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 <[email protected]> wrote: > Michael Tautschnig <[email protected]> 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 -- The Wellcome Trust Sanger Institute is operated by Genome Research Limited, a charity registered in England with number 1021457 and a company registered in England with number 2742969, whose registered office is 215 Euston Road, London, NW1 2BE.
