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

[Ruslan Ermilov]

On Tue, Feb 21, 2006 at 10:58:30AM +0100, Eivind Olsen wrote:
> Hello.
> 
> I've just configured a server to use gmirror (using FreeBSD 6.1b2). I 
> followed the tutorial on 
> http://www.onlamp.com/pub/a/bsd/2005/11/10/FreeBSD_Basics.html 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.
> 
In short: kernel (g_mirror) won't let you write to the last sector of its
media.

> 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?
> 
Your "c" partition (the whole disk) occupies 488392002 sectors.
The "h"'s partition last sector is 117440512+370951490-1=488392001.


Cheers,
-- 
Ruslan Ermilov
ru at FreeBSD.org
FreeBSD committer
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