Date: Wed, 11 May 2011 13:37:03 +0300 From: Daniel Kalchev <daniel@digsys.bg> To: freebsd-fs@freebsd.org Subject: Re: ZFS: How to enable cache and logs. Message-ID: <4DCA66CF.7070608@digsys.bg> In-Reply-To: <20110511100655.GA35129@icarus.home.lan> References: <4DCA5620.1030203@dannysplace.net> <20110511100655.GA35129@icarus.home.lan>
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On 11.05.11 13:06, Jeremy Chadwick wrote: > On Wed, May 11, 2011 at 07:25:52PM +1000, Danny Carroll wrote: >> When I move to v28 I will probably wish to enable a L2Arc and also >> perhaps dedicated log devices. >> > In the case of ZFS intent logs, you definitely want a mirror. If you > have a single log device, loss of that device can/will result in full > data loss of the pool which makes use of the log device. This is true for v15 pools, not true for v28 pools. In ZFS v28 you can remove log devices and in the case of sudden loss of log device (or whatever) roll back the pool to a 'good' state. Therefore, for most installations single log device might be sufficient. If you value your data, you will of course use mirrored log devices, possibly in hot-swap configuration and .. have a backup :) By the way, the SLOG (separate LOG) does not have to be SSD at all. Separate rotating disk(s) will also suffice -- it all depends on the type of workload. SSDs are better, for the higher end, because of the low latency (but not all SSDs are low latency when writing!). The idea of the SLOG is to separate the ZIL records from the main data pool. ZIL records are small, even smaller in v28, but will cause unnecessary head movements if kept in the main pool. The SLOG is "write once, read on failure" media and is written sequentially. Almost all current HDDs offer reasonable sequential write performance for small to medium pools. The L2ARC needs to be fast reading SSD. It is populated slowly, few MB/sec so there is no point to have fast and high-bandwidth write-optimized SSD. The benefit from L2ARC is the low latency. Sort of slower RAM. It is bad idea to use the same SSD for both SLOG and L2ARC, because most SSDs behave poorly if you present them with high read and high write loads. More expensive units might behave, but then... if you pay few k$ for a SSD, you know what you need :) Daniel
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