about zfs and ashift and changing ashift on existing zpool

[Michael Butler]

On 2019-04-08 20:55, Alexander Motin wrote:
> On 08.04.2019 20:21, Eugene Grosbein wrote:
>> 09.04.2019 7:00, Kevin P. Neal wrote:
>>
>>>> My guess (given that only ada1 is reporting a blocksize mismatch) is that
>>>> your disks reported a 512B native blocksize.  In the absence of any override,
>>>> ZFS will then build an ashift=9 pool.
>>
>> [skip]
>>
>>> smartctl 7.0 2018-12-30 r4883 [FreeBSD 11.2-RELEASE-p4 amd64] (local build)
>>> Copyright (C) 2002-18, Bruce Allen, Christian Franke, www.smartmontools.org
>>>
>>> === START OF INFORMATION SECTION ===
>>> Vendor:               SEAGATE
>>> Product:              ST2400MM0129
>>> Revision:             C003
>>> Compliance:           SPC-4
>>> User Capacity:        2,400,476,553,216 bytes [2.40 TB]
>>> Logical block size:   512 bytes
>>> Physical block size:  4096 bytes
>>
>> Maybe it't time to prefer "Physical block size" over "Logical block size" in relevant GEOMs
>> like GEOM_DISK, so upper levels such as ZFS would do the right thing automatically.
> 
> No.  It is a bad idea.  Changing logical block size for existing disks
> will most likely result in breaking compatibility and inability to read
> previously written data.  ZFS already uses physical block size when
> possible -- on pool creation or new vdev addition.  When not possible
> (pool already created wrong) it just complains about it, so that user
> would know that his configuration is imperfect and he should not expect
> full performance.

And some drives just present 512 bytes for both .. no idea if this is
consistent with the underlying silicon :-( I built a ZFS pool on it
using 4k blocks anyway.

smartctl 7.0 2018-12-30 r4883 [FreeBSD 13.0-CURRENT amd64] (local build)
Copyright (C) 2002-18, Bruce Allen, Christian Franke, www.smartmontools.org

=== START OF INFORMATION SECTION ===
Device Model:     WDC WDS100T2B0A-00SM50
Serial Number:    1837B0803409
LU WWN Device Id: 5 001b44 8b99f7560
Firmware Version: X61190WD
User Capacity:    1,000,204,886,016 bytes [1.00 TB]
Sector Size:      512 bytes logical/physical
Rotation Rate:    Solid State Device
Form Factor:      2.5 inches
Device is:        Not in smartctl database [for details use: -P showall]
ATA Version is:   ACS-4 T13/BSR INCITS 529 revision 5
SATA Version is:  SATA 3.3, 6.0 Gb/s (current: 6.0 Gb/s)
Local Time is:    Mon Apr  8 21:22:15 2019 EDT
SMART support is: Available - device has SMART capability.
SMART support is: Enabled
AAM feature is:   Unavailable
APM level is:     128 (minimum power consumption without standby)
Rd look-ahead is: Enabled
Write cache is:   Enabled
DSN feature is:   Unavailable
ATA Security is:  Disabled, frozen [SEC2]
Wt Cache Reorder: Unavailable

	imb