Re: [CentOS] Possible to use multiple disk to bypass I/O wait?

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Re: [CentOS] Possible to use multiple disk to bypass I/O wait?

Gordon Messmer
On 06/10/2011 08:17 PM, Emmanuel Noobadmin wrote:
> The irony of it was that I decided to go with qcow2 because I thought
> that would save overheads from an additional LVM layer but provided
> snapshot capabilities too :(

I read somewhere recently that people were complaining abut LVM overhead
and poor performance, but I've never seen any evidence of it.  Was there
something that made you think that LVM had significant overhead?
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Re: [CentOS] Possible to use multiple disk to bypass I/O wait?

Emmanuel Noobadmin
On 6/16/11, Gordon Messmer <[hidden email]> wrote:

> I read somewhere recently that people were complaining abut LVM overhead
> and poor performance, but I've never seen any evidence of it.  Was there
> something that made you think that LVM had significant overhead?

Looking at some very sparse notes I made on the decision, I think what
tipped the choice was that both qcow2 and lvm added overheads, but lvm
was on the whole system i.e. the host has additional processing on
every i/o whereas qcow2 overheads was only for guest i/o. More
critically my note was the thought as well that it would be easier to
move a qcow2 file to another machine/disk if necessary than to move a
partition.
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Re: [CentOS] Possible to use multiple disk to bypass I/O wait?

Gordon Messmer
On 06/15/2011 07:04 PM, Emmanuel Noobadmin wrote:
> Looking at some very sparse notes I made on the decision, I think what
> tipped the choice was that both qcow2 and lvm added overheads, but lvm
> was on the whole system i.e. the host has additional processing on
> every i/o whereas qcow2 overheads was only for guest i/o.

I think you were misinformed, or misled.  LVM should not present any
noticeable overhead on the host.  Using "raw" files to back VMs presents
a significant overhead to guests; the host performs all IO through its
filesystem.  Using "qcow2" files presents even more overhead (probably
the most of any configuration) since there are complexities to the qcow2
file itself in addition to the host's filesystem.

> More
> critically my note was the thought as well that it would be easier to
> move a qcow2 file to another machine/disk if necessary than to move a
> partition.

It shouldn't be significantly harder to copy the contents of a partition
or LV.  The block device is a file.  You can read its contents to copy
them just as easily as any other file.
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Re: [CentOS] Possible to use multiple disk to bypass I/O wait?

Emmanuel Noobadmin
On 6/16/11, Gordon Messmer <[hidden email]> wrote:
> I think you were misinformed, or misled.

That wouldn't be new for me as far as system administration is concerned :D

>LVM should not present any
> noticeable overhead on the host.  Using "raw" files to back VMs presents
> a significant overhead to guests; the host performs all IO through its
> filesystem.  Using "qcow2" files presents even more overhead (probably
> the most of any configuration) since there are complexities to the qcow2
> file itself in addition to the host's filesystem.

I was concerned about qcow2 vs raw as well since it seemed logical
that qcow2 would be slower for the added functionality. However there
was some site I found that showed that KVM with virtio, turning off
host caching (or specifying write-back instead of the default
write-through) on the file and doing preallocation on qcow2 files will
make qcow2 as fast as raw.

> It shouldn't be significantly harder to copy the contents of a partition
> or LV.  The block device is a file.  You can read its contents to copy
> them just as easily as any other file.

Although the combination of ionice and atime seemed to have stopped
things from going through the roof, I'll probably still try to convert
one of them to LVM and see if that improves things even further.
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