[CentOS] XFS on a 25 TB device

Peter Kjellstrom cap at nsc.liu.se
Wed Sep 29 13:25:11 EDT 2010

On Wednesday 29 September 2010, Boris Epstein wrote:
> On Wed, Sep 29, 2010 at 12:58 PM, Peter Kjellstrom <cap at nsc.liu.se> wrote:
> > On Wednesday 29 September 2010, Boris Epstein wrote:
> >> I am wondering if I need to worry about stripe and width though as
> >> mine resides on a logical volume residing on a hardware-controlled
> >> RAID 6 device (i.e., one slice as far as the OS is concerned).
> >
> > That is why you need to consider it. If the device is aligned on stripe
> > size (chunk size * (number of drives - 2 for raid6 parity)) and the
> > filesystem is made aware it can put stuff (files, metadata, etc.) so that
> > a minimum of stripes are touched (less I/O done).
> Well, you are interfering with the hardware RAID controller which
> copies around and stripes data as it sees fit. I am not sure with this
> many levels of abstraction I can gain any measurable performance
> improvement by adjusting the XFS to the controller's hypothetical
> behaviour.

You are a bit mistaken. The raid controller does not "copy data around as it 
sees fit". It stores data on each disk in chunk-size'ed pieces. It then 
stripes this across all drives giving you a stripe-size'ed piece of chunk 
size times the number of data drives.

Typical chunck sizes are 16, 32, 64, 128 and 256 KiB. If you created your 
raid-set with, say, 128 KiB chunk size and 16 physical drives this will give 
you a stripe size of:

 128 * (16 - 2) => 1792 KiB

Having the filesystem align its stuctures to this can (of course depending on 
work load) make a huge difference. But you won't be able to do this if your 
device isn't already aligned (unaligned use of partitions and/or LVM).

Then again, for other workloads the effect could be insignificant. YMMV.

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