Date: Thu, 4 Dec 2008 13:13:34 -0500 From: John Baldwin <jhb@freebsd.org> To: freebsd-arch@freebsd.org Cc: arch@freebsd.org, xcllnt@mac.com Subject: Re: RFC: making gpart default Message-ID: <200812041313.34565.jhb@freebsd.org> In-Reply-To: <20081203.193714.693830802.imp@bsdimp.com> References: <e7db6d980811291356w54256e6du82350baf3c57d591@mail.gmail.com> <e7db6d980812011605h18b40700v1043e376ef392365@mail.gmail.com> <20081203.193714.693830802.imp@bsdimp.com>
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On Wednesday 03 December 2008 09:37:14 pm M. Warner Losh wrote: > In message: <e7db6d980812011605h18b40700v1043e376ef392365@mail.gmail.com> > "Peter Wemm" <peter@wemm.org> writes: > : On Sat, Nov 29, 2008 at 7:30 PM, Marcel Moolenaar <xcllnt@mac.com> wrote: > : > > : > On Nov 29, 2008, at 1:56 PM, Peter Wemm wrote: > : [..] > : >> * There should be some guidance or hints on laying out disks. For > : >> example, a gpart create -s gpt on a raid volume ends up with a start > : >> sector of 34 for the free space. There should be a documentation hint > : >> to round up start sectors to a power of 2 and/or block size on a raid. > : >> eg: if you have a raid with 64K stripes, then move the start sector > : >> from 34 to 128. Otherwise we end up with file systems issuing > : >> transactions that can split across multiple raid stripes. FWIW, I > : >> conveniently filled this hole with boot code. > : > > : > Hmmm... gpart(8) typically can't store this kind > : > of information on-disk, but other than that it > : > supports alignment/padding already. We just need > : > a way to tell gpart about it. Maybe this should > : > come from the provider (i.e. underlying geom)... > : > : I was more thinking of a man page note to warn of the issue. > : > : Also, in the gpt case, it might make sense in gpt partition table case > : to round up the initial size to a power of 2. Right now we lose 34 > : sectors from the beginning. Rounding it to 64 total at least gets us > : to an even power of 2. UFS's frequent block size of 16K shouldn't > : cross any underlying stripe boundaries in the usual case. > > This likely is a hang over from the MBR code that puts the first > partition at one cylendar offset from the beginning to conform with > the MBR conventions of (some?) Bioses that use that to get the > parameters for the disk... No, the way GPT works, you have a PMBR at sector 0, then immediately following that you have the Primary partition table in the next N sectors (the first sector in the table has a header that contains the size of the table). Then you have a backup Secondary partition table in the last N sectors of the disk as well. At least with the old gpt(8) tool you could actually tell it how big of a table to make when you created a GPT, and I imagine gpart probably can do the same. -- John Baldwin
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