a few may be wiki question

a few may be wiki question

[Farkas Levente]

hi,
i try to setup a centos host server with kvm and a few guest os for the
first time. imho there is only a very limited docs about kvm (even if i
try to read them:-). so there are a few general questions:
- which is the recommended host config?:
  - should i used x86_64 or i386 host kernel? i don't know in advance,
    but probably most guest os will be 32 bit. is there any advantage or
    disadvantage to use x86_64 as a host os?
  - i'll use kernel-2.6.18-8.1.8.el5, but i read in the announcement
    for kvm-34:
    "running on a host earlier than Linux 2.6.23-rc2 will experience
     a slight performance degradation"
    what does exactly means and how much is the "slight"? or if i use
    older kernel it's better to use kvm-33?
  - i read in faq 3.6:
    "for the host. 1GB is probably a minimum configuration for the host
     OS."
    does this means i have to reserve 1GB for the host os?
    it's strange for me just because eg with xen dom0 we use 256MB and
    it was enough since dom0 don't have any real load. it's not the case
    with kvm?
    who much ram should i use for the guest os?
    eg if i use 6 guest then i need at least 6GB of ram?
  - is dynamic use of RAM for host operating system supported?
  - what is the recommended size for the host disk partition?
  - what is the recommended swap partition size for the host os ?
- which is the recommended guest config?
  - what is the recommended memory for a guest os? same as i would run
    natively?
  - what is the recommended guest disk image format? should i use an lvm
    partition as guest os partition (like in xen)? what is the
    recommended format assuming that may be later i'd like to add new
    guests and what is the performance differences (ie. which is the
    fastest format)?
  - what is the recommended swap partition format and size for the
    guest os ?
- what is the current general state of kvm ie. stability, performance
support etc? i mean for non mission critical production server it's
worth to choose in stead of xen or not?

i know most of my questions are too general, but almost all answer will
start as "it's depend on.." but still a general guideline would be useful.

last but not least may be a kvm-user list would be useful where such
questions can send and not disturbing all the developers.

thank you for your help in advance.

-- 
  Levente                               "Si vis pacem para bellum!"

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Re: a few may be wiki question

[Avi Kivity]

Farkas Levente wrote:
> hi,
> i try to setup a centos host server with kvm and a few guest os for the
> first time. imho there is only a very limited docs about kvm (even if i
> try to read them:-). so there are a few general questions:
> - which is the recommended host config?:
>   - should i used x86_64 or i386 host kernel? i don't know in advance,
>     but probably most guest os will be 32 bit. is there any advantage or
>     disadvantage to use x86_64 as a host os?
>   

If you have more than 1GB of RAM, 64-bit will be slightly faster.  
Otherwise there is no preference.

>   - i'll use kernel-2.6.18-8.1.8.el5, but i read in the announcement
>     for kvm-34:
>     "running on a host earlier than Linux 2.6.23-rc2 will experience
>      a slight performance degradation"
>     what does exactly means and how much is the "slight"? or if i use
>     older kernel it's better to use kvm-33?
>   

It's likely not measurable for most uses.  It's recommended to always 
use the latest kvm version in order to get the latest fixes and 
enhancements.

>   - i read in faq 3.6:
>     "for the host. 1GB is probably a minimum configuration for the host
>      OS."
>     does this means i have to reserve 1GB for the host os?
>   

No.  That is the total amount of RAM on the host (and it's a 
recommendation -- you can work with less if you like).

>     it's strange for me just because eg with xen dom0 we use 256MB and
>     it was enough since dom0 don't have any real load. it's not the case
>     with kvm?
>   

It depends on what the host is doing.  If it isn't running any workload, 
even 128MB should be enough.

>     who much ram should i use for the guest os?
>   

That's for you to choose; select as much memory as the guest needs and 
you can give it.

>     eg if i use 6 guest then i need at least 6GB of ram?
>   

If each guest gets 1GB, yes.  But you can give guests less memory.

>   - is dynamic use of RAM for host operating system supported?
>   

What's that?

>   - what is the recommended size for the host disk partition?
>   

Whatever is sufficient for your distro.  I use 10GB usually (FC6/F7).

>   - what is the recommended swap partition size for the host os ?
>   

The regular recommendation of your distro.  Size of RAM is usually ok.

> - which is the recommended guest config?
>   - what is the recommended memory for a guest os? same as i would run
>     natively?
>   

Yes.  Depending on workload and budget.

