Re: [CentOS-virt] img partitioning & swap

[Ben M.]

 > such as swap on ramdisk if you're running a kernel that murders 
itself when it has no swap.

Thanks for that reminder. I forget how practical that is at times.

I agree with throwing ram and trimming swap space down. On windows they 
want a huge swap so the system can dump. I think the last time I cared 
to spend the time to trouble shoot a windows machine via a dump was over 
12 years ago. Maybe more. Is a lot cheaper and quicker to just start 
rebuilding most of the time.


Christopher G. Stach II wrote:
> - "Adam"  wrote:
> 
>> There is no need for a second img to use as swap right?
> 
> Rereading your original message got me to wondering whether the examples you 
> saw were putting swap on local storage and the filesystems on remote storage. 
> You can do that for swap performance, but then you can't migrate VMs with the 
> stock tools. It's better to just allocate a decent swap space, store it over 
> the network, and avoid using swap (allocate more RAM, turn down swappiness, 
> etc.) You can also just not use a disk for swap space, such as swap on 
> ramdisk if you're running a kernel that murders itself when it has no swap.
> 

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Re: [CentOS-virt] img partitioning & swap

[Ben M.]

 > There is no need for a second img to use as swap right?

Yes and no. On physical machines I am very used to putting swaps on 
separate controllers and drives for performance reasons as am sure many 
others here are.

I have yet to see that pay off on Xen, but I really haven't had a 
"hammered on" system in production yet. All are smaller and much, much 
more focused on precisely what they need to do and not commingled as 
multipurpose machines (e.g. webserver + file server + vpn server).

It is an engineering and use question and YMMV, but you can always add 
it later if the need arises and by really measuring where you are 
bottlenecking.

Adam wrote:
> There is no need for a second img to use as swap right?
> 
> -Adam
> 
> On Sun, Nov 1, 2009 at 7:34 AM, Dennis J.  > wrote:
> 
> On 11/01/2009 10:51 AM, Manuel Wolfshant wrote:
>  > On 11/01/2009 08:37 AM, Brett Worth wrote:
>  >> Christopher G. Stach II wrote:
>  >>
>   I'd recommend not using LVM inside the images because if you
> just have
>   a raw disk image in
>   there with regular partitions you can mount it on dom0 (with
> losetup)
>   for maintenance.  I
>   don't think that would be possible with LVM.
>  
>  >>> But it is.
>  >>>
>  >>
>  >> I guess that's informative so why don't I feel informed? :-)
>  >>
>  >> OK.  I'll bite.  How?
>  > using the procedure described at
>  >
> 
> http://www.centos.org/docs/5/html/5.2/Virtualization/sect-Virtualization-How_To_troubleshoot_Red_Hat_Virtualization-Accessing_data_on_guest_disk_image.html
> 
> It should be mentioned that it's important not to accept the default
> volume
> group name when using LVM as that will lead to a collision in a case
> such
> as this where the VG name of both host and guest might end up beeing
> "VolGroup00". I hope RHEL/Centos 6 chooses better defaults based on the
> hostname for example.
> 
> Regards,
>   Dennis
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> 
> 
> 
> 
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Re: [CentOS-virt] img partitioning & swap

[Christopher G. Stach II]

- "Adam"  wrote:

> There is no need for a second img to use as swap right?

Rereading your original message got me to wondering whether the examples you 
saw were putting swap on local storage and the filesystems on remote storage. 
You can do that for swap performance, but then you can't migrate VMs with the 
stock tools. It's better to just allocate a decent swap space, store it over 
the network, and avoid using swap (allocate more RAM, turn down swappiness, 
etc.) You can also just not use a disk for swap space, such as swap on ramdisk 
if you're running a kernel that murders itself when it has no swap.

-- 
Christopher G. Stach II


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Re: [CentOS-virt] img partitioning & swap

[Christopher G. Stach II]

- "Adam"  wrote:

> There is no need for a second img to use as swap right?

Correct, unless you were in a pinch and strangely needed to add more swap, had 
no unallocated space in your image, and couldn't shut down the guest to resize 
the image.

-- 
Christopher G. Stach II


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Re: [CentOS-virt] img partitioning & swap

[Adam]

There is no need for a second img to use as swap right?

