Using 2 SSD's to create a SLOG

[krad]

An NFS server is a common task that generates lots of synchronous writes


On 8 July 2014 03:51, <kpneal at pobox.com> wrote:

> On Mon, Jul 07, 2014 at 06:06:43PM -0700, javocado wrote:
> > I am interested in adding an SSD "SLOG" to my ZFS system so as to
> > (dramatically) speed up writes on this system.
> >
> > My question is if ZFS will, itself, internally, mirror two SSDs that are
> > used as a SLOG ?
> >
> > What I mean is, if ZFS is already smart enough to create a zpool mirror
> > (or, on my case, a zpool raidz3) then perhaps ZFS is also smart enough to
> > mirror the SLOG to two individual SSDs ?
> >
> > I am hoping to dumbly plug two SSDs onto motherboard SATA ports and just
> > hand them over, raw, to ZFS.
>
> From the zpool man page:
>
>        Example 12 Creating a ZFS Pool with Mirrored Separate Intent Logs
>
>        The following command creates a ZFS storage  pool  consisting  of
>  two,
>        two-way mirrors and mirrored log devices:
>
>          # zpool create pool mirror c0d0 c1d0 mirror c2d0 c3d0 log mirror \
>             c4d0 c5d0
>
> You should be able to use that example to make the 'zpool add' command to
> add the mirrored log to an existing pool.
>
> But know that the SLOG only helps writes that are synchronous. This is in
> many workloads a small fraction of the total writes. For other workloads
> it is a large portion of the writes.
>
> Do you know for certain that you need a SLOG?
> --
> Kevin P. Neal                                http://www.pobox.com/~kpn/
>            On the community of supercomputer fans:
> "But what we lack in size we make up for in eccentricity."
>   from Steve Gombosi, comp.sys.super, 31 Jul 2000 11:22:43 -0600
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