On Tue, 18 Sep 2018, Eric Sandeen wrote: > > > On 9/18/18 9:16 AM, Eric Sandeen wrote: > > On 9/18/18 9:09 AM, Mikulas Patocka wrote: > >> > >> > >> On Tue, 18 Sep 2018, Eric Sandeen wrote: > >> > >>> On 9/18/18 7:32 AM, Dave Chinner 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? > >>> > >>> Not only that, but it must have told us that it had a /physical/ 512 sector. > >>> If it had even said physical/logical 4096/512, we would have chosen 4096. > >>> > >>> What does please check blockdev --getpbsz --getss /dev/$FOO say at mkfs time? > >> > >> On SSDs, physical sector size is not detectable - the ATA and NVME > >> standards allows reporting physical sector size, but some SSD vendors > >> report this as 512-bytes despite the fact that the SSD has 4k sectors > >> internally. > > > > There's a difference between "detecting" and "observing what the > > device reports." > > > > All we have to go on is the geometry reported by the device. > > > > # cat /sys/block/sdc/device/model > > Samsung SSD 850 > > # blockdev --getpbsz --getss /dev/sdc > > 512 > > 512 > > > > If the device lies to us, there's nothing to be done about it. > > > >> I tested 5 SSDs (Samsung SSD 960 EVO NVME, KINGSTON SKC1000240G NVME, > >> Samsung SSD 850 EVO SATA, Crucial MX100 SATA, Intel 520 SATA) - all of > >> them have 4k sectors internally (i.e. the SSDs have higher IOPS for 4k > >> writes than for 2k writes), but only the Crucial SSD reports 4096 in > >> /sys/block/*/queue/physical_block_size. Intel and Samsung report 512. > > See also > https://www.intel.com/content/www/us/en/support/articles/000006392/memory-and-storage.html > > -Eric And does it really support native 512-byte writes? Or does it emulate 512-byte writes by doing read-modify-write? That needs to be benchmarked, the paper doesn't say that. Memory is expensive and reducing SSD sector size increases memory requirement on the SSD. I doubt that any SSD vendor would want to use 8-times more memory just to support 512-byte sectors natively. Mikulas
