[clug] forget RAID?

Anthony David adavid at adavid.com.au
Thu Feb 22 04:55:28 GMT 2007

On Thu, Feb 22, 2007 at 09:57:07AM +1100, Michael Cohen wrote:
> On Wed, Feb 21, 2007 at 11:50:47PM +1100, Anthony David wrote:
> > Surely your OS write can wait for one write to disk to complete. The other
> > one can finish when it likes.
> That may be the case if you have enough buffer cache - but if you are hammering
> the disk, eventually you need to wait for that write because the cache is full.
> I was talking about write performance without cosidering caching.
> > RAID 5 is better for reading, especially as your stripe size goes
> > to 5, 6 whatever you are comfortable with and fits your application
> > profile. Writes are a real burden, especially with software RAID 5
> > as you are "flying without a parachute". Run a RAID 5 controller with
> > no write cache and see how fast your writes go. It has to do two
> > writes that HAVE to complete. Software RAID 5 has the same problem.
> Im not sure what you mean by flying without a parachute? If you have 5 disks in
> raid 5 - one is parity the rest are data, you can write 4 blocks in the same
> time it would take to write 1  so you get 4 x speed improvement. If you lose
> power you lose 4 blocks instread of 1 too - but thats not a big deal usually
> cause you already lost data anyway.  

Sorry. I meant specifically Battery-backed, Non-Volatile Cache Memory.

Anthony David

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