Date: Thu, 12 May 2011 09:44:08 +0300 From: Daniel Kalchev <daniel@digsys.bg> To: freebsd-fs@freebsd.org Subject: Re: ZFS: How to enable cache and logs. Message-ID: <4DCB81B8.6070301@digsys.bg> In-Reply-To: <4DCB455C.4020805@dannysplace.net> References: <4DCA5620.1030203@dannysplace.net> <4DCB455C.4020805@dannysplace.net>
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On 12.05.11 05:26, Danny Carroll wrote: > > - Don't use SSD for the Log device. Write speed tends to be a problem. It all depends on your usage. You need to experiment, unfortunately. > - SSD ok for cache if the sizing is right, but without TRIM, don't > expect to take full advantage of the SSD. I do not believe TRIM has any effect on L2ARC. Why? - TRIM is a technique to optimize future writes; - L2ARC is written at controlled, very low rate, I believe something like 8MB/sec. There is no SSD currently on the market, with or without TRIM that has any trouble sustaining that rate. - TRIM might introduce delays, it is very 'expensive' command. But that will surely wary by drive/manufacturer. - There is no way TRIM can influence reading from the flash media. Reading from L2ARC with low latency and high speed is it's main purpose anyway. > Remaining questions. > - Is there any advantage to using a spare partition on a SCSI or SATA > drive as L2Arc? Assuming it was in the machine already but doing nothing? Absolutely no advantage. You want L2ARC to be very low latency and high-bandwidth for random reading. Especially low-latency. This does not apply to rotating disks. Daniel
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