zvol block size mis-match
mfhwaltz opened this issue · comments
When creating a zvol on macOS, the OS always sees the resulting block device as having a block size of 512 Bytes regardless of the volblocksize for that zvol.
Mojave:~ root# uname -a|perl -p -e 's/\s\S+/ <redacted>/'
Darwin <redacted> 18.6.0 Darwin Kernel Version 18.6.0: Thu Apr 25 23:16:27 PDT 2019; root:xnu-4903.261.4~2/RELEASE_X86_64 x86_64
Mojave:~ root# zfs version
zfs-1.9.0-1
zfs-kmod-1.9.0-1
Mojave:~ root# diskutil list disk0
/dev/disk0 (internal, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *1.0 TB disk0
1: EFI EFI 209.7 MB disk0s1
2: Apple_APFS Container disk2 274.7 GB disk0s2
3: ZFS 725.3 GB disk0s3
Mojave:~ root# zpool create -m none -O compression=on testpool disk0s3
Mojave:~ root# zpool list
NAME SIZE ALLOC FREE CKPOINT EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT
testpool 672G 408K 672G - - 0% 0% 1.00x ONLINE -
Mojave:~ root# zfs create -s -V 10G testpool/testvol1
Mojave:~ root# zfs create -s -b 4k -V 20G testpool/testvol2
Mojave:~ root# zfs list
NAME USED AVAIL REFER MOUNTPOINT
testpool 556K 651G 96K none
testpool/testvol1 56K 651G 56K -
testpool/testvol2 56K 651G 56K -
Mojave:~ root# zfs get volblocksize testpool/testvol1
NAME PROPERTY VALUE SOURCE
testpool/testvol1 volblocksize 128K default
Mojave:~ root# zfs get volblocksize testpool/testvol2
NAME PROPERTY VALUE SOURCE
testpool/testvol2 volblocksize 4K -
Mojave:~ root# diskutil info disk4
Device Identifier: disk4
Device Node: /dev/disk4
Whole: Yes
Part of Whole: disk4
Device / Media Name: ZVOL testpool/testvol1
Volume Name: Not applicable (no file system)
Mounted: Not applicable (no file system)
File System: None
Content (IOContent): None
OS Can Be Installed: No
Media Type: Generic
Protocol: Disk Image
SMART Status: Not Supported
Disk Size: 10.7 GB (10737418240 Bytes) (exactly 20971520 512-Byte-Units)
Device Block Size: 512 Bytes
Read-Only Media: No
Read-Only Volume: Not applicable (no file system)
Device Location: External
Removable Media: Fixed
Solid State: Yes
Virtual: Yes
Mojave:~ root# diskutil info disk5
Device Identifier: disk5
Device Node: /dev/disk5
Whole: Yes
Part of Whole: disk5
Device / Media Name: ZVOL testpool/testvol2
Volume Name: Not applicable (no file system)
Mounted: Not applicable (no file system)
File System: None
Content (IOContent): None
OS Can Be Installed: No
Media Type: Generic
Protocol: Disk Image
SMART Status: Not Supported
Disk Size: 21.5 GB (21474836480 Bytes) (exactly 41943040 512-Byte-Units)
Device Block Size: 512 Bytes
Read-Only Media: No
Read-Only Volume: Not applicable (no file system)
Device Location: External
Removable Media: Fixed
Solid State: Yes
Virtual: Yes
Mojave:~ root#
That is correct - turns out the IO layer in XNU kernels (at least back in 10.8 when we looked at it) can only do two sizes, 512, and 4096 - everything else is an error. So you can set the recordsize that ZFS will use with the TXGs and all that logic, but when it comes to issuing IO to the devices, it has to use one of the two sizes. We should check if there has been any improvements in this area by Apple, but it seems unlikely.