>   - what is the recommended guest disk image format? should i use an lvm
>     partition as guest os partition (like in xen)? what is the
>     recommended format assuming that may be later i'd like to add new
>     guests and what is the performance differences (ie. which is the
>     fastest format)?
>   

Depends on workload.  lvm is fastest, qcow2 is most flexible and 
managable.  If your guest is not issuing significant disk I/O, I'd 
recommend qcow2.  Otherwise lvm.

>   - what is the recommended swap partition format and size for the
>     guest os ?
>   

Whatever is recommended by the guest OS vendor.

> - what is the current general state of kvm ie. stability, performance
> support etc? i mean for non mission critical production server it's
> worth to choose in stead of xen or not?
>   

I find it very stable.  Try it out!

> i know most of my questions are too general, but almost all answer will
> start as "it's depend on.." but still a general guideline would be useful.
>
> last but not least may be a kvm-user list would be useful where such
> questions can send and not disturbing all the developers.
>   

Well, my stance on this is that if a user needs to send such an email, 
then the application or documentation is missing and should be fixed.  
The developers need to be bothered until the situation is corrected.  
Having a users list is just a cover up for bad usability and documentation.

I regard this type of email as valuable feedback, not disturbance.

[now back to not writing documentation]

-- 
error compiling committee.c: too many arguments to function


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Re: a few may be wiki question

[Gregory Haskins]

On Tue, 2007-08-14 at 15:50 +0300, Avi Kivity wrote:
> Farkas Levente wrote:
> > hi,
> > i try to setup a centos host server with kvm and a few guest os for the
> > first time. imho there is only a very limited docs about kvm (even if i
> > try to read them:-). so there are a few general questions:
> > - which is the recommended host config?:
> >   - should i used x86_64 or i386 host kernel? i don't know in advance,
> >     but probably most guest os will be 32 bit. is there any advantage or
> >     disadvantage to use x86_64 as a host os?
> >   
> 
> If you have more than 1GB of RAM, 64-bit will be slightly faster.  
> Otherwise there is no preference.

One other difference is that x86_64 host can run either 32 or 64 bit
guest.  i386 can only run 32 bit.

-Greg


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Re: a few may be wiki question

[Farkas Levente]

hi,
first of all thanks for the quick reply!

Avi Kivity wrote:
> Farkas Levente wrote:
>>   - i read in faq 3.6:
>>     "for the host. 1GB is probably a minimum configuration for the host
>>      OS."
>>     does this means i have to reserve 1GB for the host os?
>>   
> 
> No.  That is the total amount of RAM on the host (and it's a
> recommendation -- you can work with less if you like).
> 
>>     it's strange for me just because eg with xen dom0 we use 256MB and
>>     it was enough since dom0 don't have any real load. it's not the case
>>     with kvm?
>>   
> 
> It depends on what the host is doing.  If it isn't running any workload,
> even 128MB should be enough.
> 
>>     who much ram should i use for the guest os?
>>   
> 
> That's for you to choose; select as much memory as the guest needs and
> you can give it.
> 
>>     eg if i use 6 guest then i need at least 6GB of ram?
>>   
> 
> If each guest gets 1GB, yes.  But you can give guests less memory.
> 
>>   - is dynamic use of RAM for host operating system supported?
>>   
> 
> What's that?

ok i try explain our plan and ask a better question about the memory
setup. this may clean things a bit. i like to setup a server for the
development dept. which compiles softwares. we need to compile on
different platforms so we need:
- one main server for the development (svn, trac, pxe boot, web, email
etc, plus compile and test) centos 5 normally just this guest have some
load.
- mandrake 8.2 and mandrake 10 (to compile and test) don't have any
load, but during compile they have high cpu and disk load.
- windows may be two different 2000 and XP (to compile and test) don't
have any load, but during compile they have high cpu and disk load.

so i need a host and about 4 guest (i assume it also not recommended to
use the host for others just to run the guests?!). i assume 4GB ram and
think about:
- 256MB for the host. is there any reason to give more memory for the
host if it's only running the guests (kernel, ntp, local mail, ssh
nothing else running on it).
- -m 2000 for centos 5
- -m 768 for each mandrake and windows
fist of all is it possible to give more dynamic memory for guests as the
sum of the whole memory? if not what is the purpose of dynamic memory?
is the host can also dynamically get/steal memory from guests? or even i
can give -m 2000 for all guest and the host and they will try to compete
for the memory?
so in short in the above setup what is the recommended config for the
host and for the guest os?

