On Wednesday 29 September 2010, Boris Epstein wrote:
On Wed, Sep 29, 2010 at 12:58 PM, Peter Kjellstrom cap@nsc.liu.se wrote:
On Wednesday 29 September 2010, Boris Epstein wrote:
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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).
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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.
/Peter