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

Gregory Gulik ggulik at reachmail.com
Mon Nov 5 11:09:36 CST 2012


Thanks for this great info.
How would I find out if the stripes line up and if they don't how does 
one go about fixing them?
I'm looking at the mkfs.ext4 man page and is it the "-b block_size" 
option or something else?


BTW, in experimenting further over the weekend I found the PERC 6/E 
firmware was out of date so I updated it and now performance is 
significantly better.  That said I want to make sure all settings are as 
optimal as they could be before we put real data on the server.



On 11/5/12 7:39 AM, L. A. Walsh, Tlinx Solutions wrote:
>
> ----
>   Couple things are in play here.
>
> 1) you need to start your pv's on a stripe unit boundary for large I/O 
> performance.
> For optimal smaller I/O, you want to start it on a stripe-width 
> boundary.   This is a parameter
> you give to pvcreate. (data offset).  So I have 12 datax64k = 768k -- 
> I start my 1st pv @ 768k.
>
> Then you layer a vg on top of that -- it has a segment size -- you 
> want that to be a multiple of
> your stripwidth for optimal I/O (I messed that up.. it's a 
> strip-multple, but not a stripe-width
> boundary.... so I get lower perf on some small writes than I might 
> otherwise.  Too much of a pain
> to reformat.
>
> Then your lv's have a start position and chunksize -- you want those 
> to multiple out as well.
>
> Then  your specify your strip width and size to as file system param 
> (or at least you do on
> file systems that support RAID -- like XFS)... I assume, since you are 
> using ext4, that it also
> has such params.
>
> While the OS knows the layout of disks in the pv -- it doesn't know 
> about the lvm layers on top of
> that... so the file systems generally don't even try to guess about 
> proper alignment -- that's all manual
> AFAIK...(at least on linux)...
>
> If you want to test disk speed -- you should pre-allocate the file as 
> contiguous (you can
> do this on xfs... dunnow about ext4)....then when you 'dd', use direct 
> I/O -- and use
> nocreat,notrunc options on 'dd', so you are reading/writing to the 
> same area and aren't
> exercising the file-system's ability to allocate space, nor the OS's 
> buffer system.
>
> Doing those things, should get you alot closer to your theoreticals...
>
> Hopefully I explained enough...if not, feel free to ask more...
>
> Linda
>

-- 
Gregory Gulik

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