you need in RAID5 before you get back to the performance level of a single drive? I have seem some reports here that 3-disk RAID5 is unexpectedly slow.


sminlal is correct, mostly. you'll see read speeds more/less equivalent to reading from a RAID 0 of 1 less number of drives, after all RAID 5 is essentially just a RAID 0 with an extra drive for the fault tolerance of parity. writing is much more difficult because parity must be calculated from an intensive XOR formula, so writing is usually limited to the speed of the processor calculating parity. a software RAID5 (that relies on the CPU) will be slower than a hardware RAID5 (controller card that has processor dedicated to handling calculations itself).

i have hardware accelerated RAID5, 4*1TB drives (3TB logical drive). i'm pushing 200MB/s both read and write.
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a c 415 G Storage
August 6, 2011 3:29:12 AM

Branden said:
writing is much more difficult because parity must be calculated from an intensive XOR formula, so writing is usually limited to the speed of the processor calculating parity. a software RAID5 (that relies on the CPU) will be slower than a hardware RAID5 (controller card that has processor dedicated to handling calculations itself).
Slow RAID-5 write speeds have nothing to do with the compute power needed to calculate parity. XOR calculations are trivial in modern processors - they're nothing compared to all of the disk I/O required to read and write the data and parity sectors. Hard drives access speeds are several milliseconds, while CPU instructions execute in fractions of nanoseconds - that's well over a million-fold difference in speed.

Dedicated RAID processors gain their speed by using a lot of cache memory to reduce the need to perform as many I/Os, not because they can calculate parity any faster.
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a b G Storage
August 6, 2011 2:09:02 PM

well i'm not going to pretend that i know more about hardware controllers or XOR than anyone else, i just recall reading somewhere that XOR was an intensive calculation and that the processor on a controller card was much faster at it than a CPU. i had assumed that was why my hardware RAID 5 is much faster than the software RAID 5 i was running previously. maybe that was incorrect.

i disabled the cache on my controller card (or rather limited it to 0% due to stability issues) and i'm still getting 190MB/s read and write. i was getting slightly faster speeds when cache was enabled, but the card tended to freeze if several gigs were being written.
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a c 415 G Storage
August 6, 2011 3:16:35 PM

Branden said:
...i just recall reading somewhere that XOR was an intensive calculation and that the processor on a controller card was much faster at it than a CPU.
That was true at one time, but that time is gone. A lot of people still repeat it as "common knowledge", though - just trying to set the record straight.
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