RFC: changing start sector in setup-storage

[Tim Cutts]

I vote for following the 1M start, unless preserving.  Alignment is particularly important in virtual machines.

On 20 Dec 2010, at 18:03, Brian Kroth <bpkroth at gmail.com> wrote:

> Michael Tautschnig <mt at debian.org> 2010-12-03 21:31:
>> Hi all,
>> 
>> Let me first give a short summary of the context: Modern storage devices more
>> and more use 4k physical sector sizes instead of the original 512B sector sizes.
>> As such a change would result in a number of incompatibilities, the OS and
>> software continue to be presented with 512B sector sizes, and hardware takes
>> care of mapping read and write requests to the underlying physical sectors. What
>> looks like some minor implementation issue turns out to be causing performance
>> problems with the original DOS disk layout: in this setting, the first partition
>> starts at sector 63, meaning 63*512B. This results in misalignment with 4k
>> physical sector sizes, which degrades performance. Some more information about
>> this problem plus a number of pointers can be found in Mika's article [1].
>> 
>> To resolve this problem, Microsoft decided to start the first partition at
>> 1M=2048*512B=256*4k. According to [2] Linux kernel folks consider it the best
>> approach to follow this decision. Assuming the list on that page is correct,
>> partitioning tools such as fdisk or parted will, by default, already follow this
>> proposal.
>> 
>> So how does this affect setup-storage, and why am I asking for additional input
>> concerning this problem?
>> 
>> setup-storage uses parted, but it forces partitions to start and end at certain
>> points instead of leaving parted to choose itself. Consequently we remain
>> independent of parted's changes regarding start sectors, but at the same time
>> also have to take this recent trend into account ourselves: Should setup-storage
>> follow the trend and make the first partition start at 1M or should we stick
>> with 63*512B? 
>> 
>> Should setup-storage be changed, this will affect anyone using preserve options.
>> It won't matter that much for fresh installs without preserved partitions, but
>> if some partition is preserved and all partitions listed before the preserved
>> partition have fixed sizes, setup-storage will fail miserably to determine a
>> viable disk layout. If anybody thinks they might be in such a situation, it
>> would be nice if they spoke up. 
>> 
>> Well, of course we can add yet another config option to specify the start sector
>> to be used. In fact the experimental builds already know an option "align-at"
>> that enforces proper alignment, e.g., for 4k sector sizes - but it won't give a
>> 1M starting gap (unless you use align-at:1M, but that might have other
>> undesirable side effects).
>> 
>> In terms of code changes the move from 63*512B starting gap to 1M is almost
>> trivial, but I feel this might break a number of setups and hence would like to
>> give a chance to everyone to provide feedback before implementing this change.
>> 
>> Best regards,
>> Michael
>> 
>> [1] http://www.infoq.com/news/2010/03/4k-sectors
>> [2] https://ata.wiki.kernel.org/index.php/ATA_4_KiB_sector_issues
> 
> Sorry for the slow response and if this is too specific to offer any
> guidance.
> 
> I tend to use a layout basically makes a small (128-256M) /boot
> partition, then carves the rest up for LVM to play with.  On that LVM I
> usually specify a single partition (eg: /home) that I want to be
> preserved, but I'll let the rest of it get blown away.  With that in
> mind, do you think setup-storage tweaks could be made to allow /boot to
> perhaps become a little smaller, but then leave enough of the other
> partition alone to allow for preserving my /home lv?
> 
> More generally, I tend to think giving people the option is best.  I
> like the notion of an align-at argument that defaults to the new value
> (4K?), and then send out a big red warning in the changes/release
> announcement that says something along the lines of "if you want the old
> behavior/preserving to work, set align-at=512B".
> 
> Then again, in my own experience preserve has always been a little
> finicky anyways.  By the time I get around to wanting to completely
> reinstall a machine, I probably want to change the layout anyways.
> 
> Thanks,
> Brian


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