[linux-lvm] LVM Overhead

[linux-lvm] LVM Overhead

[Joseph L. Casale]

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Hi,
I am wanting to use LVM behind an iet iscsi target and I am wondering how much overhead if any does LVM add? Would the performance degrade any, currently I am exporting full unpartitioned discs and am happy with the performance. From what I am reading now, I would likely partition the physical discs and use that in LVM then export a volume.

I am new to LVM but see some advantages long term to using it!

Thanks!
jlc



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Re: [linux-lvm] LVM Overhead

[Chris Cox]

Joseph L. Casale wrote:
> Hi,
> I am wanting to use LVM behind an iet iscsi target and I am wondering
> how much overhead if any does LVM add? Would the performance degrade
> any, currently I am exporting full unpartitioned discs and am happy with
> the performance. From what I am reading now, I would likely partition
> the physical discs and use that in LVM then export a volume.

Overhead is negligible.  I haven't seen any impact at all.

> 
>  
> 
> I am new to LVM but see some advantages long term to using it!

Definitely.

> 
>  
> 
> Thanks!
> jlc
> 


-- 
Chris Cox
Sr. Unix Sys Admin

Re: [linux-lvm] LVM Overhead

[Tomasz Chmielewski]

Chris Cox schrieb:
> Joseph L. Casale wrote:
>> Hi,
>> I am wanting to use LVM behind an iet iscsi target and I am wondering
>> how much overhead if any does LVM add? Would the performance degrade
>> any, currently I am exporting full unpartitioned discs and am happy with
>> the performance. From what I am reading now, I would likely partition
>> the physical discs and use that in LVM then export a volume.
> 
> Overhead is negligible.  I haven't seen any impact at all.

Quite the contrary - unless the default settings are not changed.


By default, the readahead values for LVM volumes is quite low / for 
iSCSI, I always have to change it to get acceptable performance (one 
might want to replace the $HOSTNAME with the name of the PV, or define a 
separate list; "setra" values might need some experimenting, too):


LVMS=$(/bin/ls /dev/$HOSTNAME/)

for LVM in $LVMS
     do
         blockdev --setra 16384 /dev/$HOSTNAME/$LVM
     done


-- 
Tomasz Chmielewski
http://blog.wpkg.org

Re: [linux-lvm] LVM Overhead

[Chris Cox]

Tomasz Chmielewski wrote:
> Chris Cox schrieb:
>> Joseph L. Casale wrote:
>>> Hi,
>>> I am wanting to use LVM behind an iet iscsi target and I am wondering
>>> how much overhead if any does LVM add? Would the performance degrade
>>> any, currently I am exporting full unpartitioned discs and am happy with
>>> the performance. From what I am reading now, I would likely partition
>>> the physical discs and use that in LVM then export a volume.
>>
>> Overhead is negligible.  I haven't seen any impact at all.
> 
> Quite the contrary - unless the default settings are not changed.
> 
> 
> By default, the readahead values for LVM volumes is quite low / for
> iSCSI, I always have to change it to get acceptable performance (one
> might want to replace the $HOSTNAME with the name of the PV, or define a
> separate list; "setra" values might need some experimenting, too):
> 
> 
> LVMS=$(/bin/ls /dev/$HOSTNAME/)
> 
> for LVM in $LVMS
>     do
>         blockdev --setra 16384 /dev/$HOSTNAME/$LVM
>     done
> 
> 

Hmmm, perhaps for iSCSI.  But using fibre SAN, I saturate my
2Gbps link (almost 200MB/sec).


-- 
Chris Cox
Sr. Unix Sys Admin

Re: [linux-lvm] LVM Overhead

[Chris Cox]

Chris Cox wrote:
> Tomasz Chmielewski wrote:
>> Chris Cox schrieb:
>>> Joseph L. Casale wrote:
>>>> Hi,
>>>> I am wanting to use LVM behind an iet iscsi target and I am wondering
>>>> how much overhead if any does LVM add? Would the performance degrade
>>>> any, currently I am exporting full unpartitioned discs and am happy with
>>>> the performance. From what I am reading now, I would likely partition
>>>> the physical discs and use that in LVM then export a volume.
>>> Overhead is negligible.  I haven't seen any impact at all.
>> Quite the contrary - unless the default settings are not changed.
>>
>>
>> By default, the readahead values for LVM volumes is quite low / for
>> iSCSI, I always have to change it to get acceptable performance (one
>> might want to replace the $HOSTNAME with the name of the PV, or define a
>> separate list; "setra" values might need some experimenting, too):
>>
>>
>> LVMS=$(/bin/ls /dev/$HOSTNAME/)
>>
>> for LVM in $LVMS
>>     do
>>         blockdev --setra 16384 /dev/$HOSTNAME/$LVM
>>     done
>>
>>
> 
> Hmmm, perhaps for iSCSI.  But using fibre SAN, I saturate my
> 2Gbps link (almost 200MB/sec).
> 
> 

