gmirror is confusing me, will I be bitten by the last-sector-use?

Eivind Olsen eivind at
Tue Feb 21 01:58:33 PST 2006


I've just configured a server to use gmirror (using FreeBSD 6.1b2). I 
followed the tutorial on and did 
it install-time.
Then I read in the gmirror(8) man-page that gmirror uses the last sector 
of the provider to store metadata.

My mirror consists of two IDE-disks/consumers (ad0 and ad3), both 
showing a mediasize of "250059350016 (233G)" in "gmirror list". The 
provider is shown as having a mediasize of 250059349504. Sectorsize = 512.

Now, how can I check whether my current partitions will cause trouble 
with the metadata-sector or not? I've done some searching on the net, 
but I'm still a bit confused. If I understand correctly, the consumers 
ad0/ad2 span 488397168 sectors (mediasize/sectorsize), and the 
mirror/gm0 spans 488397167 sectors.

Here's the output from bsdlabel:

vimes# bsdlabel /dev/mirror/gm0s1
# /dev/mirror/gm0s1:
8 partitions:
#        size   offset    fstype   [fsize bsize bps/cpg]
   a:  6291456        0    4.2BSD     2048 16384 28552
   b:  4194304  6291456      swap
   c: 488392002        0    unused        0     0         # "raw" part, 
don't edit
   d: 41943040 10485760    4.2BSD     2048 16384 28552
   e:  2097152 52428800    4.2BSD     2048 16384 28552
   f: 31457280 54525952    4.2BSD     2048 16384 28552
   g: 31457280 85983232    4.2BSD     2048 16384 28552
   h: 370951490 117440512    4.2BSD     2048 16384 28552

So, the "h" partition, which is the last one, has offset=117440512 and 
size=370951490, and if I add those together I get 488392002. Does this 
mean that my "h" partition ends at 488392001 (right before 488392002), 
and I'm still several sectors away from the metadata? Or am I 
misunderstanding things here?

Regards / Hilsen
Eivind Olsen
<eivind at>

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