>>   - what is the recommended guest disk image format? should i use an lvm
>>     partition as guest os partition (like in xen)? what is the
>>     recommended format assuming that may be later i'd like to add new
>>     guests and what is the performance differences (ie. which is the
>>     fastest format)?
>>   
> 
> Depends on workload.  lvm is fastest, qcow2 is most flexible and
> managable.  If your guest is not issuing significant disk I/O, I'd
> recommend qcow2.  Otherwise lvm.

what does the "flexible and managable" means?
i plan the whole system is on 2 x 250gb hdd in a raid1 and the host and
each guest has an lvm partition as /.

-- 
  Levente                               "Si vis pacem para bellum!"

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Re: a few may be wiki question

[Avi Kivity]

Farkas Levente wrote:
>>
>>>   - is dynamic use of RAM for host operating system supported?
>>>   
>>>       
>> What's that?
>>     
>
> ok i try explain our plan and ask a better question about the memory
> setup. this may clean things a bit. i like to setup a server for the
> development dept. which compiles softwares. we need to compile on
> different platforms so we need:
> - one main server for the development (svn, trac, pxe boot, web, email
> etc, plus compile and test) centos 5 normally just this guest have some
> load.
> - mandrake 8.2 and mandrake 10 (to compile and test) don't have any
> load, but during compile they have high cpu and disk load.
> - windows may be two different 2000 and XP (to compile and test) don't
> have any load, but during compile they have high cpu and disk load.
>
> so i need a host and about 4 guest (i assume it also not recommended to
> use the host for others just to run the guests?!). i assume 4GB ram and
> think about:
> - 256MB for the host. is there any reason to give more memory for the
> host if it's only running the guests (kernel, ntp, local mail, ssh
> nothing else running on it).
> - -m 2000 for centos 5
> - -m 768 for each mandrake and windows
> fist of all is it possible to give more dynamic memory for guests as the
> sum of the whole memory? if not what is the purpose of dynamic memory?
> is the host can also dynamically get/steal memory from guests? or even i
> can give -m 2000 for all guest and the host and they will try to compete
> for the memory?
>   

There are two answers to this:

- balloon driver (like in Xen) which allows the host to move memory from 
one guest to another (or from host to guest)
- swapping, which allows the host to move guest memory to disk and 
allocate it to another guest

Unfortunately the balloon driver has not been publicly released yet, and 
swapping is still in early development, so for now you will have to use 
static allocation.


> so in short in the above setup what is the recommended config for the
> host and for the guest os?
>   

It really depends on the workload.  256MB for the host is fine, though.

>   
>>>   - what is the recommended guest disk image format? should i use an lvm
>>>     partition as guest os partition (like in xen)? what is the
>>>     recommended format assuming that may be later i'd like to add new
>>>     guests and what is the performance differences (ie. which is the
>>>     fastest format)?
>>>   
>>>       
>> Depends on workload.  lvm is fastest, qcow2 is most flexible and
>> managable.  If your guest is not issuing significant disk I/O, I'd
>> recommend qcow2.  Otherwise lvm.
>>     
>
> what does the "flexible and managable" means?
> i plan the whole system is on 2 x 250gb hdd in a raid1 and the host and
> each guest has an lvm partition as /.
>
>   

Flexible means you don't need to allocate all disk space up front, it 
grows as necessary.  With lvm you can defer allocation, but you need 
manual intervention (and changes in the guest) in order to use more 
storage.  It's also easier to back up, move around, etc.

If you have enough space, then lvm can be a good choice.  Certainly it 
gives the best performance.

-- 
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Re: a few may be wiki question

[Farkas Levente]

Avi Kivity wrote:
> There are two answers to this:
> 
> - balloon driver (like in Xen) which allows the host to move memory from
> one guest to another (or from host to guest)
> - swapping, which allows the host to move guest memory to disk and
> allocate it to another guest
> 
> Unfortunately the balloon driver has not been publicly released yet, and
> swapping is still in early development, so for now you will have to use
> static allocation.

ie. the sum of the host and the guests memory should have to be equal
with the total physical memory?
but then what this faq means?
http://kvm.qumranet.com/kvmwiki/FAQ#head-03de45d48dbc4f37baa6e42b8c14a5bd0182b2e2

-- 
  Levente                               "Si vis pacem para bellum!"