I check my local drives, they all use the default 1024....
Is setting the read ahead that big of a deal with just LVM?
I would think if it's an issue, it's an issue everywhere.


-- 
Chris Cox
Sr. Unix Sys Admin

Re: [linux-lvm] LVM Overhead

[Tomasz Chmielewski]

Chris Cox schrieb:

(...)

>>> LVMS=$(/bin/ls /dev/$HOSTNAME/)
>>>
>>> for LVM in $LVMS
>>>     do
>>>         blockdev --setra 16384 /dev/$HOSTNAME/$LVM
>>>     done
>>>
>>>
>> Hmmm, perhaps for iSCSI.  But using fibre SAN, I saturate my
>> 2Gbps link (almost 200MB/sec).
>>
>>
> 
> I check my local drives, they all use the default 1024....
> Is setting the read ahead that big of a deal with just LVM?
> I would think if it's an issue, it's an issue everywhere.

For me, the default was 256.

I use that big readahead after a bit of benchmarking; perhaps it's very 
specific to my setup.


-- 
Tomasz Chmielewski
http://wpkg.org

Re: [linux-lvm] LVM Overhead

[Chris Cox]

Tomasz Chmielewski wrote:
> Chris Cox schrieb:
...
>> I check my local drives, they all use the default 1024....
>> Is setting the read ahead that big of a deal with just LVM?
>> I would think if it's an issue, it's an issue everywhere.
> 
> For me, the default was 256.
> 
> I use that big readahead after a bit of benchmarking; perhaps it's very
> specific to my setup.
> 
> 

Oh... I didn't say it wouldn't benefit, I just know in my case
it's not a HUGE win... but it is significant, significant enough
for me to want to experiment with the values and doing some
benchmarking.

256?  Whoa... that seems really low.


-- 
Chris Cox
Sr. Unix Sys Admin

RE: [linux-lvm] LVM Overhead

[Joseph L. Casale]

Thanks guys!
You'll have to bear with me here, but how and where do I run the commands you noted here to set this value?

Thanks!
jlc

-----Original Message-----
From: linux-lvm-bounces@redhat.com [mailto:linux-lvm-bounces@redhat.com] On Behalf Of Tomasz Chmielewski
Sent: Monday, September 24, 2007 9:16 AM
To: LVM general discussion and development
Subject: Re: [linux-lvm] LVM Overhead

Chris Cox schrieb:
> Joseph L. Casale wrote:
>> Hi,
>> I am wanting to use LVM behind an iet iscsi target and I am wondering
>> how much overhead if any does LVM add? Would the performance degrade
>> any, currently I am exporting full unpartitioned discs and am happy with
>> the performance. From what I am reading now, I would likely partition
>> the physical discs and use that in LVM then export a volume.
>
> Overhead is negligible.  I haven't seen any impact at all.

Quite the contrary - unless the default settings are not changed.


By default, the readahead values for LVM volumes is quite low / for
iSCSI, I always have to change it to get acceptable performance (one
might want to replace the $HOSTNAME with the name of the PV, or define a
separate list; "setra" values might need some experimenting, too):


LVMS=$(/bin/ls /dev/$HOSTNAME/)

for LVM in $LVMS
     do
         blockdev --setra 16384 /dev/$HOSTNAME/$LVM
     done


--
Tomasz Chmielewski
http://blog.wpkg.org

_______________________________________________
linux-lvm mailing list
linux-lvm@redhat.com
https://www.redhat.com/mailman/listinfo/linux-lvm
read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/

Re: [linux-lvm] LVM Overhead

[Chris Cox]

Joseph L. Casale wrote:
> Thanks guys!
> You'll have to bear with me here, but how and where do I run the commands you noted here to set this value?

I'd probably create an init script and have it executed for a given
runlevel after the mounts are done.

Some distros have a mechanism to add "user" things to end of
the init process, but if not, just grab an init script and
mod it to do the blockdev commands you need (you could make
it smart so that it does some sort of checking for valid
names, etc.)

