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Re: [Cluster-devel] [PATCH 2/4] mkfs.gfs2: Align resource groups to RAID stripes



On 06/06/13 14:11, Steven Whitehouse wrote:
It has never been valid to assume that all the rgrps are the same size.
It may be useful as a hint, but we should not be relying on that being
true. Obviously it makes sense to try and keep them to an even spacing
where possible but we must allow for them to be placed and sized
independently as required for alignment, etc.

There are some restrictions on rgrp length as they need to be aligned
such as to give an integer number of bitmap bytes, and in fact it would
be good if that could be further enforced to ensure that all rgrp
bitmaps are an integer number of 64 bit words in length (so a multiple
of 32 blocks, excluding the headers). So depending on the various
restrictions, there may be a few unused blocks between rgrps in some
cases.

This is on my to-do list and should be a case of changing the "& ~(uint32_t)3" parts of rgblocks2bitblocks() but I wanted to make sure that won't break fsck for older file systems first.

It would be worth abstracting the details about alignment from mkfs in
due course, so that fsck also has access to the same information about
where rgrps are likely to have been put, I think.

Yes, I need to move the new rgrp creation bits from mkfs into libgfs2 now so that's going to mean updating the other tools to use them eventually (hold onto your hats!).

Some further thoughts:

  - Would it be useful to introduce a flag to show the source of an rgrp
(whether mkfs or gfs2_grow) ?
  - Would it be useful to add a creation date stamp to each rgrp so that
we can see when things have happened in the past?

Both sound good to me.

Andy


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