On Wed, Apr 21, 2021 at 09:56:59AM +0100, Tom Smyth wrote:
> Hello Otto, Christian,
>
> I was relying on that paper for the pictures of the alignment issue,
>
> VMFS (vmware file system) since version 5 of vmwarehas allocation
> units of 1MB each
>
> https://kb.vmware.com/s/article/2137120
>
> my understanding is that SSDs have a similar allocation unit setup of 1MB,
>
> and that aligning your file system to 1MB would improve performance
>
>
> |OpenBSD Filesystem --------------| FFS-Filesystem
> |VMDK Virtual Disk file for Guest | OpenBSD-Gusest-Disk0.vmdk
> |vmware datastore------ ------------| 1MB allocation
> |Logical Storage Device / RAID---|
> |SSD or DISK storage --------------| 1MB allocation unit (on some SSDs)
>
> Figure 2 of the following paper shows what
> https://www.usenix.org/legacy/event/usenix09/tech/full_papers/rajimwale/rajimwale.pdf
> as your writes start to cross another underlying block boundary you
> see a degradation of performance
> largest impact is on a write o1 1MB (misaligned) across 2 blocks,
Max unit OpenBSD writes in one go is 64k. So the issue is not that
relevant. Only 1 in 16 blocks would potentially cross a boundary.
You are free to setup your disks in a way that suits you, but in
general I don't think we should enforce 1Mb alignment of start of
partition and/or size because *some* *might* get a benefit.
-Otto
> but it repeats as you increase the number of MB in a transaction but
> the % overhead
> reduces for each additional 1MB in the Transaction.
>
> If there is no downside to allocating /Offsetting filesystems on 1MB
> boundaries,
> can we do that by default to reduce wear on SSDs, and improve performance
> in Virtualized Environments with large allocation units on what ever storage
> subsystem they are running.
>
> Thanks for your time
>
> Tom Smyth
>
>
>
>
> On Wed, 21 Apr 2021 at 08:49, Otto Moerbeek <o...@drijf.net> wrote:
> >
> > 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.
> > >
>
>
>
> --
> Kindest regards,
> Tom Smyth.
