[lustre-discuss] Overstriping setting

[Andreas Dilger]

You should be able to use "-C N" to overstripe a file without specifying the OST indexes with "-o ...". 

For handling this via llapi_layout commands, I believe it is necessary to set llapi_layout_pattern_set(LLAPI_LAYOUT_OVERSTRIPING) flag on the component, and then specify a stripe count > OSTCOUNT.  I see this isn't documented in the llapi_layout_pattern_set(3) man page (along with LLAPI_LAYOUT_FOREIGN), so please file a Jira ticket for this (and ideally also submit a patch to the man page).

The flag will be cleared if the stripe count <= OSTCOUNT, for improved compatibility with older clients that do not understand overstriping (though that is unlikely these days).

The patch https://review.whamcloud.com/54192 ("LU-16938 utils: setstripe overstripe multiple OST count") along with a few follow-on fixes in Lustre 2.16+ also allows specifying:

     lfs setstripe -C -N ... FILE|DIR

(or llapi equivalent) to create 'N' stripes per OST for the file, instead of having to know the exact OST count, if that is more convenient.

Cheers, Andreas

> On Dec 20, 2025, at 18:52, Wei-Keng Liao via lustre-discuss <lustre-discuss at lists.lustre.org> wrote:
> 
> When setting the overstriping for a new file, is it possible to let
> the MDS to choose the OST indices?
> 
> I was able to use lfs command to set an overstiping for a new file.
> For example, to overstripe a file over 4 OSTs with 2 stripe per OST,
> I am using this command:
> 
> %  lfs setstripe -c 4 -C 8 -o 10-13,10-13 $SCRATCH/dummy
> 
> %  lfs getstripe $SCRATCH/dummy | grep lmm
> lmm_stripe_count:  8
> lmm_stripe_size:   1048576
> lmm_pattern:       raid0,overstriped
> lmm_layout_gen:    0
> lmm_stripe_offset: 10
> lmm_pool:          original
> 
> My understanding is when without overstriping, the default is that
> the OSTs are selected by Lustre MDS based on some policy (maybe OST
> usage). I wonder if this can also apply to overstriping, i.e. using
> lfs command options '-c' and '-C' without option '-o'.
> 
> I am also wondering how this can be achieved using the Lustre user
> C APIs, when making calls to llapi_layout_ost_index_set().
> 
> 
> Wei-keng
> 
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