On 2016-08-19 10:13, Warner Losh wrote: > 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 >
Maybe instead we just change gpart to default to 4k alignment, but users can always override with -a 512 or some other value? Then the installer behaves the same as a user typing 'gpart', but we don't mess with the entire geom layer? -- Allan Jude
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