Re: Device Mapper as a cache for SAN?

[Ming Zhang <mingz@ele.uri.edu>]

On Tue, 2006-04-11 at 10:55 +0200, Jure Pečar wrote:
> On Mon, 10 Apr 2006 17:25:25 -0400
> Ming Zhang <mingz@ele.uri.edu> wrote:
> 
> > still, what is the main benefit of this cache? what u mean "local disk
> > page cache"? use "local disk" as page cache or page cache for "local
> > disk" or something else?
> 
> 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. 

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