[linux-lvm] Re: LVM Performance effects?

[Lars Ellenberg]

/ 2006-07-13 17:56:55 +0200
\ Michael Heyse:
> (sorry for disrupting the thread - copied this message from the archives)
> 
> > I have set up a file server with LVM on top of RAID 5, and seem to be
> > having LVM related performance issue.
> 
> Me too.
> 
> > raid device: 118.2 MB/s
> > lvm device: 49.43 MB/s
> > file system: 40.83 MB/s
> 
> Have you found an answer to that problem? I couldn't find anything
> helpful in the archives. Is this a general LVM on software RAID issue?

maybe lvm (device mapper) does not propagate the read-ahead
parameters of the physical device to the upper layers
(we had a similar problem with drbd, which in respect to this problem is
"just an other" virtual block device)

once we did (block device kernel code during device creation)
+
+       if( q->backing_dev_info.ra_pages != b->backing_dev_info.ra_pages) {
+               INFO("Adjusting my ra_pages to backing device's (%lu -> %lu)\n",
+                    q->backing_dev_info.ra_pages,
+                    b->backing_dev_info.ra_pages);
+               q->backing_dev_info.ra_pages = b->backing_dev_info.ra_pages;
+       }
+

we suddenly got basically all the bandwidth the box could deliver.
without that patch and that particular storage backend we had only
about 1/6 of the performance. 
other storage backends don't care at all.

you could verify this with "blockdev --getra /dev/whatever",
and try to tune it with "blockdev --setra 1024 /dev/whatever".

-- 
: Lars Ellenberg                                  Tel +43-1-8178292-0  :
: LINBIT Information Technologies GmbH            Fax +43-1-8178292-82 :
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