[Xen-users] Re: XEN - networking and performance

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Jeff Sturm wrote:

> Anyone tried SSD with good results?  I'm sure capacity requirements 
> can make it cost-prohibitive for many.
Interesting that a product I recall from 'some years ago' doesn't 
seem to have popped up again - or perhaps it has and I never noticed 
since I'm not into high end storage. This device looked to the host 
like a standard SCSI disk, but internally it had a load of DRAM, a 
small (2 1/2" ?) disk, a controller, and a small battery.
Basically it was a big RAM disk with a SCSI interface, but when the 
power went off it would write everything to disk. I suspect it 
probably had a continuous process of writing dirty blocks to disk.
Mind you, I suppose RAM does still cost somewhat more than disk.

fpt stl wrote:

> >  Also, copying 8 GB of data from one disk to another takes 50 
> > (fifty) minutes !!! - both LVMs attached separately to the DomU 
> > as two independent volumes [xvda - /dev/mapper/VG1-VM1 and xvdb - 
> > /dev/mapper/VG2-VM1_home].
> Unfortunately, at this point I cannot reconfigure the host's 
> storage. But what would be the course of action taking in 
> consideration the exiting storage configuration? Any tips to improve 
> performance? 50 minutes for 8GB is rather slooooow.
What is most likely happening here is that while your OS sees the 
storage as two devices, in fact they are on the same disk (or set of 
disks). So the copy becomes :
read a bit - seek - write a bit - seek write some metadata - seek - 
read a bit - seek - write a bit ...
That's a lot of seeking and seeks kill performance really badly.

It also depend son what that 8G is. A small number of big files 
stands a half decent chance of using some write cache to buffer some 
of the seeks, but if it's lots of small files then there'll be a huge 
amount of filesystem metadata to be updated as well.
And it also depend on what you are using for the copy. Some programs 
(such as dd and cpio) allow you to set a blocksize. Increasing this 
as far as your memory allows will help as that would mean reading a 
big chunk of data before seeking elsewhere to write it. Less seeks = 
better performance.
--
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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