From mboxrd@z Thu Jan 1 00:00:00 1970 From: Doug Ledford Subject: Re: bitmap chunk size Date: Tue, 10 Nov 2009 13:01:54 -0500 Message-ID: <4AF9AA92.7030604@redhat.com> References: Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enigDF0E77F8E1BC78B2DB9DCA40" Return-path: In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: "Darius S. Naqvi" Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enigDF0E77F8E1BC78B2DB9DCA40 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable On 11/10/2009 11:39 AM, Darius S. Naqvi wrote: > Is there any possibility of having a bitmap chunk size of 512 bytes? > I know that mdadm rejects anything under 4k. I fear that the > assumption of the 4k minimum is embedded fairly strongly in the code. > Can my fear be alleviated? >=20 If you're putting any normal filesystem (with a block size of 4k) on this, then it makes absolutely no sense to have a bitmap size less than 4k as any given filesystem block is either dirty or clean, sub-block semantics make no sense in this scenario. That said, unless you have a specific need for this level of granularity, it is a really bad idea (performance wise and space wise) to go with anything even close to resembling the granularity you are requesting. I usually go with --bitmap-chunk=3D32768 (which since that's expressed in k means 32Megabytes). I would actually suspect that if you have a truly pressing need for a 512byte bitmap chunk, then you probably don't need a bare raid, you need some sort of database underlying your data or something else. --=20 Doug Ledford GPG KeyID: CFBFF194 http://people.redhat.com/dledford Infiniband specific RPMs available at http://people.redhat.com/dledford/Infiniband --------------enigDF0E77F8E1BC78B2DB9DCA40 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) iEYEARECAAYFAkr5qpIACgkQg6WylM+/8ZS3zACfXbdD8GPfy8XqHGSYDJViOepn 1tAAoIpcd8kHeJR26MaBwqJAaw42d7ND =1imn -----END PGP SIGNATURE----- --------------enigDF0E77F8E1BC78B2DB9DCA40--