Date: Wed, 19 Aug 2015 12:06:13 -0400 From: PK1048 <paul@pk1048.com> To: FreeBSD Filesystems <freebsd-fs@freebsd.org> Subject: Re: Optimizing performance with SLOG/L2ARC Message-ID: <3C6E3C03-9A64-4CEC-8238-2A73F4EE26D1@pk1048.com> In-Reply-To: <20150819154650.GA78333@neutralgood.org> References: <CAP1HOmTidC3%2BG4XfhvkQxieo%2BSYMq-JWiXF9Cs4FSW2VqkktWA@mail.gmail.com> <023F881D-CCC5-4FCA-B09D-EB92C3BFBC03@pk1048.com> <20150819154650.GA78333@neutralgood.org>
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On Aug 19, 2015, at 11:46, kpneal@pobox.com wrote: > On Wed, Aug 19, 2015 at 11:29:44AM -0400, PK1048 wrote: >> Someone commented on the size of the ZIL/SLOG=85 it needs to hold all = of the write data that arrives between TXG commits, which happen at = least every 5 seconds (it used to be 30 seconds, but that scared too = many people :-). SU a sync write arrives and it _must_ be committed to = permanent storage, so ZFS writes it to the ZFS Intent Log (ZIL) which = may or may not be a separate device (vdev). When the TXG that contains = that data is committed to the pool itself the data can be flushed from = the ZIL. If your source of sync writes is network shares, and you have a = 1 Gbps link, then your maximum ZIL will be 5 seconds x 1 Gbps or 5 = Gigabits. >=20 > That was me. By your figures 5 Gigabits is small compared to the size = of > SSD these days. If the SLOG ends up being that important to = performance > then it may make sense to buy small, excellent quality SSD. Exactly, unless your sync write data is coming from a _local_ = application or you have multiple 10 G ethernet connections :-) When I asked about SSD recommendations and sizing over on the OpenZFS = list, the consensus was that a 32 GB log device was probably big enough = for any rational load. I ordered 200 GB 3710=92s =85 sometimes = performance also scales with capacity, so be careful buying the smallest = SSD that fits the strict size needs. I will partition them and use some = for LOG and some for L2ARC (if I need it). I know this is not = recommended, but if it works, why not (and I do understand the = limitations of such a configuration). I only have 24 GB RAM in this box = and can probably benefit from some L2ARC. > I believe they even make PCIe battery-backed up RAM for this use. I = have > no idea about the price, though. Probably a lot. But maybe "a lot" = isn't > much depending on the value of the service provided by the machine. I have seen the RAM based battery-backed-up drives, and some are not = even ludicrous in terms of price, but I did not see any with FreeBSD = driver support. Since they are generally PCIe based (for speed), drives = are needed.
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