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SubjectRe: [PATCH 5/5] brd: Add getgeo to block ops for fdisk
On Sun, Nov 09, 2014 at 06:57:27PM +0200, Boaz Harrosh wrote:
> with a small 4M disk

This is the problem, for small disks (<= 4MiB) we don't use 1MiB
grain because it does not make sense. For so small devices the grain
(and the first partition offset) is the same as physical sector size.

Try something bigger in your tests :-)

> I see brd_getgeo() getting called on fdisk load and when pressing
> g or o. But it no longer has any effect at all if I have it defined
> returning CHS(4,64,32) or returning CHS(1,1,1) or not defined at all
> I get the same exact below experience:

The alignment and topology code is generic, it gathers all
information about the device, but it's fine if the device does not
provide HDIO_GETGEO. The geometry is currently used for DOS
compatible mode, or SGI and SUN only.

> ======== 4k physical_block_size PATCH ===============================================================
> Command (m for help): g
> Command (m for help): n
> First sector (34-8158, default 40):
> Last sector, +sectors or +size{K,M,G,T,P} (40-8158, default 8158): 1717
> Command (m for help): n
> First sector (34-8158, default 1720):
> # NOTE 34-8158 again, only with gpt

It seems like a fdisk bug, the gap between 34-40 is smaller than phy sector
size.. I'll fix it, thanks!

> Dave, Karel, what would you say fdisk should do? do you think it behaves correctly
> to only align with the 4k-physical_block_size or must it always align ?

If you expect partitions aligned to 4K (~pagesize?) then you have to
provide proper information to userspace (sector size or min/opt_io),
so from my point of view the patch makes sense.

Note that for example zram uses 4K logical and physical sector sizes
as well as all I/O limits are aligned to 4K at all:

# modprobe zram
# zramctl --find --size 4MiB

# fdisk -l /dev/zram0
Disk /dev/zram0: 4 MiB, 4194304 bytes, 1024 sectors
Units: sectors of 1 * 4096 = 4096 bytes
Sector size (logical/physical): 4096 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes

Is there a reason for brd to behave differently?


Karel Zak <>

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