On Fri, Aug 19, 2016 at 12:51 AM, Dag-Erling Smørgrav <d...@des.no> wrote: > Warner Losh <i...@bsdimp.com> writes: >> Allan Jude <allanj...@freebsd.org> writes: >> > Which makes more sense: >> > >> > A) If stripesize == 0, use some sane value like 4096 >> >> I don't like this. >> >> > B) Some other combination that uses the reported stripe size, unless it >> > is 0, in which case it uses 4096 (or some other value controlled by a >> > different new sysctl) >> >> Don't like this so much. >> >> > C) create kern.geom.min_stripe_size with a default of 512, but users can >> > set 4096 if they use only 4k devices. (doesn't really solve the problem >> > for the installer) >> >> Default it to 4k, and allow users to set it to 512. If the drive >> reports < this value >> report this value instead. > > I don't like either option. Option D (which I don't like either, but > which should at least work in most cases) is a sysctl that specifies a > minimum factor, and set the reported stripe size to the least common > multiple of that number and the actual stripe or sector size. This is > what my bsdinstall patch does. However, I think that pushing this down > to a layer where it will affect all applications is a terrible idea, > because we have no way of knowing what will break[*], and it can > seriously mislead users and hinder troubleshooting - especially if it is > enabled by default rather than only when necessary.
I took a look into the implications of doing a 4k stripesize 'automatically' this morning. I found a few places in g_part where it would actively hurt when coupled with gpart's insistence on aligning things. So I now think it's a bad idea. This will make it harder for FreeBSD to generate arbitrary disk layouts. And I'm not too sure about what things like gstripe would report as a result and if this would actually interfere if you had a large, but not power of two stripe size. > I don't think it's a good idea to enforce stripe alignment everywhere, > either. It works for partitions because they are very large relative to > the stripe size, and at worst we will waste a few millionths of the disk > on inter-partition gaps, which should only occur between the partition > table and the boot partition, and possibly, if the stripe size is very > large, between the boot partition and the swap partition. But forcing > filesystems to respect the stripe size will lead to no end of trouble, > because RAID volumes can have stripe sizes of 16 kB or more. I think it > is important to align partitions during installation because of the huge > performance impact of misaligned partitions on AF disks, but despite > what Nathan claims, I never advocated applying the same logic > everywhere. Yea, having poked at it for just a little while, I agree. The installer is the right place to make sure we don't cross-thread the 4k sectors. Stripe size means too many other things to have it be useful in that context. Warner _______________________________________________ svn-src-head@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
