> There is no such thing as WRITE UNAVAILABLE in NVMe.
Apologize, that is WRITE UNCORRECTABLE. Chapter 3.2.7 of
NVM Express NVM Command Set Specification 1.0b
> That being siad you still haven't actually explained what problem
> you're even trying to solve.
* There is an thinpool over a physical device
* There are multiple logical volumes over the thin pool
* Each logical volume has an independent file system and an
independent application running over it
* Each application is potentially allowed to consume the entirety
of the disk space - there is no strict size limit for application
* Applications need to pre-allocate space sometime, for which
they use fallocate. Once the operation succeeded, the application
assumed the space is guaranteed to be there for it.
* Since filesystems on the volumes are independent, filesystem
level enforcement of size constraints is impossible and the only
common level is the thin pool, thus, each fallocate has to find its
representation in thin pool one way or another - otherwise you
may end up in the situation, where FS thinks it has allocated space
but when it tries to actually write it, the thin pool is already
exhausted.
* Hole-Punching fallocate will not reach the thin pool, so the only
solution presently is zero-writing pre-allocate.
* Not all storage devices support zero-writing efficiently - apart
from NVMe being or not being capable of doing efficient write
zero - changing which is easier said than done, and would take
years - there are also other types of storage devices that do not
have WRITE ZERO capability in the first place or have it in a
peculiar way. And adding custom WRITE ZERO to LVM would be
arguably a much bigger hack.
* Thus, a provisioning block operation allows an interface specific
operation that guarantees the presence of the block in the
mapped space. LVM Thin-pool itself is the primary target for our
use case but the argument is that this operation maps well to
other interfaces which allow thinly provisioned units.
--Daniil
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