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



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 -
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.

       -eric


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