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Date: Mon, 24 Sep 2007 11:39:21 -0500
From: Chris Cox <chris_cox@stercomm.com>
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Subject: Re: [linux-lvm] LVM Overhead
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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