From mboxrd@z Thu Jan 1 00:00:00 1970 From: NeilBrown Subject: Re: very large data-offset? Date: Wed, 06 Dec 2017 08:53:12 +1100 Message-ID: <878tegu9iv.fsf@notabene.neil.brown.name> References: <87po7vtiw2.fsf@notabene.neil.brown.name> <216131fc-ef9d-2650-18a4-d2512e8b6f81@evenson.tk> Mime-Version: 1.0 Content-Type: multipart/signed; boundary="=-=-="; micalg=pgp-sha256; protocol="application/pgp-signature" Return-path: In-Reply-To: <216131fc-ef9d-2650-18a4-d2512e8b6f81@evenson.tk> Sender: linux-raid-owner@vger.kernel.org To: Duane , linux-raid List-Id: linux-raid.ids --=-=-= Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable On Mon, Dec 04 2017, Duane wrote: > 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:=C2=A0 9.64-1-lts I don't know what that means? Maybe 4.9.64-1-lts. That's nice and recent. > mdadm:=C2=A0 mdadm - v4.0 - 2017-01-09 > action:=C2=A0 reduce the number of raid devices Ahh. Reducing the number of devices doesn't use the head-space, it used the end-space. As you reduce the size of the array when doing this, there is always lots of end-space. So I'm surprised that it would want a backup file. However without specifics (mdadm -E of devices before the reshape, and exact command given) I won't be looking into why it might. Thanks, NeilBrown >>> >>> 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 --=-=-= Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEEG8Yp69OQ2HB7X0l6Oeye3VZigbkFAlonFUkACgkQOeye3VZi gblESA//Y5Gry8EXDEoTSl2GuPSu607BsTQ5Krin7Nt+Qhb7GU6kHXwyf8c4c/BW gBK97GZpeFzs/eJSh8wuqyIS82d5aLuonr8xVMaxc92w6KWzTgL8G+TpfvRzoVN2 RtceIL5Uv/oTlvgGx8dfHYgiQbTs7sxXhfQ7+TnLgNV1mSIReOVfM86M+5PdSeUQ fkufxuapfpPa9oUu6n0CGCRMezQJ7wR134kcGpGH8nLa7A3p01EXnFcV0BJpToc+ 7vO+1GeD9RVme7XbuKfKOcax24FAVl5gNFscSTCRSZzAMQsluxMDQrO+Q2oy6XHi 9polQzsxhLUHjFXehbK0gvnYydxpV5Hgylwu6nNstpN+gDTM9MEYC7d43KWPR2b0 jHJTe6ejRKX4XGbZ5hsT53nit7GS6wGl+KlqzJsQR1H2q7B8DxlIgP3n6dHdgp/e f3Yji4p5FifshrbXOgfTnDDHR0KQYQqoQEP2QhrMEiIpckoQQPJ3A60kDKIwdV5n 8vy3pWCjWZNwbJ1/n0wkiIrHSBBlw2/cP3rdDQep7Dhi0bAFp/uDexAAj/BTiUW+ HVz2oSEPJfCUXpDOlJ+hXp6DqY1Vw8PWeS/l/oPIN4TztVLkf0yb85O0bAgbsror QPCiM4UtLFUn9WAEmFwwEZsF+EEscuoSVA0P4I4k/pcHI+qU3/Y= =Q0BJ -----END PGP SIGNATURE----- --=-=-=--