beyond the end of disk. r298900 added code to prevent this. Some BIOSes
cause significant delays if asked to read past end-of-disk.
We never trusted the BIOS to accurately report the sectorsize of disks
before and this set of changes. Unfortuately they interact badly with
the infamous >2TB wraparound bugs. We have a number of relatively-recent
machines in the FreeBSD.org cluster where the BIOS reports 3TB disks as 1TB.
With pre-r298900 they work just fine. After r298900 they stop working if
the boot environment attempts to access anything outside the first 1TB on
the disk. 'ZFS: I/O error, all block copies unavailable' etc. It affects
both UFS and ZFS if they try to boot from large volumes.
This change replaces the blind trust of the BIOS end-of-disk reporting
with a read-ahead clip to prevent reads crossing the of end-of-disk
boundary. Since 2^32 (2TB) size reporting truncation is not uncommon,
the clipping is done on 2TB aliases of the reported end-of-disk.
ie: a 3TB disk reported as 1TB has readahead clipped at 1TB, 3TB, 5TB, ...
as one of them is likely to be the real end-of-disk.
This should make the loader on these broken machines behave the same as
traditional pre-r298900 loader behavior, without disabling read-ahead.
PR: 212139
Discussed with: tsoome, allanjude
99cbd70f0b