From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andreas Klauer Subject: Re: "bitmap" line in /proc/mdstat Date: Fri, 14 Oct 2016 16:10:00 +0200 Message-ID: <20161014141000.GA23227@metamorpher.de> References: <5800AE09.70608@bluewin.ch> <8922e2e1-b612-fa32-6932-9f4158f52fe6@turmel.org> <5800D045.9060602@bluewin.ch> <5800E4B8.9040205@bluewin.ch> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <5800E4B8.9040205@bluewin.ch> Sender: linux-raid-owner@vger.kernel.org To: Ramon Hofer Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids On Fri, Oct 14, 2016 at 03:59:20PM +0200, Ramon Hofer wrote: > Still I would imagine if I assume that a bit per chunck is needed and I > use 128kB chuncks on the 11720662272 blocks of the 12TB RAID5 this is: > log(11720662272)/log(2) = 34 Bytes > > Or am I totally wrong? It's negligible, a few bytes in the metadata somewhere. The bitmap has its own size (--bitmap-chunk option). It's fine to set this to a large value (128MiB, 1GiB, ...). That might help performance and reduce the bytes required for the bitmap itself (not that it matters). The main issue is performance impact... it's a tradeoff for additional bitmap updates each write, or long resync time after a power loss or similar event. If performance is not critical (e.g. it's a NAS, not a database) then you can enable it without worries. Regards Andreas Klauer