Date: Thu, 24 Apr 2008 11:00:12 +0100 From: Dieter <freebsd@sopwith.solgatos.com> To: freebsd-performance@freebsd.org Subject: vfs.write_behind Message-ID: <200804241800.SAA11746@sopwith.solgatos.com>
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The handbook says: The vfs.write_behind sysctl variable defaults to 1 (on). This tells the file system to issue media writes as full clusters are collected, which typically occurs when writing large sequential files. The idea is to avoid saturating the buffer cache with dirty buffers when it would not benefit I/O performance. However, this may stall processes and under certain circumstances you may wish to turn it off. Looking through the documentation, I can't find an explaination of what a cluster is. Multiple blocks? How would this stall processes? Seems backwards. If you don't have write-behind, a process would block until the data gets written to the media. So if vfs.write_behind is 0, then a larger number of smaller writes are issued?
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