[Tmem-devel] [Xen-devel] Queries on Filesystem Support

[Dan Magenheimer]

Hi Ashwin --

Yes, the Linux-side tmem support is specific
to certain Linux filesystems.  For an explanation
of this, see FAQ #2 in:

http://lkml.org/lkml/2010/9/3/383 

Vasiliy is correct that ext3, ext4, btrfs, and ocfs2
are currently supported.  Other filesystems should
be easy to add, but must be extensively tested.
The xfs filesystem would be next, but I haven't
tried it.

As you know, tmem supports ephemeral pages and
peristent pages.  In Linux these are called
"cleancache" and "frontswap" (as the Linux patches
are layered so they can be used for functionality
other than tmem).  Cleancache works for any clean
pages that are "evicted" by Linux, so this may
be code or data pages, but only "mapped" pages
(i.e. pages that are obtained from disk or match
pages that have been written to disk).

If many VMs on the same host share a clustered
filesystem, the entire ephemeral pool corresponding
to that filesystem is shared.  If the VMs do not
share a clustered filesystem and "tmem-dedup" is
enabled, identical pages are deduplicated.  Since
ephemeral pages in tmem are essentially read-only,
this is different from classic "transparent page
sharing".

Thanks,
Dan

> -----Original Message-----
> From: ash win [mailto:ash_win012006 at yahoo.com]
> Sent: Saturday, September 11, 2010 1:45 PM
> To: tmem-devel at oss.oracle.com
> Cc: Kurt Hackel; Dan Magenheimer; Xen-devel at lists.xensource.com; Chris
> Mason; Dave Mccracken
> Subject: [Xen-devel] Queries on Filesystem Support
> 
> I read this paper "Transcendent Memoryand Linux" by Dan
> Magenheimer,Chris Mason,Dave McCracken,Kurt Hackel.
> After reading this ,I think that tmem implementation is specific to
> filesystem as if changes are also done in fs/.
> which are the filesystems supported by tmem? and which are the
> upcoming filesystems getting tmem support ?
> So does it shares only data segment and not the code segment?as if
> many VMs of same type running on same host can also share Code-segment-
> pages.
> Let me correct if I am wrong.
> 
> --
> With Regards,
> Ashwin Vasani
> B.E. (Fourth Year)
> Computer  Engineering,
> Pune Institute of Computer Technology.
> +91 9960405802
> 
> 
> 
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