On Tue, Sep 18, 2018 at 10:22:15AM -0400, Mikulas Patocka wrote: > > On Tue, Sep 18, 2018 at 07:46:47AM -0400, Mikulas Patocka wrote: > > > I would ask the XFS developers about this - why does mkfs.xfs select > > > sector size 512 by default? > > > > Because the underlying device told it that it supported a > > sector size of 512 bytes? > > SSDs lie about this. They have 4k sectors internally, but report 512. SSDs can't lie about the sector size because they don't even have sectors in the disk sense, they have program and erase block size, and some kind of FTL granularity (think of it like a file system block size - even a 4k block size file can do smaller writes with read-modify-write cycles, so can SSDs). SSDs can just properly implement the guarantees they inherited from disk by other means. So if an SSD claims it supports 512 byte blocks it better can deal with them atomically. If they have issues in that area (like Intel did recently where they corrupted data left right and center if you actually did 512byte writes) they are simply buggy. SATA and SAS SSDs can always use the same trick as modern disks to support 512 byte access where really needed (e.g. BIOS and legacy OSes) but give a strong hint to modern OSes that they don't want that to be actually used with the physical block exponent. NVMe doesn't have anything like that yet, but we are working on something like that in the NVMe TWG.