-Adam

On Sun, Nov 1, 2009 at 7:34 AM, Dennis J.  wrote:

> On 11/01/2009 10:51 AM, Manuel Wolfshant wrote:
> > On 11/01/2009 08:37 AM, Brett Worth wrote:
> >> Christopher G. Stach II wrote:
> >>
>  I'd recommend not using LVM inside the images because if you just have
>  a raw disk image in
>  there with regular partitions you can mount it on dom0 (with losetup)
>  for maintenance.  I
>  don't think that would be possible with LVM.
> 
> >>> But it is.
> >>>
> >>
> >> I guess that's informative so why don't I feel informed? :-)
> >>
> >> OK.  I'll bite.  How?
> > using the procedure described at
> >
> http://www.centos.org/docs/5/html/5.2/Virtualization/sect-Virtualization-How_To_troubleshoot_Red_Hat_Virtualization-Accessing_data_on_guest_disk_image.html
>
> It should be mentioned that it's important not to accept the default volume
> group name when using LVM as that will lead to a collision in a case such
> as this where the VG name of both host and guest might end up beeing
> "VolGroup00". I hope RHEL/Centos 6 chooses better defaults based on the
> hostname for example.
>
> Regards,
>Dennis
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Re: [CentOS-virt] img partitioning & swap

[Dennis J.]

On 11/01/2009 10:51 AM, Manuel Wolfshant wrote:
> On 11/01/2009 08:37 AM, Brett Worth wrote:
>> Christopher G. Stach II wrote:
>>
 I'd recommend not using LVM inside the images because if you just have
 a raw disk image in
 there with regular partitions you can mount it on dom0 (with losetup)
 for maintenance.  I
 don't think that would be possible with LVM.

>>> But it is.
>>>
>>
>> I guess that's informative so why don't I feel informed? :-)
>>
>> OK.  I'll bite.  How?
> using the procedure described at
> http://www.centos.org/docs/5/html/5.2/Virtualization/sect-Virtualization-How_To_troubleshoot_Red_Hat_Virtualization-Accessing_data_on_guest_disk_image.html

It should be mentioned that it's important not to accept the default volume 
group name when using LVM as that will lead to a collision in a case such 
as this where the VG name of both host and guest might end up beeing 
"VolGroup00". I hope RHEL/Centos 6 chooses better defaults based on the 
hostname for example.

Regards,
   Dennis
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Re: [CentOS-virt] img partitioning & swap

[Manuel Wolfshant]

On 11/01/2009 08:37 AM, Brett Worth wrote:
> Christopher G. Stach II wrote:
>   
>>> I'd recommend not using LVM inside the images because if you just have
>>> a raw disk image in
>>> there with regular partitions you can mount it on dom0 (with losetup)
>>> for maintenance.  I
>>> don't think that would be possible with LVM.
>>>   
>> But it is.
>> 
>
> I guess that's informative so why don't I feel informed? :-)
>
> OK.  I'll bite.  How?
using the procedure described at 
http://www.centos.org/docs/5/html/5.2/Virtualization/sect-Virtualization-How_To_troubleshoot_Red_Hat_Virtualization-Accessing_data_on_guest_disk_image.html
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Re: [CentOS-virt] img partitioning & swap

[Brett Worth]

Christopher G. Stach II wrote:
>> I'd recommend not using LVM inside the images because if you just have
>> a raw disk image in
>> there with regular partitions you can mount it on dom0 (with losetup)
>> for maintenance.  I
>> don't think that would be possible with LVM.
> 
> But it is.

I guess that's informative so why don't I feel informed? :-)

OK.  I'll bite.  How?

Brett


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Re: [CentOS-virt] img partitioning & swap

[Christopher G. Stach II]

- "Brett Worth"  wrote:

> Adam wrote:
> 
> > What do you guys use for your partition scheme for .img files? I
> see
> > examples where people create another .img to use it as swap. I was
> gonna
> > put lvm inside the .img and create separate filesystems in logical
> > volumes(/var, /home, /opt, /tmp and swap).
> 
> I'd recommend not using LVM inside the images because if you just have
> a raw disk image in
> there with regular partitions you can mount it on dom0 (with losetup)
> for maintenance.  I
> don't think that would be possible with LVM.

But it is.

-- 
Christopher G. Stach II


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Re: [CentOS-virt] img partitioning & swap

[Brett Worth]

Adam wrote:

> What do you guys use for your partition scheme for .img files? I see
> examples where people create another .img to use it as swap. I was gonna
> put lvm inside the .img and create separate filesystems in logical
> volumes(/var, /home, /opt, /tmp and swap).

I'd recommend not using LVM inside the images because if you just have a raw 
disk image in
there with regular partitions you can mount it on dom0 (with losetup) for 
maintenance.  I
don't think that would be possible with LVM.

Brett
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[CentOS-virt] img partitioning & swap

[Adam]

Hello everyone,

I am moving a 2 node cluster from XenServer + iSCSI with lvm to CentOS
5.4+Xen on NFS using .img files.

What do you guys use for your partition scheme for .img files? I see
examples where people create another .img to use it as swap. I was gonna put
lvm inside the .img and create separate filesystems in logical volumes(/var,
/home, /opt, /tmp and swap).


-Adam
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