On 10/4/18 12:58 PM, Ilya Dryomov wrote:
> rbd devices report the following geometry:
>
> $ blockdev --getss --getpbsz --getiomin --getioopt /dev/rbd0
> 512
> 512
> 4194304
> 4194304
>
> (4M is unnecessarily high and will probably be made configurable and
> changed to 64K in the future. By default, the new bluestore backend
> does double-write for I/Os smaller than 64K.)
>
> If pbsz != iomin, mkfs.xfs goes into multidisk mode and, under the
> assumption that larger multidisk filesystems will have more devices,
> chooses a higher agcount. Though rbd devices are indeed backed by
> multiple OSD devices, it appears that high agcount actually degrades
> the performance with multiple rbd devices on the same host.
>
> Commit 9a106b5fbb88 ("mkfs.xfs: Don't stagger AG for a single disk")
> has set a precedent for treating sunit == swidth specially. Take it
> one step further.
>
> Signed-off-by: Ilya Dryomov <idryomov@xxxxxxxxx>
> ---
> mkfs/xfs_mkfs.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mkfs/xfs_mkfs.c b/mkfs/xfs_mkfs.c
> index 2e53c1e83b6a..c3efa30005a2 100644
> --- a/mkfs/xfs_mkfs.c
> +++ b/mkfs/xfs_mkfs.c
> @@ -2650,8 +2650,8 @@ _("agsize (%s) not a multiple of fs blk size (%d)\n"),
> (cfg->dblocks % cfg->agcount != 0);
> } else {
> calc_default_ag_geometry(cfg->blocklog, cfg->dblocks,
> - cfg->dsunit, &cfg->agsize,
> - &cfg->agcount);
> + cfg->dsunit != cfg->dswidth,
> + &cfg->agsize, &cfg->agcount);
> }
> }
I think this makes reasonable sense - it's tough to argue that storage is
advertising parallelism (multidisk) if swidth == sunit, IMHO. I'd be curious
to know what others think though, and I may go back and review the definitions
of what these values actually mean and how we choose to interpret them :)
TBH though if the functionality is OK (and I think it is) I'd rather pass
dsunit and dswidth both to the function, and let /it/ make the determination
about multidisk as opposed to leaving that up to the callers. Seems like
I had some other reason to do that as well, though I'm not remembering it
right now ...
-Eric
