Date: Thu, 27 Aug 2015 20:12:54 -0400 From: Quartz <quartz@sneakertech.com> To: freebsd-fs@freebsd.org Subject: Re: Options for zfs inside a VM backed by zfs on the host Message-ID: <55DFA786.8090809@sneakertech.com> In-Reply-To: <CALcn87yArcBs0ybrZBBxaxDU0y6s=wM8di0RmaSCJCgOjUHq9w@mail.gmail.com> References: <CALd%2BdcfJ%2BT-f5gk_pim39BSF7nhBqHC3ab7dXgW8fH43VvvhvA@mail.gmail.com> <20150827061044.GA10221@blazingdot.com> <20150827062015.GA10272@blazingdot.com> <1a6745e27d184bb99eca7fdbdc90c8b5@SERVER.ad.usd-group.com> <55DF46F5.4070406@redbarn.org> <453A5A6F-E347-41AE-8CBC-9E0F4DA49D38@ccsys.com> <CALcn87yArcBs0ybrZBBxaxDU0y6s=wM8di0RmaSCJCgOjUHq9w@mail.gmail.com>
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> I am right now exploring the question: are SSD ZILs necessary in an all SSD > pool? Something mentioned in another recent thread on this list (or maybe it was -questions?) was that yes, you really should consider a separate ZIL if you're using primarily SSDs. Without a separate disk, log writes have to steal blocks from the pool itself which then have to be deleted afterwards to let go of the space. Besides causing excess file fragmentation, the write-delete cycle doesn't play well with SSDs and trim and can seriously hamper performance. With a dedicated disk, it writes and then just leaves it there, only overwriting later if necessary.
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