How big can ZFS L2ARC grow?

krad kraduk at gmail.com
Fri Oct 24 06:52:36 UTC 2014


doesn l2arc have compression these days?

On 23 October 2014 14:29, Karli Sjöberg <Karli.Sjoberg at slu.se> wrote:

> Hey!
>
> As the topic states, I´m wondering about the size of L2ARC and if there
> is a limit to how big it is able to grow.
>
> Why I´m asking is because I´ve always thought that if you add a cache
> device to the pool, the maximum size of L2ARC would be the size of the
> disk you´ve added, but recently I´ve come to know that´s not the case.
>
> Here´s a 9.3-RELEASE system that has 64 GB RAM and two 256 GB large
> SSD's added as cache, that I would´ve thought only could´ve grown to
> about 512 GB:
> # sysctl -n kstat.zfs.misc.arcstats.l2_size
> 7696626233344
>
> Another system running 9.2-RELEASE with 32 GB RAM + 240 GB SSD:
> # sysctl -n kstat.zfs.misc.arcstats.l2_size
> 1400038980608
>
> The servers are running a software for graphing so I have seen that the
> size numbers can go up and down over time, but clearly goes over the
> size of the SSD's that have been added.
>
> We have two more systems configured with cache devices and yet another
> two systems configured without.
>
> The problem we have is that the four systems with cache devices (our
> primary storage systems) goes completely unresponsive after different
> periods of time, depending on how much RAM they have and the load they
> ´ve been under, I guess. The less RAM, the more frequent they stall, and
> I´m starting to wonder if what´s common between them is L2ARC, because
> the other two systems without cache devices doesn´t have those issues,
> although they aren´t under the same kind of load either, it´s a disaster
> recovery system receiving zfs snapshots and the other one is our syslog
> server, but still...
>
> What do you think, are the size numbers for L2ARC unusual, and could it
> be related to the stalls we´ve been experiencing? And if the size
> numbers really are unusual, is there a way to handle it, like limit how
> large the L2ARC is able to grow somehow?
>
>
>
> --
>
> Med Vänliga Hälsningar
>
>
> -------------------------------------------------------------------------------
> Karli Sjöberg
> Swedish University of Agricultural Sciences Box 7079 (Visiting Address
> Kronåsvägen 8)
> S-750 07 Uppsala, Sweden
> Phone:  +46-(0)18-67 15 66
> karli.sjoberg at slu.se
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