On Tue, 18 Sep 2018, 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? SSDs lie about this. They have 4k sectors internally, but report 512. > > If a filesystem created with the default 512-byte sector size is activated > > on a device with 4k sectors, it results in mount failure. > > Yes, it does, but mkfs should also fail when it tries to write 512 > byte sectors to a 4k device, too. > > > On Tue, 11 Sep 2018, David Teigland wrote: > > > > > Hi Mikulas, > > > > > > Am I doing something wrong below or is there a bug somewhere? (I could be > > > doing something wrong in the lvm activation code, also.) > > > Thanks > > > > > > > > > [root@null-05 ~]# lvs foo > > > LV VG Attr LSize > > > fast foo -wi------- 32.00m > > > main foo -wi------- 200.00m > > > > > > [root@null-05 ~]# lvchange -ay foo/main > > > > > > [root@null-05 ~]# mkfs.xfs /dev/foo/main > > > meta-data=/dev/foo/main isize=512 agcount=4, agsize=12800 blks > > > = sectsz=512 attr=2, projid32bit=1 > > > = crc=1 finobt=0, sparse=0 > > > data = bsize=4096 blocks=51200, imaxpct=25 > > > = sunit=0 swidth=0 blks > > > naming =version 2 bsize=4096 ascii-ci=0 ftype=1 > > > log =internal log bsize=4096 blocks=855, version=2 > > > = sectsz=512 sunit=0 blks, lazy-count=1 > > > realtime =none extsz=4096 blocks=0, rtextents=0 > > > > > > [root@null-05 ~]# mount /dev/foo/main /mnt > > > [root@null-05 ~]# cp /root/pattern* /mnt/ > > > [root@null-05 ~]# umount /mnt > > > [root@null-05 ~]# lvchange -an foo/main > > > > > > [root@null-05 ~]# lvconvert --type writecache --cachepool fast foo/main > > > Logical volume foo/main now has write cache. > > > > > > [root@null-05 ~]# lvs -a foo -o+devices > > > LV VG Attr LSize Origin Devices > > > [fast] foo -wi------- 32.00m /dev/pmem0(0) > > > main foo Cwi------- 200.00m [main_wcorig] main_wcorig(0) > > > [main_wcorig] foo -wi------- 200.00m /dev/loop0(0) > > Yeehaw! > > I'm betting that the underlying device advertised a logical/physical > sector size of 512 bytes to mkfs, and then adding pmem as the cache > device changed the logical volume from a 512 byte sector device to a > hard 4k sector device. > > If so, this is a dm-cache bug. dm-writecache can run with 512-byte sectors, but it increases metadata overhead 8 times and degrades performance. My question is - what's the purpose of using a filesystem with 512-byte sector size? Does it really improve performance? > Filesystems don't support changing the logical/physical sector sizes of ext4 uses 4096 byte sectors by default (except for devices smaller than 512MiB). > the block device dynamically. Filesystems lay out the filesystem > structure at mkfs time based on the assumption that the sector size of > the block device is fixed and will never change for the life of that > filesystem. > > Indeed, if the sector size of the block device is not fixed and can > change dynamically, then the block device also violates the > assumptions that the filesystem journalling algorithms make about > the atomic write size of the underlying device.... > > Cheers, > > Dave. > -- > Dave Chinner > david@xxxxxxxxxxxxx Mikulas