> 
> Thanks!
> jlc
> 
> -----Original Message-----
> From: linux-lvm-bounces@redhat.com [mailto:linux-lvm-bounces@redhat.com] On Behalf Of Tomasz Chmielewski
> Sent: Monday, September 24, 2007 9:16 AM
> To: LVM general discussion and development
> Subject: Re: [linux-lvm] LVM Overhead
> 
> Chris Cox schrieb:
>> Joseph L. Casale wrote:
>>> Hi,
>>> I am wanting to use LVM behind an iet iscsi target and I am wondering
>>> how much overhead if any does LVM add? Would the performance degrade
>>> any, currently I am exporting full unpartitioned discs and am happy with
>>> the performance. From what I am reading now, I would likely partition
>>> the physical discs and use that in LVM then export a volume.
>> Overhead is negligible.  I haven't seen any impact at all.
> 
> Quite the contrary - unless the default settings are not changed.
> 
> 
> By default, the readahead values for LVM volumes is quite low / for
> iSCSI, I always have to change it to get acceptable performance (one
> might want to replace the $HOSTNAME with the name of the PV, or define a
> separate list; "setra" values might need some experimenting, too):
> 
> 
> LVMS=$(/bin/ls /dev/$HOSTNAME/)
> 
> for LVM in $LVMS
>      do
>          blockdev --setra 16384 /dev/$HOSTNAME/$LVM
>      done
> 
> 
> --
> Tomasz Chmielewski
> http://blog.wpkg.org
> 
> _______________________________________________
> linux-lvm mailing list
> linux-lvm@redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
> 
> 
> 
> 
> 
> _______________________________________________
> linux-lvm mailing list
> linux-lvm@redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
> 

RE: [linux-lvm] LVM Overhead

[Joseph L. Casale]

Thank you Chris!
jlc

-----Original Message-----
From: linux-lvm-bounces@redhat.com [mailto:linux-lvm-bounces@redhat.com] On Behalf Of Chris Cox
Sent: Wednesday, September 26, 2007 10:11 AM
To: LVM general discussion and development
Subject: Re: [linux-lvm] LVM Overhead

Joseph L. Casale wrote:
> Thanks guys!
> You'll have to bear with me here, but how and where do I run the commands you noted here to set this value?

I'd probably create an init script and have it executed for a given
runlevel after the mounts are done.

Some distros have a mechanism to add "user" things to end of
the init process, but if not, just grab an init script and
mod it to do the blockdev commands you need (you could make
it smart so that it does some sort of checking for valid
names, etc.)

>
> Thanks!
> jlc
>
> -----Original Message-----
> From: linux-lvm-bounces@redhat.com [mailto:linux-lvm-bounces@redhat.com] On Behalf Of Tomasz Chmielewski
> Sent: Monday, September 24, 2007 9:16 AM
> To: LVM general discussion and development
> Subject: Re: [linux-lvm] LVM Overhead
>
> Chris Cox schrieb:
>> Joseph L. Casale wrote:
>>> Hi,
>>> I am wanting to use LVM behind an iet iscsi target and I am wondering
>>> how much overhead if any does LVM add? Would the performance degrade
>>> any, currently I am exporting full unpartitioned discs and am happy with
>>> the performance. From what I am reading now, I would likely partition
>>> the physical discs and use that in LVM then export a volume.
>> Overhead is negligible.  I haven't seen any impact at all.
>
> Quite the contrary - unless the default settings are not changed.
>
>
> By default, the readahead values for LVM volumes is quite low / for
> iSCSI, I always have to change it to get acceptable performance (one
> might want to replace the $HOSTNAME with the name of the PV, or define a
> separate list; "setra" values might need some experimenting, too):
>
>
> LVMS=$(/bin/ls /dev/$HOSTNAME/)
>
> for LVM in $LVMS
>      do
>          blockdev --setra 16384 /dev/$HOSTNAME/$LVM
>      done
>
>
> --
> Tomasz Chmielewski
> http://blog.wpkg.org
>
> _______________________________________________
> linux-lvm mailing list
> linux-lvm@redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>
>
>
>
>
> _______________________________________________
> linux-lvm mailing list
> linux-lvm@redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>

_______________________________________________
linux-lvm mailing list
linux-lvm@redhat.com
https://www.redhat.com/mailman/listinfo/linux-lvm
read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/