Re: cpu/memory use

[NeilBrown <neilb@suse.de>]

On Wed, 19 Jan 2011 22:14:45 -0200 Roberto Spadim <roberto@spadim.com.br>
wrote:

> Hi, i wass thinking about cpu/memory use
> i have a dual (three four) cpu
> could i make linux to only use cpu1 for raid? and others to anythink
> else, don´t migrate raid between cpus... and don´t allow others
> programs to use raid cpu...
> is it possible?
> is it dificult?
> it´s more linux related feature, not raid related, but could we implement it?
> what about memory usage? how many memory software raid use? is it per
> device, per raid, does it have a hard limit (offcourse)? could we
> calculate it? for example i want raid1 with two disks of 1tb, how many
> memory should i buy?
> 

For levels other than RAID4/5/6, md/raid does not use any significant amount
of CPU or memory.

For RAID4/5/6, md's use for CPU is single-threads so it will only use a
single CPU - which ever one the scheduler allocates it to from time to time.

The only room for improvement that I can see would be to allow the 'xor'
calculation to be run in parallel on multiple CPUs and that would only help
if the storage devices were nearly as fast as a CPU.  Where we have tried
parallelising xor, it has only made things slower. 


For your particular question about RAID1 - use a RAID1 across two devices
would use less than 100K more than using just one of the devices.
During resync it might use as much as a couple of megabytes of extra memory.

NeilBrown
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