Date: Thu, 23 Oct 2014 13:29:25 +0000 From: =?utf-8?B?S2FybGkgU2rDtmJlcmc=?= <Karli.Sjoberg@slu.se> To: "freebsd-fs@freebsd.org" <freebsd-fs@freebsd.org> Subject: How big can ZFS L2ARC grow? Message-ID: <5F9E965F5A80BC468BE5F40576769F099DF6E88A@exchange2-1>
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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@slu.se
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