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Re: [dm-devel] Device Mapper as a cache for SAN?



On Mon, 2006-04-10 at 19:28 -0500, Eric Van Hensbergen wrote:
> On 4/10/06, Ming Zhang <mingz ele uri edu> wrote:
> > On Mon, 2006-04-10 at 14:46 -0500, Eric Van Hensbergen wrote:
> > > On 4/10/06, Ed Wilts <ewilts ewilts org> wrote:
> > > >
> > > > The obvious question is why?  What would this provide that the kernel
> > > > and file systems don't already provide?
> > > >
> > >
> > > Well, kernel provides in-memory page cache, not local disk page cache.
> > >  There is some ongoing remote file system local disk cache work being
> > > done, but it applies primarily to AFS and NFS - not block-based
> > > transports such as iSCSI and AOE.  The various cluster file systems
> > > may provide something more akin to this, but again, this doesn't
> > > really cover SAN technologies -- not in any sort of a generic way.
> > > Let me know if I'm missing something.
> >
> > 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?
> >
> 
> Okay, I suppose I wasn't exactly clear.  What I want is something that
> will let me use a local disk (or local disks) as a cache for a storage
> area network volume (or volumes).  There are a variety of scenarios -

so a disk as cache for another (remote) disk(s).

> my primary interest is in using it for large clusters with shared
> read-only volumes or with exclusive-access read/write volumes.   So,
> local disk page cache is probably not the right terminology -- it is
> simply a local block cache for a remote block device.

yes, "page cache" has its meaning in linux.

as i know, no stable implementation for this.

> 
>        -eric


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