Date: Thu, 14 Feb 2019 16:47:22 +0100 From: Andrea Venturoli <ml@netfence.it> To: freebsd-questions@freebsd.org Subject: Re: ZFS and a spare SSD: ZIL or L2ARC? Message-ID: <f9145f98-0679-045e-081c-8a25d2f075a4@netfence.it> In-Reply-To: <CALfReydr8srcZYdntQv-xxp=UONrx2tPbAS6a5wr0cxioGM2cQ@mail.gmail.com> References: <c296b7b3-8418-3966-66ed-502d25a848dd@netfence.it> <20190213171142.33b7c430c5ec256b1cbafa42@sohara.org> <CALfReydr8srcZYdntQv-xxp=UONrx2tPbAS6a5wr0cxioGM2cQ@mail.gmail.com>
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On 2/14/19 3:48 PM, krad wrote: > You need to be careful with L2ARC as under certain conditions it can > actually degrade performance. This is down to you needing a data structure > in ram to manage the l2arc. This comes at the expense of ARC. However this > is more of an edge case when you are adding huge amounts of L2ARC compared > to system ram, but important in the enterprise world. This is interesting. I understand precise measurement would be required: are there proper tools to do this? Lacking them, are there "known cases" described? Like 16GiB RAM + 128 GiB SSD (which would be my case)? > From your described workload ZiL might be better suited as databases and > virtualisation often cause lots of sync writes. This is where Zil can help. > However the picture is never simple. If you are going to have a zil you > really want it mirrored This was also my impression: I would probably benefit more from a ZiL than an L2ARC, but as I said, I've only got a spare SSD (no mirror). Since this is more of a curiosity than a real need, I'm not risking data. bye & Thanks av.
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