[Xen-users] Re: Mounting domU filesystem on Dom0. (xm mount)

[Ligesh <myself@xxxxxxxxxx>]

On Wed, Mar 14, 2007 at 07:50:57PM +0700, Fajar A. Nugraha wrote:
> >
> >  With LVM, you get filesystem (kernel) level corruption. With nfs, you get 
> > application level corruption. The first is obviously graver. 
> I have to disagree.
> 
> When you restore something backed up from LVM snapshot, it is as if the
> system was powered off at the time of the snapshot. As Siim mentioned,
> some applications (e.g : Oracle or MySQL with correct settings) can
> handle that nicely and provides a consistent, recovered, usable data.

The point is that the corruption is at the kernel level itself, and if at all 
any recovery can be made, it has to be done at the file system level (ext3, or 
reiserfs or whatever recovery tools). Applications don't actually come into the 
picture. We have already discussed this earlier. The ideal would be a 
'sync-and-save' command which would actually be equivalent to a 'kill -9'ing 
the application, which is something all applications can recover from. But LVM 
data corruption happens at the file system level, and has to be recovered first 
using e2fsck, before mysql can even have a go at it fixing the rest of the 
problems.

 So LVMs have two levels of corruption: File system, and 'kill -9'ing the 
application. Mysql can recover the latter. An e2fsck on the LVM snapshot before 
tarring up might actually reduce the chances of corruption, but I am not sure.

[quote] 
> 
> Nfs backups, on the other hand, has no way guaranting consistent data.
> An extreme example, think of a 100-GB MySQL Innodb datafile. If you copy
> that file (via nfs or whatever) when MySQL server is running, the first
> part and last part will most likely come from different points in time.
> Good luck recovering that.
[/quote] 

 This might appear grave, but in practice it is not. Of course, the scenario I 
am talking about is 10s of thousands of small domUs running small workloads. As 
I have pointed out in the beginning, the purpose is mass management of LARGE 
number of light workloads. If you have heavy workload, then individual 
management of each domU becomes practical, and then we are talking about 
different scenarios altogether.

 Further, in practice, this particular method has been employed for 5 years by 
virtuozzo service providers to deliver managed services to their clients. And 
again, please note, we are talking about large number of small workloads. If 
you have small number of large workloads, then you don't need the system at 
all. You can run a full fledged backup system on each domU.



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