[dm-devel] [PATCH RFC 0/8] Introduce provisioning primitives for thinly provisioned storage

[Mike Snitzer]

On Tue, Sep 20 2022 at  5:48P -0400,
Daniil Lunev <dlunev at google.com> wrote:

> > 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.
> 
> The specific problem is the following:
> * 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.

Thanks for this overview. Should help level-set others.

Adding fallocate support has been a long-standing dm-thin TODO item
for me. I just never got around to it. So thanks to Sarthak, you and
anyone else who had a hand in developing this.

I had a look at the DM thin implementation and it looks pretty simple
(doesn't require a thin-metadata change, etc).  I'll look closer at
the broader implementation (block, etc) but I'm encouraged by what I'm
seeing.

Mike