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Re: [dm-devel] Re: [PATCH] dm-striped device sizes must be multiple of chunk-size



Kevin,

I'm not convinced that a more complex mapping is needed.

Haven't seen the reported case so far with a lot of RAID0 metadata samples
users provided to me for various ATARAID flavours.

The RAID device size (which is smaller than the underlying device)
needs to be divisable w/o rest by the stride size IMO.

Guess I'm missing something about setting up the device size (rd->sectors)
correctly in the metadata format handler.

Investigating...

Heinz

On Fri, Mar 24, 2006 at 10:46:25PM -0600, Kevin Corry wrote:
> On Wed March 22 2006 11:28 pm, Joseph Yasi wrote:
> >     I am using the dmraid utility to detect and configure my Intel
> > Software Raid RAID0 array.  When upgrading to 2.6.16 from 2.6.15, the
> > system failed to boot because the device-mapper array was not built.
> > I have traced the problem to this patch that limits dm-stripe to
> > targets that are multiples of the chunk size.  I don't know whether to
> > consider this a problem with device-mapper or with the Intel Software
> > Raid BIOS that built the array.  Dmraid is giving me a target of
> > 980460038 blocks with a chunk size of 256 blocks.  Is this a problem
> > with device-mapper, the dmraid utility or the raid bios?  Should
> > device-mapper handle striped targets that are not multiples of the
> > chunk size?
> 
> Hmm...I obviously didn't expect dmraid to be creating such devices.
> 
> The kernel patch that was added to 2.6.16 is definitely correct. Dm-stripe 
> should not allow creating a device with a size that's not a multiple of the 
> chunk-size. To do so leaves a partial chunk at the end of each device. While 
> at first glance this might not seem like a big deal, the striping 
> calculations in the I/O path do not handle this situation (at least, not in 
> the way you would think it should be handled), and cause I/O errors if you 
> try to access the end of the stripe device.
> 
> > dmesg gives these pertinent lines:
> > device-mapper: dm-stripe: Target length not divisible by chunk size.
> > device-mapper: error adding target to table.
> >
> > dmsetup status
> > isw_caejhbcidd_Volume03: 0 976173660 linear
> > isw_caejhbcidd_Volume02: 0 4209030 linear
> > isw_caejhbcidd_Volume01: 0 64197 linear
> > isw_caejhbcidd_Volume0: 0 980460038 striped
> >
> > dmraid -s
> > *** Group superset isw_caejhbcidd
> > --> Active Subset
> > name   : isw_caejhbcidd_Volume0
> > size   : 980460038
> > stride : 256
> > type   : stripe
> > status : ok
> > subsets: 0
> > devs   : 2
> > spares : 0
> 
> In your example, your stripe device is 980460038 sectors, with a 256 sector 
> chunk-size.This means the stripe device has 3829922 chunks, with 6 sectors 
> left over (3 on each of the two devices, presumably). I would think that the 
> BIOS intends that these last 3 sectors on each device should be striped with 
> an effective chunk-size of 3 sectors. To do this, dmraid should set up a 
> device with two targets, one with a chunk-size of 256, and one with a 
> chunk-size of 3. The map would look something like:
> 
> 0 980460032 striped 2 256 /dev/hda 0 /dev/hdb 0
> 980460032 6 striped 2 3 /dev/hda /dev/hdb 490230016
> 
> Unfortunately, this doesn't work either, because the chunk-size must be a 
> power-of-2. So...since we know the second target in the map really represents 
> only one "stripe", we could treat them as a concatenation of independent 
> linear targets. So we end up with a map like this:
> 
> 0 980460032 striped 2 256 /dev/hda 0 /dev/hdb 0
> 980460032 3 linear /dev/hda 490230016
> 980460032 3 linear /dev/hdb 490230016
> 
> I'm thinking that this mapping would do what the BIOS is actually intending. 
> It ought to be relatively straight-forward to recognize this situation in 
> dmraid and set up a mapping similar to the one above.
> 
> -- 
> Kevin Corry
> kevcorry us ibm com
> http://www.ibm.com/linux/
> http://evms.sourceforge.net/
> 
> --
> dm-devel mailing list
> dm-devel redhat com
> https://www.redhat.com/mailman/listinfo/dm-devel

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Heinz Mauelshagen                                 Red Hat GmbH
Consulting Development Engineer                   Am Sonnenhang 11
Storage Development                               56242 Marienrachdorf
                                                  Germany
Mauelshagen RedHat com                            PHONE +49  171 7803392
                                                  FAX   +49 2626 924446
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