Re: [linux-lvm] Testing the new LVM cache feature

[Mike Snitzer <snitzer@redhat.com>]

On Fri, May 30 2014 at 10:51am -0400,
Richard W.M. Jones <rjones@redhat.com> wrote:

> On Fri, May 30, 2014 at 10:44:54AM -0400, Mike Snitzer wrote:
> > I'm not talking about alignment relative to the physical device's
> > limits.  I'm talking about alignment of ext4's data areas relative to
> > the 64K block boundaries.
> > 
> > Also a point of conern would be: how fragmented is the ext4 space?  It
> > could be that it cannot get contiguous 64K regions from the namespace.
> > If that is the case than a lot more IO would get pulled in.
> 
> I would be surprised if it was fragmented, since it's a recently
> created filesystem which has only been used to store a few huge disk
> images ...
> 
> > Can you try reducing the cache blocksize to 32K (lowest we support at
> > the moment, it'll require you to remove the cache and recreate) to see
> > if performance for this 64K random IO workload improves?  If so it does
> > start to add weight to my alignment concerns.
> 
> ... nevertheless what I will do is recreate the origin LV, ext4
> filesystem, and change the block size.

You don't need to recreate the origin LV or FS.
If anything that'd reduce our ability to answer what may be currently
wrong with the setup.  I was just suggesting removing the cache and
recreating the cache layer.  Not saure how easy it is to do that with
the lvm2 interface.  Jon and/or Kabi?
 
> What is the command to set the cache blocksize?  It doesn't seem to be
> covered in the documentation anywhere.

I would think it is lvconvert's --chunksize...