From mboxrd@z Thu Jan 1 00:00:00 1970 From: NeilBrown Subject: Re: very large data-offset? Date: Mon, 04 Dec 2017 11:51:41 +1100 Message-ID: <87po7vtiw2.fsf@notabene.neil.brown.name> References: Mime-Version: 1.0 Content-Type: multipart/signed; boundary="=-=-="; micalg=pgp-sha256; protocol="application/pgp-signature" Return-path: In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: Duane , linux-raid List-Id: linux-raid.ids --=-=-= Content-Type: text/plain Content-Transfer-Encoding: quoted-printable On Thu, Nov 30 2017, Duane wrote: > Why is the data offset set so big? I created a 3x4TB RAID5 array and the= =20 > 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=20 > offset need only be at most the chunk size plus the space (1 sector) for= =20 > 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=20 > don't see that mdadm ever uses the space. Citation needed.... what version of mdadm, what kernel? What reshape command? > > > I tried making some test arrays and got much smaller sizes. A 3x1GB=20 > 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=20 > setting the data offset to around 2MB? Only that it might reduce your options in the future, though probably not by much. NeilBrown --=-=-= Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEEG8Yp69OQ2HB7X0l6Oeye3VZigbkFAloknB0ACgkQOeye3VZi gblzKQ/+P65rCrOXYjdZb07kUBzCdGUYHah7wa5WEqU6g9zLLnrLTQJuVlES0/Cr g1L4JmT+gn1mpYgtVdyyXLUz5H3iGVJ6BhoUpWpOLg5k3fxWDrwZJZdrxQ75rL0q cWqNmTvgRNQV9l68LKdbYLjQFgyCXvQ9seJad//r18OJJlakCDwerKmAaJfONowd KFCHCd8hFNxAyk+gNUEmljj6sadBZEtrOP4DxmU0xUtan/2uWTA9JhB5Nyth+6yK SM3RoiFNQiiqghMbzGeOsejb2mo+Y2lxp7YcCvikmaox/OLrnZduBkCkZthD1HcF gdg8Ec+uCsHtVxlziWZk2OYlDc4IbK5tv/StIgzGCaanIKnhopwKkwhnvttryImJ vZhrb0ZyVPoq5n2UkBR3r1JfH78keX9trFP5sMts73o7lb/nJgpYm0YJmdo7yta2 pfIy19rzT0oKSO/iHAVzrMcmSG3Fjv00Fb0Ib7WQnL1xMTOgUqMVjHSL1urqwNDL FJ4VKsDSR7PLUkUVa8Q0SXkdVED/FF44q/SnZ+6E7G5wHOUUi9+hntOoAjvxr2rC nOKiEZVLYNVuhra7R0AP3DC/ZkHOoqXxiK4e692lx/KKGcu8675wrcCpB3fUufmt lfLBDbClK+zyMMH2FswFKUF3+92/Sr3Fn9Ws2jyLhhLaUKa0aJ8= =Apg8 -----END PGP SIGNATURE----- --=-=-=--