Re: XFS journaling position

[Michael Monnerie <michael.monnerie@is.it-management.at>]

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On Freitag, 29. Oktober 2010 Stan Hoeppner wrote:
> So as we move to a totally virtualized guest OS, we then lose the
> stripe width and stripe size information.  How much performance does
> this really cost us WRT XFS filesystem layout?  

This is still a very straightforward config. When you use a NetApp 
storage with virtualization, you normally use thin provisioning. So you 
put one VM with it's root FS on that system, and assign a data disk of 
1.4TB. You then "flexclone" that VM 9 times, so you have 10 VMs running 
but they only use the diskspace of one. Then you upgrade or modify one 
VM, and only the blocks that are modified get copied on the disks. 
You fill 5 VMs in parallel with data, and it gets stored totally 
"fragmented" on the storage. Now do a snapshot of a VM, so it's old 
contents are frozen and every write gets written to a new place.
There are companies who do a snapshot of every VM every hour, so their 
users can by themself recover files that they deleted/modifed wrongly.

And now take into account that NetApp uses WAFL - write anywhere file 
layout. It means that even if you have a VM straight on disk, as soon as 
you modify a block it can be written in a totally different place on the 
storage.

I'd say this is such a "total mess" where you know exactly *nothing* 
about the layout of anything. You cannot even optimize for stripe size.

> How many folks are running their
> critical core business databases in virtual machine guests?  How
> about core email systems? Other performance/business critical
> applications?

I don't know which country you are from, I'm from Austria/Europe (not 
the kangaroo country :-). On every single tech talk and presentation I 
visited this year, every single speaker talked about virtualization. 
Depending on who spoke, either 2009 or 2010 were the years where more 
virtual servers were deployed than physical ones, with a sharp increase 
each year. We didn't sell a single server running bare-metal OS, all had 
VMware or XenServer.

And for business critical: SAP uses virtualization everywhere in-house, 
ÖBB (Austrian Railways) use Xen for virtualized central accounting 
program since 2001 exclusively. These are just the 2 companies who were 
speaking on last weeks presentation, there are far more.

-- 
mit freundlichen Grüssen,
Michael Monnerie, Ing. BSc

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