autodetect ashift better

[ek4]

Autodetect on SSD results in ashift=9 but this is obviously nonsense. Please check whether the drive is rotational or not before propagating this brokenness onto my pool.

[amotin]

Do you propose to enforce some ashift for all non-rotatinf devices? I think it is very wrong assumption. If flash block us 32KB, it may have no big difference whether ashift is 9 or 12, both are equally bad, while bumping ashift to 12 may reduce space ZFS efficiency and increase disk and flash traffic without need.

[gdevenyi]

If there is more than one way to detect the underlying device sector geometries, perhaps ZFS should use all of the along with rotational vs ssd, and provide a warning/error if there are discrepancies, forcing the user to explicitly set a value based on research of the device?

[shodanshok]

Do you propose to enforce some ashift for all non-rotatinf devices? I think it is very wrong assumption. If flash block us 32KB, it may have no big difference whether ashift is 9 or 12, both are equally bad, while bumping ashift to 12 may reduce space ZFS efficiency and increase disk and flash traffic without need.

This is not totally correct: while ashift=12 does not means aligning to flash page nowadays, SSD's controllers (and FTLs) are way more optimized to manage 4K writes rather than 512B ones.

I would propose a much simpler approach: simply let zfs create ashift=12 pools by default, leaving the choice of 512B (or other values) at user requests. This will solve 99% of user issues, leaving other users/sysadmins the power to specify different ashift when it make sense.

Note: I propose the above for both rotational (ie: HDD) and non-rotational (SSD, Optane, etc) storage devices. This will even have the good side effect of removing the device blacklist once included (still present?) in zfs.

[ek4]

#As of 2017, NAND-flash SSDs are tuned for 4096-byte IOs. Matching the flash page size is unnecessary and ashift=12 is usually the correct choice.

Even if the code is still setting ashift based on legacy nonsense the docs have it straight.

[kftsehk]

I am sorry but I seem learnt the hard way, plan for and set a fixed ashift when creating the pool manually, do not autodetect

#15352