linux-kernel - Re: [PATCH 2/5] Add aix lvm partitions support files

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Date:	Tue, 30 Apr 2013 08:41:52 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Philippe De Muyter <phdm@...qel.be>
Cc:	Karel Zak <kzak@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] Add aix lvm partitions support files

On Mon, Apr 29 2013, Philippe De Muyter wrote:
> Hi Karel
> 
> On Mon, Apr 29, 2013 at 02:36:51PM +0200, Karel Zak wrote:
> > On Mon, Apr 29, 2013 at 01:40:41PM +0200, Philippe De Muyter wrote:
> > > > why not memset(pps_found, ....)? I also see magical constant 16
> > > 
> > > Actually 16 is the maximum partition count allowed in a disk by linux,
> > > or should it be 15 ?  Is there already a constant for that ?
> > > The AIX disk I tested with had only :) 11 partitions.
> > 
> > I don't think it's correct to expect any hardcoded limit. 
> >  
> > The struct parsed_partitions->parts is allocated according to
> > disk_max_parts() where the limit depends on number of minor numbers or
> > it's DISK_MAX_PARTS (=256).
> > 
> > There is no problem to create disk with many partitions:
> > 
> >  # modprobe scsi_debug dev_size_mb=300
> >  # (echo -e 'g\n'; for i in {1..100}; do echo -e "n\n\n\n+1M"; done; \
> >     echo -e 'w\nq\n') | fdisk /dev/sdb
> > 
> >  # lsblk -n /dev/sdb | wc -l
> >  101
> 
> how are they named then ?
> 
> on my system (a 2.6.24 kernel which is the last one that support my
> powerpc PReP machine because PReP support got removed with the merge
> of /arch/ppc and /arch/powerpc :( ), I get :
> 
> root:~# ls -l /dev/sd[ab]*
> brw-r----- 1 root disk 8,  0 Apr 25 22:22 /dev/sda
> brw-r----- 1 root disk 8, 10 Apr 25 22:22 /dev/sda10
> brw-r----- 1 root disk 8, 11 Apr 25 22:22 /dev/sda11
> brw-r----- 1 root disk 8,  3 Apr 25 22:22 /dev/sda3
> brw-r----- 1 root disk 8,  4 Apr 25 22:22 /dev/sda4
> brw-r----- 1 root disk 8,  5 Apr 25 22:22 /dev/sda5
> brw-r----- 1 root disk 8,  6 Apr 25 22:22 /dev/sda6
> brw-r----- 1 root disk 8,  7 Apr 25 22:22 /dev/sda7
> brw-r----- 1 root disk 8,  8 Apr 25 22:22 /dev/sda8
> brw-r----- 1 root disk 8,  9 Apr 25 22:22 /dev/sda9
> brw-r----- 1 root disk 8, 16 Apr 25 22:22 /dev/sdb
> brw-r----- 1 root disk 8, 26 Apr 25 22:22 /dev/sdb10
> brw-r----- 1 root disk 8, 27 Apr 25 22:22 /dev/sdb11
> brw-r----- 1 root disk 8, 19 Apr 25 22:22 /dev/sdb3
> brw-r----- 1 root disk 8, 20 Apr 25 22:22 /dev/sdb4
> brw-r----- 1 root disk 8, 21 Apr 25 22:22 /dev/sdb5
> brw-r----- 1 root disk 8, 22 Apr 25 22:22 /dev/sdb6
> brw-r----- 1 root disk 8, 23 Apr 25 22:22 /dev/sdb7
> brw-r----- 1 root disk 8, 24 Apr 25 22:22 /dev/sdb8
> brw-r----- 1 root disk 8, 25 Apr 25 22:22 /dev/sdb9
> root:~#
> 
> so sda is 8,0 and sdb is 8,16
> 
> and if, while discovering partitions of /dev/sda, I try to make a
> partition 16 or higher, it is silently discarded by 'put_partition'.
> 
> Is that changed ?

That's a set limitation of sd, it does not apply to other devices. The
legacy IDE code used 64 for max partitions, for instance. So Karel is
right, you should not make any assumptions about the max number of
partitions, it is driver dependent.

-- 
Jens Axboe

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