[zfs-discuss] zfs caching question

covici at ccs.covici.com covici at ccs.covici.com
Tue Feb 2 12:15:29 EST 2016


OK, thanks.  I have a second pool for my hds -- I was originally
thinking of using the ssd as a cache, but people here indicated that it
would not work well, so I have two separate pools.

Gregor Kopka via zfs-discuss <zfs-discuss at list.zfsonlinux.org> wrote:

> Sadly the reads will be served from both the SSD and the HDD, which will
> lead to it being slower than needed - especially in case ZFS decides
> that it could be a good idea to pull demanded metadata from the HDD
> (stalling everything else while waiting for the seek).
> 
> There are some issues (open and closed) about this, but as of now ZFS is
> neither smart enough to realize that the SSD side of a mirror is several
> orders of magnitude faster so to ignore the HDD (resulting in an order
> of magniture performance degradion compared to pure single-SSD vdev),
> nor does an administrator have a way to instruct ZFS to not read from
> the HDD side of a mirror (except for scrub/resilver).
> 
> Some tuning options are in the pipeline from illumous though (IIRC), so
> it hopefully will get better with time.
> 
> Gregor
> 
> Am 01.02.2016 um 22:38 schrieb Andrew Carlson via zfs-discuss:
> > I believe the way it works is that if you mirror the ssds to hd, and
> > leave it that way, it will be fairly slow.  The reads would probably
> > get read from ssd, but writes have to go to both.  I would not use
> > mixed ssd/hd in a pool.
> >
> > On Mon, Feb 1, 2016 at 3:35 PM, John Covici via zfs-discuss
> > <zfs-discuss at list.zfsonlinux.org
> > <mailto:zfs-discuss at list.zfsonlinux.org>> wrote:
> >
> >     On 01/31/2016 11:21 PM, John Covici via zfs-discuss wrote:
> >     > > Right now I have a non-redundantpool entirely on two ssd drives.
> >     > ...
> >     > > I will use a mirrored configuration soon thereafter all this is
> >     > > done.
> >     >
> >     > To be clear, you're intending on ending up with 4 spinning disks
> >     in two
> >     > mirrored vdevs?
> >     >
> >     > > Would I still get the nice performance out of the ssds and what
> >     > > would be the proper procedure to do this?  From what I have
> >     read in
> >     > > the
> >     > > man page, I would attach the new hd space and once it resilvers,
> >     > > detach
> >     > > the ssd drives and add them back as cache devices.
> >     >
> >     > For the design you have proposed, yes, that's the procedure to get
> >     > there.
> >     >
> >     > Design-wise, another option would be separate pools. That would
> >     get you
> >     > pure SSD performance for some datasets and pure HDD for others. What
> >     > you're proposing will get you somewhere in the middle.
> >     >
> >     > Keep in mind that until persistent L2ARC is merged (which I
> >     don't think
> >     > has happened yet upstream, much less in ZoL), the contents of
> >     the cache
> >     > are lost on every reboot. So L2ARC is okay for a server, but
> >     probably
> >     > not on a laptop.
> >     >
> >     > Also, L2ARC requires RAM to keep its metadata, so there's a
> >     trade-off
> >     > there too.
> >     >
> >     > I don't feel qualified enough to say whether L2ARC is good or
> >     bad for
> >     > you overall.
> >     >
> >     > --
> >     > Richard
> >
> >     Yes, the equivalent of that, actually two disks  in one mirror and one
> >     larger disk in the other.  But, maybe I should just mirror the ssd
> >     pool
> >     onto hd and  do a separate pool for the other hd stuff, or leave the
> >     rest alone using lvm  Would mirroring the ssd's decrease the
> >     performance?
> >
> >
> >      --
> >      Your life is like a penny.  You're going to lose it.  The
> >     question is:
> >      How do
> >      you spend it?
> >
> >               John Covici
> >               covici at ccs.covici.com <mailto:covici at ccs.covici.com>
> >
> >     _______________________________________________
> >     zfs-discuss mailing list
> >     zfs-discuss at list.zfsonlinux.org
> >     <mailto:zfs-discuss at list.zfsonlinux.org>
> >     http://list.zfsonlinux.org/cgi-bin/mailman/listinfo/zfs-discuss
> >
> >
> >
> >
> > -- 
> > Andy Carlson
> > ---------------------------------------------------------------------------
> > Gamecube:$150,PSO:$50,Broadband Adapter: $35, Hunters License:
> > $8.95/month,
> > The feeling of seeing the red box with the item you want in it:Priceless.
> >
> >
> > _______________________________________________
> > zfs-discuss mailing list
> > zfs-discuss at list.zfsonlinux.org
> > http://list.zfsonlinux.org/cgi-bin/mailman/listinfo/zfs-discuss
> 
> 
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-- 
Your life is like a penny.  You're going to lose it.  The question is:
How do
you spend it?

         John Covici
         covici at ccs.covici.com


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