From owner-svn-doc-head@FreeBSD.ORG Tue Nov 20 02:55:19 2012 Return-Path: Delivered-To: svn-doc-head@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id EC4803FE; Tue, 20 Nov 2012 02:55:19 +0000 (UTC) (envelope-from eadler@FreeBSD.org) Received: from svn.freebsd.org (svn.freebsd.org [IPv6:2001:1900:2254:2068::e6a:0]) by mx1.freebsd.org (Postfix) with ESMTP id D170D8FC12; Tue, 20 Nov 2012 02:55:19 +0000 (UTC) Received: from svn.freebsd.org (localhost [127.0.0.1]) by svn.freebsd.org (8.14.5/8.14.5) with ESMTP id qAK2tJ6X005741; Tue, 20 Nov 2012 02:55:19 GMT (envelope-from eadler@svn.freebsd.org) Received: (from eadler@localhost) by svn.freebsd.org (8.14.5/8.14.5/Submit) id qAK2tJg5005739; Tue, 20 Nov 2012 02:55:19 GMT (envelope-from eadler@svn.freebsd.org) Message-Id: <201211200255.qAK2tJg5005739@svn.freebsd.org> From: Eitan Adler Date: Tue, 20 Nov 2012 02:55:19 +0000 (UTC) To: doc-committers@freebsd.org, svn-doc-all@freebsd.org, svn-doc-head@freebsd.org Subject: svn commit: r40102 - head/en_US.ISO8859-1/books/faq X-SVN-Group: doc-head MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-BeenThere: svn-doc-head@freebsd.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: SVN commit messages for the doc tree for head List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 20 Nov 2012 02:55:20 -0000 Author: eadler Date: Tue Nov 20 02:55:19 2012 New Revision: 40102 URL: http://svnweb.freebsd.org/changeset/doc/40102 Log: Add a section in the FAQ about ZFS Reviewed by: peter, peterj, jpaetzel, kpneal@pobox.com, Reviewed by: zbeeble@gmail.com, mcdouga9@egr.msu.edu, many others Approved by: bcr (mentor) Modified: head/en_US.ISO8859-1/books/faq/book.xml Modified: head/en_US.ISO8859-1/books/faq/book.xml ============================================================================== --- head/en_US.ISO8859-1/books/faq/book.xml Tue Nov 20 02:07:15 2012 (r40101) +++ head/en_US.ISO8859-1/books/faq/book.xml Tue Nov 20 02:55:19 2012 (r40102) @@ -5328,6 +5328,107 @@ C:\="DOS" + + + ZFS + + + + + What is the minimum amount of RAM one should have to + run ZFS? + + + + A minimum of 4GB of RAM is required for comfortable + usage, but individual workloads can vary widely. + + + + + + What is the ZIL and when does it get used? + + + + The ZIL ((ZFS + intent log) is a write log used to implement posix write + commitment semantics across crashes. Normally writes + are bundled up into transaction groups + and written to disk when filled (Transaction Group + Commit). However syscalls like &man.fsync.2; + require a commitment to stable storage before returning. + The ZIL is needed for writes that have been acknowledged + as written but which are not yet on disk as part of a + transaction. The transaction groups are timestamped. + In the event of a crash the last valid timestamp is + found and missing data is merged in from the ZIL. + + + + + + Do I need a SSD for ZIL? + + + + By default, ZFS stores the ZIL in the pool with all + the data. If your application has a heavy write load, + storing the ZIL in a separate device that has very fast + synchronous, sequential write performance can improve + overall system. For other workloads, a SSD is unlikely + to make much of an improvement. + + + + + + What is the L2ARC? + + + + The L2ARC is a read cache stored + on a fast device such as an SSD. + This cache is not persisent across + reboots. Note that RAM is used as the first layer + of cache and the L2ARC is only needed if there is + insufficient RAM. + + L2ARC needs space in the ARC to index it. So, + perversely, a working set that fits perfectly in the + ARC will not fit perfectly any more if a L2ARC is used + because part of the ARC is holding the L2ARC index, + pushing part of the working set into the + L2ARC which is slower than RAM. + + + + + + Is enabling deduplication advisable? + + + + Generally speaking, no. + + Deduplication takes up a significant amount + of RAM and may slow down read and write + disk access times. Unless one is storing data that is + very heavily duplicated (such as virtual machine images, + or user backups) it is possible that deduplication will + do more harm than good. Another consideration is the + inability to revert deduplication status. If + deduplication is enabled, data written, and then dedup + is disabled, those blocks which were deduplicated will + not be duplicated until they are next modified. + + Deduplication can also lead to some unexpected + situations. In particular deleting files may become much + slower. + + + +