Re: [PATCH] md: 'size_limit' attribute

[Neil Brown <neilb@suse.de>]

On Friday February 13, dan.j.williams@intel.com wrote:
> Subject: md: 'size_limit' attribute
> From: Dan Williams <dan.j.williams@intel.com>
> 
> Provide a sysfs attribute to allow a raid array to be truncated to an
> arbitrary size.  This functionality is needed to support imsm raid
> arrays where the metadata format expects that the size of some arrays is
> rounded down to the nearest 1MB boundary.

Well it's not April 1st, so I assume you are serious.

It really truncates the array, not the individual drives?
So you could have e.g. a raid0 in which only some of the last stripe
was used?

Can you give me a concrete example of an array where this will make a
required difference?  I just want to be sure I understand.

I guess you couldn't just add an 'array_size' attribute which gave
direct access to mddev->array_size because that gets set when the
array is started, and we want to be able to impose the limit before
starting the array....

How about a semantic where starting the array will only modify
->array_size if it's value is zero of if it would reduce the value.

How might this interact with array resizing?  You add a drive to an
array, reshape it, and then it doesn't get any bigger until the
size_limit is updated?   I guess that could work but it might be
confusing.... though presumably mdadm/mdmon would know to look after
all the details. 


What would you think of renaming the attribute to 'array_size' with
the semantic of "once user-space sets it, the kernel will never change
it" ??

NeilBrown