[lustre-discuss] How to obtain the total number of available OSTs when poolname is empty

[Wei-Keng Liao]

Many thanks, Andreas!
That is exactly what I need.

Wei-keng

On Dec 20, 2025, at 2:13 PM, Andreas Dilger <adilger at thelustrecollective.com> wrote:

There is a function to return the number of OSTs at a specific directory (it does not need to be the mount point of the filesystem):

int llapi_get_obd_count(char *dir, int *count, int is_mdt);

that looks like what you want.  It appears there is no man page for this, so it should be added.

Cheers, Andreas

On Dec 20, 2025, at 12:45, Wei-Keng Liao via lustre-discuss <lustre-discuss at lists.lustre.org> wrote:


I am trying to get the total number of available OSTs given
a folder name, through calls of Luster user C APIs, but failed
to do so, when a layout has no pool name. I have tried the followings.

At first, call llapi_layout_get_by_fd() to obtain a folder's 'layout'.

llapi_layout_pool_name_get() returns successfully, but the returned
pool_name is empty.

llapi_getname() returns a valid fsname.

llapi_get_poolmembers() returns -22 (i.e. -EINVAL), when using
the 'fsname' as the argument 'poolname.'

If I appended a dot, '.', to fsname, llapi_get_poolmembers() returns
successfully, but the returned value is 0 (number of members).

When doing my experiments on Polaris at ALCF, I have also tried
the following lfs commands at the prompt of login node.

% lfs  pool_list /eagle
Pools from eagle:

% lfs getname -n /eagle
eagle

% lfs osts /eagle |wc -l
161

Command 'lfs osts /eagle' shows there are 160 OSTs.

On Perlmutter at NERSC, its Lustre has a pool name, and thus
I was able to make the above Lustre user API calls to retrieve
the total number of available OSTs.

In summary, my question is how do I properly make calls to Lustre
user C APIs when a layout has no pool name?


Wei-keng Liao
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