Re: Shrinking number of devices on a RAID-10 (near 2) array

[NeilBrown <neilb@suse.de>]

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On Fri, 29 Aug 2014 04:42:26 +0000 Andy Smith <andy@strugglers.net> wrote:

> Hi Neil,
> 
> On Fri, Aug 29, 2014 at 02:35:28PM +1000, NeilBrown wrote:
> > On Fri, 29 Aug 2014 04:26:18 +0000 Andy Smith <andy@strugglers.net> wrote:
> > > Sorry, still no joy:
> > > 
> > > # echo 309363696 > /sys/block/md2/md/dev-sda3/size
> > > # cat /sys/block/md2/md/dev-sda3/size
> > > 309363264
> > 
> > Try use "/bin/echo" so you get an error message.
> > Also try smaller numbers.  Can you write anything larger than what is already
> > there?  Increasing by 64 (match the 128 sectors that mdadm wants for
> > new_offset) should  be enough.
> 
> I can't even increase it by 64 from what it is now:
> 
> # /bin/echo 309363328 > /sys/block/md2/md/dev-sda3/size
> # cat /sys/block/md2/md/dev-sda3/size
> 309363264
> 
> In fact I can't increase it by 1, and /bin/echo doesn't say why.
> 

Hmm.... I've obviously been assuming something that was wrong.

So I did some experiments myself....

If you apply the following patch to mdadm, stop the array, and then assemble
with --update=devicesize, you should then be able to reshape the array to
fewer devices.

This isn't a final solution, I need to think about the whole thing more
carefully.  But it will work for you.

Thanks for your patience.

NeilBrown

diff --git a/super1.c b/super1.c
index 727a08a110e0..efa1042ab47a 100644
--- a/super1.c
+++ b/super1.c
@@ -1251,6 +1251,17 @@ static int update_super1(struct supertype *st, struct mdinfo *info,
 			misc->device_size - __le64_to_cpu(sb->data_offset));
 		printf("Size is %llu\n", (unsigned long long)
 		       __le64_to_cpu(sb->data_size));
+	} else if (strcmp(update, "devicesize") == 0 &&
+		   !(__le32_to_cpu(sb->feature_map)&MD_FEATURE_BITMAP_OFFSET) &&
+		   __le64_to_cpu(sb->super_offset) >
+		   __le64_to_cpu(sb->data_offset)) {
+		/* set data_size to device size less data_offset */
+		printf("Size was %llu\n", (unsigned long long)
+		       __le64_to_cpu(sb->data_size));
+		sb->data_size = __cpu_to_le64(
+			__le64_to_cpu(sb->super_offset) - __le64_to_cpu(sb->data_offset));
+		printf("Size is %llu\n", (unsigned long long)
+		       __le64_to_cpu(sb->data_size));
 	} else if (strcmp(update, "revert-reshape") == 0) {
 		rv = -2;
 		if (!(sb->feature_map & __cpu_to_le32(MD_FEATURE_RESHAPE_ACTIVE)))

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