On Feb 7, 2021, at 8:50 AM, Abner Gershon <6731...@gmail.com> wrote: > Disclaimer, I am not a programmer but have tinkered with shell and perl > scripts. Having said that, I think many would appreciate the option of > using gmirror with disks larger than 2T. > > The reason this is currently impossible is due to GPT and gmirror both > trying to store metadata in the last disk sector. > > MBR does not allow for partitioning of disk greater 2T. > > Workaround to gmirror partitions rather than whole disk is less than ideal.
Personally, I don't find mirroring individual partitions "less than ideal" or at least onerous enough to enforce "whole-disk" semantics. For one, it lets you use different balance algorithms for different partitions, e.g., "prefer" on mirrored swap (so crash dumps work without extra effort) and other balance algorithms for other file systems. For another, you might potentially have to rebuild less data, depending on the failure that leads to loss of mirror synchronisation. In a whole-disk mirror, you are always going to have to resynchronise the whole disk, whereas with partition-level mirrors you might be lucky and have to resynchronise only a single partition. Also, you could turn automatic resynchronisation off where it's not needed, e.g., for auto-encrypted swap partitions. Gmirror's naive resilvering is also why I prefer to use ZFS for mirroring: it resilvers (resynchronises) only actual data, and so is much faster for pools that are emptier. (I only use gmirror for swap nowadays and use ZFS for everything else.) > Lines 254-293 of geom_mirror.c « mirror « geom « lib - src - FreeBSD source > tree > <https://cgit.freebsd.org/src/tree/lib/geom/mirror/geom_mirror.c?h=releng/13.0> > relate > to clearing and storing metadata on last sector of disk. > > What would be the ramifications of altering gmirror to store metadata on > the 2nd to last sector? This is basically what it ends up doing already. You make a GPT covering the whole disk and the GPT uses the last sector for the backup. You then make a gmirror inside this GPT container and it uses the last sector of the container (second-to-last sector of the drive) for its gmirror stored metadata. Also, couldn't you achieve your end using BSD partitions, or are they disallowed inside GPT partitions? (I.e., create a GPT covering the whole disk; create a GPT partition covering the entire GPT; create a gmirror out of that; create a BSD partition on the gmirror for your file systems.) Cheers, Paul. _______________________________________________ freebsd-geom@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/freebsd-geom To unsubscribe, send any mail to "freebsd-geom-unsubscr...@freebsd.org"
