From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from frost.carfax.org.uk ([85.119.82.111]:37283 "EHLO frost.carfax.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753300Ab3A3KCE (ORCPT ); Wed, 30 Jan 2013 05:02:04 -0500 Date: Wed, 30 Jan 2013 10:02:01 +0000 From: Hugo Mills To: Roger Binns Cc: linux-btrfs@vger.kernel.org Subject: Re: RAID 0 across SSD and HDD Message-ID: <20130130100201.GN16285@carfax.org.uk> References: MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="nRwNdQxTdQ7rZk9A" In-Reply-To: Sender: linux-btrfs-owner@vger.kernel.org List-ID: --nRwNdQxTdQ7rZk9A Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Wed, Jan 30, 2013 at 01:27:37AM -0800, Roger Binns wrote: > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > I've been unable to find anything definitive about what happens if I use > RAID0 to join an SSD and HDD together with respect to performance > (latency, throughput). The future is obvious (hot data tracking, using > most appropriate device for the data, data migration). > > In my specific case I have a 250GB SSD and a 500GB HDD, and about 250GB of > files (constantly growing). One message I saw said that new blocks are > allocated on the device with the most free space which implies the SSD > would be virtually unused in my case, except for metadata which would only > be used half the time. That would be the case with "single" mode, not with RAID-0. With RAID-0, you'd get data striped equally across all (in this case, both) the devices, up to the size of the second-largest one, at which point it'll stop allocating space. > At the moment I have two independent filesystems (one per device) and > manually move data files between them using symlinks to keep pathnames the > same. This requires keeping lots of slop free space on the SSD as well as > administration whenever it runs out of space. > > My hope would be overall performance between that of the two devices, and > closer to that of the SSD. We don't have any kind of hot-data management yet, but it's on the list of things we'd like to have at some point. Hugo. -- === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === PGP key: 515C238D from wwwkeys.eu.pgp.net or http://www.carfax.org.uk --- Some days, it's just not worth gnawing through the straps. --- --nRwNdQxTdQ7rZk9A Content-Type: application/pgp-signature; name="signature.asc" Content-Description: Digital signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (GNU/Linux) iQIVAwUBUQjvmb9z9OVl50rAAQLNUhAAnA96XGL75BQ7UkCLFqrjI9IlxciQ9UNG psm5IWaiBdgFaOT/ygOfT6VJjH8RbYNU93P1NUyQO4e/rSyEjHqZkxjkUUtLCTZq nd3u9+btwvTeP8mZtNLiqb4ISUq7/O1rGeqZy/0l/xminOLMDfpZmJ1vRRgAFwBj CUTDM2QimPejunI2du/s4dHLmcIrFe3u9A6TqkKDV6t//xm0tb3MAMVfpWbPlKnI b0IILPfM57/gb/piT7DYp81k6nF+j/OKkoMrUBUfcZrmE5cBq6CiZF1dXFOSfTd2 zv0EN5M7zA5SVMDtA5Jiz4tAzlIGSvsVwXu9rgZ7FQDy6ol7Dqk72qyLKft+DIdI TojudMtykLipenWFYangnYoAQH1idcdrQuaC2kbsZ0ism+XSWWOuQuyz2R8oICsc Cxx/7fEKYhoLVXE7N/7xUIP66Td4qdfW0Q0D6ZWdDtXTGDsTs/P77uA73/Ur+ZkQ 2luqQplaC5ghezDMbWzVHGUxBnUqch/Wr2tx78ZJQ6Eq3QRQ9pqPTP0VbV890MCM B1SKTO1QsGiVzl1Ktq7ckyNdYNWVEcFX00Fe9r2kLcaDSDl7mptybymgKYQx0m8j V9qGXcRuoRjFC8QlpWuOK5hJ51jCs0ipRqcvQ0zOqvGFuUCf/0v/Xlq/yh9qZolc L4HqlvOfGEg= =xSHw -----END PGP SIGNATURE----- --nRwNdQxTdQ7rZk9A--