On 2011-12-08 20.11, Josh Grosse wrote:
> Pavel Shvagirev <pavel.shvagirev <at> gmail.com> writes:
>> Thank you for the reply. Unfortunately RAID0 is not exactly what I was
>> looking for 'cause it does not really concatenate disks - it stripes as
>> you've mentioned. And two disks, 80 and 120 Gb, that were to be
>> concatenated will never give ~200Gb with RAID0.
> Sure they will. Just factor the size. In your example, use 5 x 40GB
> partitions:
> wd1 = 80 GB, two 40GB partitions
> wd2 = 120 GB, three 40GB partitions
> Something like this should work:
> # bioctl -c 0 -l /dev/wd1a,/dev/wd1d,/dev/wd2a,/dev/wd2d,/dev/wd2e softraid0
Out of curiosity, have you actually tried something like this? While I'm
sure it works technically, I'd imagine the performance would be abysmal.
Think about it: When writing a chunk of data, the first part goes to one
part of the first disk, the next part goes to another part, 40 gigs away,
then the second disk gets three writes, all separated by long platter
distances requiring large seek times for *every* write.
Also, striping or concatentation without redundancy is generally a very,
very bad idea for anything but temporary data you can live without, for
two reasons:
First of all, in the event of a single disk failure you lose the entire
volume (although with concatenation you have a fair chance of recovering
half your data).
Second, and in my experience this is something people just don't think to
consider, with each disk you add to the stripe/concat you greatly reduce
MTTF (mean time to failure), according to this formula: t=m/n, where m
is MTTF for one drive and n is the number of drives.
Meaning, with two drives in a stripe, you've doubled the chance of total
loss of your data. With four drives, the risk is four times that of a
single drive.
In the majority of use cases, one is better off using two drives
separately, each with its own file system, than striping or concatenating
them. Possible exceptions to this is where performance is very important
(and then never ever put more than one stripe in a set on the same drive)
and where a very large single volume is required (but then a RAID 5
volume is the way to go).
Regards,
/Benny
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