RE: Wheezy Installer Auto-Partition Oddity
>From: Patrick Bartek [mailto:bartek047@yahoo.com]
>> From: Tom H <tomh0665@gmail.com>
>> On Fri, Jan 4, 2013 at 1:25 PM, Patrick Bartek <bartek047@yahoo.com>
wrote:
>>> From: Roger Leigh <rleigh@codelibre.net>
>>>> LVM does not use unpartitioned space for anything TTBOMK. It uses
>>>> physical volumes (PVs) which are block devices (either partitions or
>>>> whole disks or RAID arrays etc.). These are entirely self-contained.
>>>> Internally, the PV contains its own metadata and extents which are
>>>> allocated to individual logical volumes within the volume group
>>>> containing the PV. It's simply impractical and fragile to use
>>>> unpartitioned space, and LVM only uses the devices (partitions) you
>>> put the PVs on.
>>>
>>> That was what I read--somewhere?--in an article on LVM. It was just
>>> one sentence mentioned in passing and was never detailed.
>>>
>>> If using unpartitioned space is so "fragile" Why do the MBR or
>> GPT,
>>> etc. use it? Seems to be a great place to "hide" data about
>> something
>>> like a LVM partition that's not going to change frequently, and is
>>> beyond normal filesystem access. Just a thought.
>>
>> The MBR, whether on msdos or gpt, is a well-defined area at the
>> beginning of a disk, not a random space between partitions.
>>
>> There's no LVM data held off a PV, whether it's a partition or a disk.
>> The LVM metadata of a PV is stored in the second sector of that PV and
>> its LV "usable area" follows. Your article might have been referring
>> to this separation.
>
> As I said in a previous reply, I could have misinterpreted. I was
skimming
> the article, not studying.
>
> So, again I ask: Why that 1MiB unpartitioned space before the beginning
of
> a new partition? Both Debian 6 and 7 installer partitioner insert it
(when
> you choose Auto-partition; don't know whether it does with Custom) as
> does gparted (Discovered that when I resized three existing contiguious
> primary partitions [no gaps added after resizing] and added two new
logical
> partitions [gaps added automatically] on a 7 year old 512 byte sector
160GB
> drive). Got to be a reason. I don't think it's a bug.
You are correct, there is a reason. Previously, I said that LVM needs it.
I read something that explained it, but I may have misremembered what I
read. Perhaps it had something to do with RAID, perhaps it was something
else. I remember several different web sites telling me to leave space,
although I seem to recall that they were Ubuntu-related.
However, there is also a performance gain to be had when aligning partitions
to 1MB boundaries. This is true for both 512- and 4096-byte sectors. Of
course, since both 512 bytes and 4096 bytes divides evenly into 1MB, adding
at least 1MB before and after the partition kills two birds with one stone.
This I read on the GParted forums.
At any rate, let the tool add the space; it even will calculated the correct
size and sector alignment for you. It may be untidy, but it does no harm.
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