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. Then that's what they'll get from us. > The SSDs use 4k sectors to reduce the size of the mapping table (hardly > any SSD vendor would want to use real 512-byte sectors and increase the > size of the table 8 times) and they do read-modify-write for sub-4k > writes. So, why do you want to do sub-4k writes in XFS? - they are slower. > > For example, the Kingston NVME SSD has 5-times lower IOPS for 2k writes > than for 4k writes. And if I use mkfs.xfs directly on it, it selects > sectsz=512 for both metadata and log. If a device tells us that it has 512 sector granularity, that's all we have to go on. We can't second guess what the device reports to us. How could we do so safely? -Eric