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Re: a few may be wiki question

[Avi Kivity]

Farkas Levente wrote:
> Avi Kivity wrote:
>   
>> There are two answers to this:
>>
>> - balloon driver (like in Xen) which allows the host to move memory from
>> one guest to another (or from host to guest)
>> - swapping, which allows the host to move guest memory to disk and
>> allocate it to another guest
>>
>> Unfortunately the balloon driver has not been publicly released yet, and
>> swapping is still in early development, so for now you will have to use
>> static allocation.
>>     
>
> ie. the sum of the host and the guests memory should have to be equal
> with the total physical memory?
>   

At present, yes.

> but then what this faq means?
> http://kvm.qumranet.com/kvmwiki/FAQ#head-03de45d48dbc4f37baa6e42b8c14a5bd0182b2e2
>
>   

No idea.  I didn't add it :)

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Re: a few may be wiki question

[Farkas Levente]

Avi Kivity wrote:
> There are two answers to this:
> 
> - balloon driver (like in Xen) which allows the host to move memory from
> one guest to another (or from host to guest)
> - swapping, which allows the host to move guest memory to disk and
> allocate it to another guest
> 
> Unfortunately the balloon driver has not been publicly released yet, and
> swapping is still in early development, so for now you will have to use
> static allocation.

which will be available sooner? and an estimate when?
this rise another question if swapping will be used the it moves the
guest memory to the guest's swap or the host's swap? if to the host's
swap then this implies i should have to allocate large enough swap for
the host. ie. even if i give only 256mb to the host still have to gives
6-8GB swap partition to the host.

still believe that these memory, swap, and other disk partitions should
have to be explain in the faq.

-- 
  Levente                               "Si vis pacem para bellum!"

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Re: a few may be wiki question

[Avi Kivity]

Farkas Levente wrote:
> Avi Kivity wrote:
>   
>> There are two answers to this:
>>
>> - balloon driver (like in Xen) which allows the host to move memory from
>> one guest to another (or from host to guest)
>> - swapping, which allows the host to move guest memory to disk and
>> allocate it to another guest
>>
>> Unfortunately the balloon driver has not been publicly released yet, and
>> swapping is still in early development, so for now you will have to use
>> static allocation.
>>     
>
> which will be available sooner? and an estimate when?
>   

We already have a balloon driver, however it is not yet ready for 
inclusion.  Perhaps Dor will comment.   Swapping is some way off; there 
is a fairly complete patch from Shaohua Li that does this, but it 
requires kernel changes and we are investigating other approaches.

> this rise another question if swapping will be used the it moves the
> guest memory to the guest's swap or the host's swap? if to the host's
> swap then this implies i should have to allocate large enough swap for
> the host. ie. even if i give only 256mb to the host still have to gives
> 6-8GB swap partition to the host.
>   

Yes, when swapping is implemented then guest memory will be swapped to 
the host swap.  Of course, if the guest has its own swap file, then it 
will swap to the guest's swap independently.

> still believe that these memory, swap, and other disk partitions should
> have to be explain in the faq.
>   

Yes.

-- 
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Re: a few may be wiki question

[Gregory Haskins]

On Wed, 2007-08-15 at 11:07 +0300, Avi Kivity wrote:
> Farkas Levente wrote:
> > this rise another question if swapping will be used the it moves the
> > guest memory to the guest's swap or the host's swap? if to the host's
> > swap then this implies i should have to allocate large enough swap for
> > the host. ie. even if i give only 256mb to the host still have to gives
> > 6-8GB swap partition to the host.
> >   
> 
> Yes, when swapping is implemented then guest memory will be swapped to 
> the host swap.  Of course, if the guest has its own swap file, then it 
> will swap to the guest's swap independently.

And to clarify: FYI, I believe the guest-swap option should actually
work today.  If you assign a partition as a swap device in the guest it
should use it.  kvm-host will simply see it as disk io as any other.
This is in contrast to the host-swap and/or balloon driver which is
still a work-in-progress.

On thing that is interesting about this (to me) is that, in a way it
kind of is a "poor mans" swap for the guests memory on the host.  E.g.
you could give your guests a really small amount of "physical" ram (say,
I dunno, 64MB/ea) and a large swap file (say several gigs).  Since the
disk-io emanating from the guest would likely be mediated by the hosts
buffercache, its kind of like you just gave the guest a large chunk of
(indirectly accessible) pageable ram.

