On Wed, Apr 21, 2021 at 08:20:10AM +0100, Tom Smyth wrote:
> Hi Christian,
>
> if you were to have a 1MB file or a database that needed to read 1MB
> of data, i
> f the partitions are not aligned then
> your underlying storage system need to load 2 chunks or write 2
> chunks for 1 MB of data, written,
>
> So *worst* case you would double the workload for the storage hardware
> (SSD or Hardware RAID with large chunks) for each transaction
> on writing to SSDs if you are not aligned one could *worst *case
> double the write / wear rate.
>
> The improvement would be less for accessing small files and writing small
> files
> (as they would need to be across 2 Chunks )
>
> The following paper explains (better than I do )
> https://www.vmware.com/pdf/esx3_partition_align.pdf
>
> if the cost is 1-8MB at the start of the disk (assuming partitions are sized
> so that they dont loose the ofset of 2048 sectors)
> I think it is worth pursuing. (again I only have experience on amd64
> /i386 hardware)
Doing a quick scan trhough the pdf I only see talk about 64k boundaries.
FFS(2) will split up any partiition in multiple cylinder groups. Each
cylinder group starts with a superblock copy, inode tables and other
meta datas before the data blocks of that cylinder group. Having the
start of a partion a 1 1MB boundary does not get you those data blocks
at a specific boundary. So I think your resoning does not apply to FFS(2).
It might make sense to move the start to offset 128 for big
partitions, so you align with the 64k boundary mentioned in the pdf,
the block size is already 64k (for big parttiions).
-Otto
>
> Thanks
> Tom Smyth
>
> On Tue, 20 Apr 2021 at 22:52, Christian Weisgerber <na...@mips.inka.de> wrote:
> >
> > Tom Smyth:
> >
> > > just installing todays snapshot and the default offset on amd64 is 64,
> > > (as it has been for as long as I can remember)
> >
> > It was changed from 63 in 2010.
> >
> > > Is it worth while updating the defaults so that OpenBSD partition
> > > layout will be optimal for SSD or other Virtualized RAID environments
> > > with 1MB Chunks,
> >
> > What are you trying to optimize with this? FFS2 file systems reserve
> > 64 kB at the start of a partition, and after that it's filesystem
> > blocks, which are 16/32/64 kB, depending on the size of the filesystem.
> > I can barely see an argument for aligning large partitions at 128
> > sectors, but what purpose would larger multiples serve?
> >
> > > Is there a down side to moving the default offset to 2048 ?
> >
> > Not really. It wastes a bit of space, but that is rather insignificant
> > for today's disk sizes.
> >
> > --
> > Christian "naddy" Weisgerber na...@mips.inka.de
> >
>
>
> --
> Kindest regards,
> Tom Smyth.
>
