Re: Problem identified: WAPL/RAIDframe performance problems

[Michael van Elst <mlelstv%serpens.de@localhost>]

On Wed, Nov 28, 2012 at 10:18:04PM +0100, J. Hannken-Illjes wrote:
> On Nov 28, 2012, at 10:13 PM, Michael van Elst <mlelstv%serpens.de@localhost> 
> wrote:
> 
> > gdt%ir.bbn.com@localhost (Greg Troxel) writes:
> > 
> >> I wonder if it's possible (easily) to make the log only use fsbize
> >> boundaries, (maybe forcing it to be bigger as a side effect.)
> > 
> > Writing filesystem blocks won't help. RAIDframe needs writes as large
> > as a stripe.
> 
> The file system block size should match the raid stripe size or you
> have much more problems than flushing the log.

True. Still difficult to do, in particular for metadata which is written
in fragsized blocks. Best for speed is probably to use fragsize=blocksize=64k.


> > The log itself could write much larger chunks but flushing is done
> > in a series of writes as small as a single physical block. I think
> > the only way to improve that is to copy everything first into a
> > large buffer. Not very efficient.
> 
> Needing to copy say 8 Mbytes of data and writing it in big chunks
> will be much faster than writing it many smaller unaligned segments.

One or two MB is probably good enough. A quick test of unpacking
base.tgz produces transactions of ~3MB and 1.5MB and a few smaller
ones of 30-50kB.


-- 
                                Michael van Elst
Internet: mlelstv%serpens.de@localhost
                                "A potential Snark may lurk in every tree."