On Wednesday 29 September 2010, Boris Epstein wrote: > On Wed, Sep 29, 2010 at 12:58 PM, Peter Kjellstrom <cap@xxxxxxxxxx> 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. /Peter
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