Date: Wed, 11 May 2011 15:16:07 +0200 From: Alexander Leidinger <Alexander@Leidinger.net> To: Jeremy Chadwick <freebsd@jdc.parodius.com> Cc: freebsd-fs@freebsd.org Subject: Re: ZFS: How to enable cache and logs. Message-ID: <20110511151607.105949eypk3ed3c4@webmail.leidinger.net> In-Reply-To: <20110511100655.GA35129@icarus.home.lan> References: <4DCA5620.1030203@dannysplace.net> <20110511100655.GA35129@icarus.home.lan>
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Quoting Jeremy Chadwick <freebsd@jdc.parodius.com> (from Wed, 11 May 2011 03:06:56 -0700): > L2ARC devices should primarily be something with extremely fast read > rates (e.g. SSDs). USB1.x and 2.x memory sticks do not work well for > this purpose given protocol and bus speed limits + overhead. (I only > mention them because people often think "Oh, USB flash would work great > for this!" I disagree.) Using USB flash may work acceptable. It depends upon the rest of the system. If you have very fast harddisks (or only USB 1 hardware), USB flash will not give you a faster FS. If you have slow (and low-power) desktop disks, a fast USB flash (attention, there are also slow ones) connected via USB 2 (or 3) will give you a speed improvement you notice. As a matter of fact, I have this: - Pentium 4 - 1 GB RAM - 1 Western Digital Caviar Blue - 2 Seagate Barracuda 7200.10 - an ICH5 controller (no NCQ) - no name cheap give-away 1 GB USB flash (so not a very fast one) The disks are used in a RAIDZ, with the USB flash as a cache device. My use case was connecting to a webmail system over a slow line (ADSL 224 kilobit/s). I noticed directly when the cache was in use or not. I also have another system, ICH 10 with NCQ, 5 disks (WD RE4 RAID) in RAIDZ2, Intel Xeon 4-core, 12 GB RAM. There USB flash does not make sense at all (and the SSD makes sense if you compare the price of the entire system with the price of a small or medium SSD). For the first system, it does not make sense to spend 200 units of money for a SSD, the system itself is not worth much more now. Spending 5-10 units of money for this system is ok, and gives a speed improvement. Bye, Alexander. -- Even God cannot change the past. -- Joseph Stalin http://www.Leidinger.net Alexander @ Leidinger.net: PGP ID = B0063FE7 http://www.FreeBSD.org netchild @ FreeBSD.org : PGP ID = 72077137
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