From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ramon Hofer Subject: Re: "bitmap" line in /proc/mdstat Date: Fri, 14 Oct 2016 14:32:05 +0200 Message-ID: <5800D045.9060602@bluewin.ch> References: <5800AE09.70608@bluewin.ch> <8922e2e1-b612-fa32-6932-9f4158f52fe6@turmel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <8922e2e1-b612-fa32-6932-9f4158f52fe6@turmel.org> Sender: linux-raid-owner@vger.kernel.org To: Phil Turmel , linux-raid@vger.kernel.org List-Id: linux-raid.ids Thanks for your answer, Phil. On 14/10/16 13:53, Phil Turmel wrote: > On 10/14/2016 06:06 AM, Ramon Hofer wrote: > >> From what I was reading [1,2] it is useful for faster rebuilds. >> But why is it not present for md[1234] as I used the same command to >> create these RAIDs? >> Is it only possible for the last of the linear devices, or is it only >> used for almost empty devices, or was the bitmap introduced in a recent >> update? > > Bitmaps have been available for a long time, but including them on a new > array by default is recent. They are most useful if a device is > unexpectedly disconnected but otherwise OK. mdadm --re-add will allow > that device to rejoin the array and only the missing data (tracked by > the bitmap) will have to be written to sync it up. What could be an unexpected disconnect? If a disc is accidentally removed from a slot? >> I wonder if I should and could add a bitmap for the other devices as well? > > There is a performance hit, so it should be considered on a case by case > basis. My uneducated guess would be that I don't really need it. So I should probably remove the bitmap for the newly created device. Although the performance hit is not really an issue in my case, I guess. How much disk space is used for a 12 TB RAID5 bitmap? Best regards, Ramon