[Beowulf] Looking for block size settings (from stat) on parallel filesystems

Greg Lindahl lindahl at pbm.com
Thu Jun 17 21:29:44 EDT 2010


On Thu, Jun 17, 2010 at 02:59:13PM -0600, Craig Tierney wrote:

> Why do I care?  Because in netcdf, when nf_open or nf_create are
> called, it will use the blocksize that is found in the stat structure.  On
> lustre it is 2M so writes are very fast.  However, if the number comes
> back as 4k (which some filesystems do), then writes are slower than 
> they need to be.  This isn't just a  netcdf issue.  The Linux tool cp does 
> the same thing, it will use a block  size that matches the specified 
> blocksize of the destination filesystem.

Craig,

On-node filesystems merge writes in the guts of the block device
system, so I wouldn't be surprised if 4k buffers and 2M buffers were
about the same with ext3. To get an idea if this is the case with
parallel filesystems, if people could measure the speed of dd with
various blocksizes, that would tell you a better answer than just the
blocksize.

But, of course, you will run into the usual issue of write buffering.

-- greg



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