SATA controllers

[Chan Chung Hang Christopher]

Liam Proven wrote:
> On Wed, Mar 10, 2010 at 3:19 PM, Dave Howorth
> <dhoworth at mrc-lmb.cam.ac.uk> wrote:
>> I'm speccing a new machine and as part of it I'd like to have a
>> linux-controlled 4-disk RAID 6 array using SATA 3 Gbps disks (aka STA 2).
> 
> You want more than 4 disks for RAID6. Seriously.
> 
> RAID6 uses 2 parity drives; this means you get the capacity of (N-2)
> drives, where N is the number of drives. Ergo, use 4 drives, you only
> get the capacity of 2. This is pointless, because if you lose half the
> capacity, you would get /much/ better performance from RAID10 (a
> mirror of stripes) or RAID 0+1 (a stripe set of mirror pairs).

md raid10 != md raid1+0. You can do a lot of fancy configs with the 
raid10 module that is not quite the same as nested raid1+0. Anyway, 
raid6 can lose any two drives whereas raid1+0 allows you to lose a 
specific combination of two drive loss. So you choose between 
performance or higher chance of survival. Given that disks of the same 
batch can have the tendency of dying together it is a risk that is worth 
considering.


> 
> (I may have got the definitions of 0+1 and 10 transposed, but it's not
> really important at this point!)
> 
> RAID levels 0+1 and 10 use simple mirroring and striping, requiring
> little CPU, therefore making them very fast, whereas with RAID6 you
> impose 2 sets of parity calculations on the system, making writes
> slow.

How much cpu is used is irrelevant because raid5 has been easily catered 
for (given no really heavy duty cpu chewing service like 
spamassassin+amavisd) since the AMD Duron. The real performance breaker 
for raid5/6 is the bus traffic necessary to read from disk before parity 
calculations can be done and the bus contention. Therefore, hardware 
raid cards that use any processor better than an Intel i960 with a 
sufficiently large cache will blow Linux software raid out of the water 
- sometimes even if the md array is raid1+0.


> 
> This means the *minimum* number of drives for a RAID6 is 5 drives,
> which will give you the capacity of 3× a single drive.

RAID5 = 3. I do not see why RAID6 has to jump to 5.


> 
> I don't know how smart the Linux software RAID system is, but if it
> has good rules built in, it won't let you create a RAID 6 out of <5
> drives and should outright block 4. It's an invalid config.
> 

Outright block 3.