RE: [PATCH] mdadm.8: Disk coercion with IMSM metadata.

-----Original Message-----
From: dan.j.williams@gmail.com [mailto:dan.j.williams@gmail.com] On Behalf Of Dan Williams
Sent: Saturday, July 16, 2011 3:10 AM
To: NeilBrown
Cc: Naruszewicz, Maciej; Neubauer, Wojciech; Kwolek, Adam; Wojcik, Krzysztof; Ciechanowski, Ed; linux-raid@vger.kernel.org
Subject: Re: [PATCH] mdadm.8: Disk coercion with IMSM metadata.

On Wed, Jul 13, 2011 at 7:26 PM, NeilBrown [off-list ref] wrote:

quoted

On Wed, 13 Jul 2011 12:17:02 +0200 "maciej.naruszewicz"
[off-list ref] wrote:

quoted

From: maciej.naruszewicz <redacted>

Actual sizes of disks from different producers may vary,
even though they are claimed to have the same amount of
space

Are you sure about this?

It was my understanding that some industry association has arranged an
agreement so that this does not happen.

http://www.idema.org/wp-content/plugins/download-monitor/download.php?id=1223

Do you have actual evidence that different drives from different
manufacturers have similar but not identical sizes?

Hmm, seemed to be a case that needed handling when requirements were
being gathered, but perhaps recent drives don't do this any more?

Probably the more important part of this commit is that the man page
currently says that --size can not be used with container metadata...
but now that I have taken two seconds to think about it the light bulb
goes off...

--size is indeed irrelevant for:
   mdadm --create /dev/md/imsm /dev/sd[a-d] -e imsm

but my first reading of comment was that:
  mdadm --create /dev/md/vol0 /dev/md/imsm --size=$size
...is somehow not valid.

How about something like "For CONTAINER metadata --size is valid when
creating and growing subarrays, when creating a new container set
--size is irrelevant"

--
Dan

Do you have actual evidence that different drives from different
manufacturers have similar but not identical sizes?

I'm afraid I do... For instance I have two disks, first one from Western Digital (model WDC WD2500YS-01SHB1), the second one from Seagate (model ST3250410AS). Both are said to have 250 GB maximum data space- however, the OS doesn't agree. LBA count for WD is 490234752, while for ST it's 488397168- that makes 251000193024 against 250059350016 bytes, nearly 1GB of difference!

I'm not sure when those disks were produced- maybe newest disks are manufactured according to the IDEMA standard, but this example shows that there definitely are differences.

How about something like "For CONTAINER metadata --size is valid when
creating and growing subarrays, when creating a new container set
--size is irrelevant"

Indeed, the --size option is irrelevant for containers and valid for subarrays in the container; however, the manpage stated: "This value cannot be used with .B CONTAINER metadata such as DDF and IMSM.". By "this", the author meant that "--grow --size" cannot be used, he / she didn't mean the "--size" alone. This sentence is deleted in the patch I sent earlier and explained more clearly in its description. Of course, we could add the information Dan mentioned to make it even more understandable.

Best wishes
Maciek

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