From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ming Zhang Subject: Re: Device Mapper as a cache for SAN? Date: Tue, 11 Apr 2006 08:57:39 -0400 Message-ID: <1144760259.26235.9.camel@localhost.localdomain> References: <20060410184531.GA12908@www.ewilts.org> <1144704325.8591.170.camel@localhost.localdomain> <20060411105552.3f32c4ef.pegasus@nerv.eu.org> Reply-To: mingz@ele.uri.edu, device-mapper development Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: <20060411105552.3f32c4ef.pegasus@nerv.eu.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com To: device-mapper development List-Id: dm-devel.ids On Tue, 2006-04-11 at 10:55 +0200, Jure Pe=C4=8Dar wrote: > On Mon, 10 Apr 2006 17:25:25 -0400 > Ming Zhang wrote: >=20 > > still, what is the main benefit of this cache? what u mean "local dis= k > > page cache"? use "local disk" as page cache or page cache for "local > > disk" or something else? >=20 > I recently did some tests with ext3 data journaling and discovered that > large journals eat almost as much system memory as its size, which is a > bit crazy. Some might argue that configuring 4gb journal is crazy too, > but there are scenarios where this might be beneficial (think large > mail servers with gazillion small writes, which journal turns into one > big sequential write). So if something like external journal can be > implemented in dm, fs-agnostic and not as memory hungry, I'd be very > interested. even u journal is a big sequential write, u data write are not. so the combined performance will not have big difference.=20 u mean u journal contain everything? including log and data? you might want to ask ext3 list and ask them why take so many memory with a 4GB journal. Also i think journal will write to disk very fast, so after written to disk, the ram can be reclaimed. also i do not see how a DM disk cache can help you on this. ming