[lustre-discuss] Group Lock Semantics

[Oleg Drokin]

Hello!

On Fri, 2026-02-06 at 18:09 -0800, Freddie Witherden via lustre-discuss
wrote:

> So, we reworked our code slightly to ensure that each page is only
> ever 
> written to by a single rank.   However, even here we find data to 
> occasionally be missing from the file with the offsets corresponding
> to 
> boundaries between hosts.  We have even tried increasing the size up
> to 
> the stripe size for the file (so each N MiB stripe is only ever
> written 
> to by a single rank) but to no avail.
> 
> Hence, I am wondering what the specific semantics are for writes
> under a 
> group lock?  Do we have to use O_DIRECT and bypass the page cache,
> are 
> there more significant alignment requirements than pages?

I think O_DIRECT was the primary idea for using as otherwise same host
mixed io might be confused about which pages are covered by what locks,
but in general it's still supposed to work without any particular
alignment requirements.

Do you happen to have a simplistic test case demonstrating the problem
by any chance?

Bye,
   Oleg