[Linux-PowerEdge] Extremely poor performance with LVM vs. RAW disc
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firmware was out of date so I updated it and now performance is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
> 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...
email | delivered <https://www.facebook.com/ReachMail>
REACHMAIL Inc. | Main 888-947-3224 | 208 S. LaSalle | Suite1427 |
Chicago, IL 60604
ggulik at reachmail.com <mailto:ggulik at reachmail.com> | Direct 312-229-0132
| http://www.reachmail.com <http://www.reachmail.com/>
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the Linux-PowerEdge