OTOH, virtualized disk-io is pretty slow compared to bare-metal io.  As
I said, this is a "poor mans" solution ;)  What it really comes down to
is: does having emulated disk-io to a buffercache offer adequate
performance for your applications running in the guest?  If so, this may
be a good interim solution to get a larger number of guests running on a
given host with "dynamic ram".

Regards,
-Greg



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Re: a few may be wiki question

[Farkas Levente]

Gregory Haskins wrote:
> On Wed, 2007-08-15 at 11:07 +0300, Avi Kivity wrote:
>> Farkas Levente wrote:
>>> this rise another question if swapping will be used the it moves the
>>> guest memory to the guest's swap or the host's swap? if to the host's
>>> swap then this implies i should have to allocate large enough swap for
>>> the host. ie. even if i give only 256mb to the host still have to gives
>>> 6-8GB swap partition to the host.
>>>   
>> Yes, when swapping is implemented then guest memory will be swapped to 
>> the host swap.  Of course, if the guest has its own swap file, then it 
>> will swap to the guest's swap independently.
> 
> And to clarify: FYI, I believe the guest-swap option should actually
> work today.  If you assign a partition as a swap device in the guest it
> should use it.  kvm-host will simply see it as disk io as any other.
> This is in contrast to the host-swap and/or balloon driver which is
> still a work-in-progress.
> 
> On thing that is interesting about this (to me) is that, in a way it
> kind of is a "poor mans" swap for the guests memory on the host.  E.g.
> you could give your guests a really small amount of "physical" ram (say,
> I dunno, 64MB/ea) and a large swap file (say several gigs).  Since the
> disk-io emanating from the guest would likely be mediated by the hosts
> buffercache, its kind of like you just gave the guest a large chunk of
> (indirectly accessible) pageable ram.

i just imagine th opposite setup. suppose i've got 4 guest and 4gb ram.
most of the time guest are idle they just used as a compile server for
different platforms (centos, mandrake, fedora, windows etc). i can
schedule the compiles (ie. 1:00 guest1, 2:00 guest 2 etc). in this case
i gives the host 1-2gb and all guest 2-3gb logical ram. which is in sum
9-14gb ram even i have only 4gb physical ram.
_BUT_ as i know that the guests are usually idle _and_ they have high
resource requirement at different time, probably all of them can use
2-3gb real ram as memory. and in this setup if all other time most of
the guest's ram are swapped out it's not really bother me since they are
idle.
or did is something misunderstood?

-- 
  Levente                               "Si vis pacem para bellum!"

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Re: a few may be wiki question

[Gregory Haskins]

On Wed, 2007-08-15 at 15:52 +0200, Farkas Levente wrote:
> Gregory Haskins wrote:
> > 
> > On thing that is interesting about this (to me) is that, in a way it
> > kind of is a "poor mans" swap for the guests memory on the host.  E.g.
> > you could give your guests a really small amount of "physical" ram (say,
> > I dunno, 64MB/ea) and a large swap file (say several gigs).  Since the
> > disk-io emanating from the guest would likely be mediated by the hosts
> > buffercache, its kind of like you just gave the guest a large chunk of
> > (indirectly accessible) pageable ram.
> 
> i just imagine th opposite setup. suppose i've got 4 guest and 4gb ram.
> most of the time guest are idle they just used as a compile server for
> different platforms (centos, mandrake, fedora, windows etc). i can
> schedule the compiles (ie. 1:00 guest1, 2:00 guest 2 etc). in this case
> i gives the host 1-2gb and all guest 2-3gb logical ram. which is in sum
> 9-14gb ram even i have only 4gb physical ram.
> _BUT_ as i know that the guests are usually idle _and_ they have high
> resource requirement at different time, probably all of them can use
> 2-3gb real ram as memory. and in this setup if all other time most of
> the guest's ram are swapped out it's not really bother me since they are
> idle.
> or did is something misunderstood?

I think you are just misunderstanding me.  We are saying the same thing
(I think) :)

What it sounds like you are talking about is oversubscribing your ram.
IIUC this is not possible today unless you employ a guest-swap scheme
like I mentioned.  Until the balloon/swap support is in, the guest "ram"
allocation aggregated across all VMs cannot exceed host-physical ram (in
your case, 4GB).  In fact, it has to be a little less of course so the
host still has some to play with ;)




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