Re: dm-writecache issue

[David Teigland <teigland@redhat.com>]

On Tue, Sep 18, 2018 at 07:48:04AM -0500, Eric Sandeen wrote:
> 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.

Right, just after asking I discovered that the advertised
physical_block_size changes after attaching dm-writecache:

Before:

# dmsetup ls
foo-main        (253:3)
foo-fast        (253:4)

# cat /sys/block/dm-3/queue/physical_block_size 
512
# cat /sys/block/dm-4/queue/physical_block_size 
4096

After:

# dmsetup ls
foo-main_wcorig (253:4)
foo-main        (253:5)
foo-fast        (253:3)

# cat /sys/block/dm-3/queue/physical_block_size 
4096
# cat /sys/block/dm-5/queue/physical_block_size 
4096


> What does please check blockdev --getpbsz --getss /dev/$FOO say at mkfs time?

Before:

# blockdev --getpbsz --getss /dev/foo/main
512
512

After:

# blockdev --getpbsz --getss /dev/foo/main
4096
4096


> > Yes, it does, but mkfs should also fail when it tries to write 512
> > byte sectors to a 4k device, too.

By default it uses the 4k sectors in that case:

# mkfs.xfs -f /dev/foo/main
meta-data=/dev/foo/main          isize=512    agcount=4, agsize=13056 blks
         =                       sectsz=4096  attr=2, projid32bit=1
         =                       crc=1        finobt=0, sparse=0
data     =                       bsize=4096   blocks=52224, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0 ftype=1
log      =internal log           bsize=4096   blocks=1605, version=2
         =                       sectsz=4096  sunit=1 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0


But you are correct if I request 512 on a 4k device:

# mkfs.xfs -f -s size=512 /dev/foo/main
illegal sector size 512; hw sector is 4096

> > 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.

Yes

I've been dealing with this by just using -s size=4096.