On Sun, Sep 05, 2010 at 07:12:41PM +0300, Anssi Hannula wrote:
> First, if I create a new partition on an empty device, fdisk appropriately
> uses 1MB offset from beginning by default:
> "First sector (2048-10239999, default 2048):"
>
> However, shouldn't it still allow using a lower starting sector, even if not
> selected by default? Like this:
> "First sector (63-10239999, default 2048):"
>
> Or is the intent to protect against users messing up the alignment, and users
> should use dos compatibility mode to avoid the limitation? I guess I would be
> ok with that :)
Yes, this is default. Anyway, you can move the begin of the first
partition to sector <1..N> in expert menu (commands 'x' and 'm') where
<N> is the end of the first partition.
> Ok, I then create the primary partition starting in sector 2048 and and select
> partition size '+5500K' for a 5,5MB partition:
> Device Boot Start End Blocks Id System
> /dev/loop0p1 2048 13047 5500 83 Linux
>
> Looks ok. Now, when I ask fdisk to create another partition, fdisk offers to
> start it immediately after the end of the previous partition:
> "First sector (13048-10239999, default 13048):"
> Is it intended that fdisk doesn't align the default value to 14336?
The begin of the (next) partition should be always aligned and
without extra gaps between partitions. We use strictly 1MiB alignment
for the begin of the first partition only. For the next partitions
fdisk cares about alignment to the physical block boundary only.
If the next LBA (the begin of the next partition) is not aligned,
then fdisk uses 1MiB grain as a default. For example:
Sector size (logical/physical): 512 bytes / 4096 bytes
Device Boot Start End Blocks Id System
/dev/sdb1 2048 11109 4531 83 Linux
...
First sector (11110-2097151, default 12288):
the next sector (11110) is not aligned, so fdisk is suggesting 12288,
this LBA is aligned to 1MiB grain and aligned to phy block boundary.
Anyway, it's very unusual to use +<size>K to define partition size.
I think it's more generic to use +<size>{M,G} and then the layout
is always aligned without gaps.
> I've gotten bitten by this several times and I've created unaligned partitions
> on 4k sector devices.
All defaults and all +<size>{K,M,G,...} offsets should be aligned,
but you can still explicitly define exact LBAs for begin/end of your
partition. Fdisk is low-level tool and it assumes that you know what
are you doing. Anyway, it prints warning for misaligned partitions.
> I.e. what I would've expected:
> "First sector (13048-10239999, default 14336)":
Please, always copy & past the "Sector size (logical/physical):" line
from fdisk output to your emails/bug reports.
> Thirdly, I create an extended partition with one logical partition as follows:
> Device Boot Start End Blocks Id System
> /dev/loop0p1 2048 13047 5500 83 Linux
> /dev/loop0p2 13048 14335 644 83 Linux
> /dev/loop0p3 14336 10239999 5112832 5 Extended
> /dev/loop0p5 16384 65534 24575+ 83 Linux
>
> Now, I wish to add another logical partition:
>
> "First sector (67583-10239999, default 67583):"
>
> fdisk suggests 67583 as the starting point, which is exactly 1MB after the end
> of the previous logical partition, but in no way aligned. Why is there such an
> offset, and why does it happen only with logical partitions?
logical partition: EBR (partition table) + partition.
It means that for every logical partition you need extra space where
is EBR. The traditional concept is to use the same offset for the
EBR as for MBR, it means 1MiB. For more details see:
http://en.wikipedia.org/wiki/Extended_boot_record
The begin of the logical partition should be aligned of course. I
guess that "67583" in your example is aligned (loop dev uses 512-byte
sector).
Now you know why GPT partition table is better ;-)
Hint: use scsi_debug, it's better than loopdev. For example:
# modprobe scsi_debug dev_size_mb=1024 sector_size=512 physblk_exp=3
to create 1GiB (allocated in RAM!) disk with 4096-byte phy.sector size.
Karel
--
Karel Zak <kzak@xxxxxxxxxx>
http://karelzak.blogspot.com
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