Re: [Nbd] bcache on NBD block devices

[Juan Antonio Martinez <jantonio@...1345...>]

El mié, 26-02-2014 a las 11:21 +0100, Wouter Verhelst escribió:
> Op woensdag 26 februari 2014 09:31:50 schreef Juan Antonio Martinez:
> > El mar, 25-02-2014 a las 11:37 -0500, Paul Clements escribió:
> > > On Tue, Feb 25, 2014 at 9:40 AM, Juan Antonio Martinez
> > > 
> > > <jantonio@...1345...> wrote:
> > > > Hi all
> > > > (This is my first post on this list. I'm still a novice on bcache and
> > > > proper nettiquete on this list, so apologize for my mistakes)
> > > > Scenario: several NBD LTSP Fat Clients on (a bit) obsolete hardware
> > > > kernel 3.11.0-15 on Ubuntu 13.10
> > > > /dev/sda1 as local cache device
> > > > /dev/nbd1 as remote NBD backing bcache device
> > > > - I've created and registered /dev/sda1 as cache device without problems
> > > > - To create nbd file to be exported I've typed following sequence:
> > > > # create an empty file
> > > > root# dd if=/dev/zero of=bcache_test.img bs=1M count=64
> > > > # use it as loop block device
> > > > root# losetup /dev/loop0
> > > > # make it a backing bcache device
> > > > root# make-bcache -B /dev/loop0
> > > 
> > > have you tried doing this step from the client machine instead, i.e.:
> > > make-bache -B /dev/nbd1
> > > after the nbd connection is up? does that work?
> > 
> > Just tested: works fine.... but only in the LTSP client where I did the
> > changes, and had to tell nbd-server to export imagefile in read-write
> > mode
> 
> In that case, I'm not entirely sure what you're trying to do.
> 
> Can you explain what you are trying to cache? How do you expect performance 
> improvements by that caching?

I'll try to explain: (excuses for my poor english)

- This is an student's lab in our university, with 200+ students working
and learning with development tools: c/c++,java,nodejs, eclipse, android
sdk, and so
- NBD Fat clients are really old PC's: little memory (1Gb). For security
(and load) reasons we don't want to use thin clients
- clients have a separate NFS /home. Using LDAP for authentication
- Image files are really large (8Gb) and contains an entire Linux
system, with complete development tools and utilities. When an student
start working in a client, we can see a very low performance due to
little memory for caching and subsecuent increasing of network load to
get data from NBD server

In an ideal scheme, just increasing memory in the clients will be
enought. As the clients are so old ,has no sense to spend money on them.
So we are looking for a method to decrease network load by mean of
caching nbd disk access into client's local (and unused) drive... and
perhaps a little improvement in client performance

My tests says that client responsiveness doesn't increase noticeabilly,
(PC's are old, disk are also old) but at this moment decreasing network
load is enought for us

Juan Antonio