[zfs-discuss] Re: ZFS memory usage

Gordan Bobic gordan.bobic at gmail.com
Wed Sep 4 11:06:42 EDT 2013


Umm... That's eyewateringly poor. There has to be a better way to do this.



On Wed, Sep 4, 2013 at 4:00 PM, Niels de Carpentier
<niels at decarpentier.com>wrote:

> > There might also be an alignment bug lurking about. I remember looking
> > into memory usage on a ZFS system previously, and it seemed as though
> > certain SPL slab caches were not aligned properly. That could cause, for
> > example, a 512B buffer to expand to a 1024B allocation. Unfortunately, I
> > haven't had time to dig into this.
>
> That is not a bug, but a feature of the SPL slab.
>
> A 512B object + header (spl_kmem_obj_t), with a 512B alignment, will
> effectively use 1024B.
> Because of the header the objects cannot be neatly packed, and because of
> the alignment a huge amount of space is wasted.
> The same is true for 4kB objects, which effectively use 8kB per object.
>
> Niels
>
>
>
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