From mboxrd@z Thu Jan 1 00:00:00 1970 From: Duane Subject: Re: very large data-offset? Date: Mon, 4 Dec 2017 20:33:07 -0700 Message-ID: <216131fc-ef9d-2650-18a4-d2512e8b6f81@evenson.tk> References: <87po7vtiw2.fsf@notabene.neil.brown.name> Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="------------78D917A462A34623BECE67B4" Return-path: In-Reply-To: <87po7vtiw2.fsf@notabene.neil.brown.name> Content-Language: en-CA Sender: linux-raid-owner@vger.kernel.org To: linux-raid List-Id: linux-raid.ids This is a multi-part message in MIME format. --------------78D917A462A34623BECE67B4 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit On 2017-12-03 05:51 PM, NeilBrown wrote: > On Thu, Nov 30 2017, Duane wrote: > >> Why is the data offset set so big? I created a 3x4TB RAID5 array and the >> data offset was 128MB. Chunk size was the default 512kB. > It is less than 0.1% of the device... > >> I cannot see why such a large offset is used. I would think the data >> offset need only be at most the chunk size plus the space (1 sector) for >> the superblock and bitmap. > It is insurance. If you want to change the chunksize later, having a > lot of head-room will allow the reshape to go much faster. > >> When reshaping the array, I am prompted to use an external file, so I >> don't see that mdadm ever uses the space. > Citation needed.... what version of mdadm, what kernel? What reshape > command? kernel:  9.64-1-lts mdadm:  mdadm - v4.0 - 2017-01-09 action:  reduce the number of raid devices >> >> I tried making some test arrays and got much smaller sizes. A 3x1GB >> RAID5 array with 64k chunks had a 1MB data offset. >> >> >> If I make a 7x4TB RAID5 array with 64kB chunks, is there a problem with >> setting the data offset to around 2MB? > Only that it might reduce your options in the future, though probably > not by much. > > NeilBrown --------------78D917A462A34623BECE67B4 Content-Type: text/x-vcard; name="duane.vcf" Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename="duane.vcf" bnVsbA== --------------78D917A462A34623BECE67B4--