[Linux-PowerEdge] Extremely poor performance with LVM vs. RAW disc

L. A. Walsh, Tlinx Solutions dell at tlinx.org
Wed Nov 7 01:35:03 CST 2012



Sabuj Pattanayek wrote:
> linda(I) asked:
>> And what do you mean by whole disk lvm?
>>     
>
> Adding the entire disk as the pv to the vg rather than a partition of it.
>   
----
    Ahhh....  I use entire disks for lvm, but inside 1 large partition 
-- primarily for safety purposes.  If I have a disk and want to ID it, 
I'll run parted -l (used to fdisk -l).  If it's formatted with lvm raw, 
I don't think that works.  Losing 1 data disk with the 'wrong data' on 
it, will cost me more than I'll ever save by not putting some OS 
readable label on it to tell me what it is...

    Windows, Mac, Linux all recognize a guid-based table and will tell you
there is data on the disk, but unlabeled, you have to have the right util
running on linux, only, to tell you something.
>   
>> What constraints do partitions put on you?...(using guid...not old
>> PC-bios partitioning)?
>>     
>
> It's just another thing to have to resize if the original partition 
> hadn't filled up the disk if you want to keep resizing the same 
> partition rather than adding another one. I think the more partitions 
> you have for LVM on a single disk the worse it's performance becomes.
>   
----
    Well, I can think of reasons why that would be the case -- if they are
discontiguous and/or not aligned with any RAID boundaries... and 
certainly it's
not as efficient with multiple tables, but if all things were aligned 
performance should be equal.    That said, I would consider it "bad 
practice"
to setup separate lvm partitions that you combine later on the same disk --
too easy to NOT align them correctly (hard enough to make sure 1 is aligned
correctly), AND it would waste space.  My only "nit" in all this is throwing
on the partition table as a label that all the bit OS's will recognize.  
To my
mind, that's a good thing.

>   
>> And with partitions -- I have extended XFS .. you make a partition
>> larger, and tell XFS... "growfs"...
>> and it gets bigger.   NTFS does too.   Dunno...does extX or others have
>> that option?
>>     
>
> lvextend -r
>
> takes care of the FS as well after your vg's are extended .
>   
----
    That only works for 'extX' file systems, though it deceptively
isn't named in a way that implies that.  I.e. it wouldn't work for NTFS or
xfs. 




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