Re: RAID10: how much does chunk size matter? Can partial chunks be written?

[Andras Korn <korn@raidlist.elan.rulez.org>]

On Fri, Jan 04, 2013 at 06:46:20PM -0600, Stan Hoeppner wrote:

> >> 1.  What filesystem are you using?
> > 
> > The filesystem is the "application": it's zfsonlinux. I'm putting it on
> > RAID10 instead of using the disks natively because I want to encrypt it
> > using LUKS, and encrypting each disk separately seemed wasteful of CPU (I
> > only have 3 cores).
> 
> I just love when the other shoe drops and it turns out to be a size 13
> boot... filled with lead.
> 
> Any reason why you intentionally omitted these critical details from
> your initial post?

Yes: I thought I was asking a theoretical question, not for advice on tuning
my specific setup, and thought - apparently correctly, I might add :) - that
including these details would only get everyone sidetracked into trying to
optimise for a specific application. I don't have a specific application
with a specific access pattern because I have to run all sorts of
applications on this box, on top of this array, simultaneously.

But meanwhile I think I have received an answer: the fact that zfs uses
blocks of 128k or less does not automatically mean that write performance
would suffer on a 512k-chunk RAID10 array because the Linux RAID10
implementation, very sensibly, doesn't insist on writes being aligned to
chunk boundaries.

So thanks.

-- 
                     Andras Korn <korn at elan.rulez.org>
         Reality is merely an illusion, albeit a very persistent one.