[dm-devel] Shared Memory Pages for Same Base Device

[Shao Miller]

On 9/22/2014 9:08 AM, Mike Snitzer wrote:
>
> Unfortunately device-mapper thin provisioning doesn't offer shared
> pagecache pages across snapshot volumes.  This is a block layer
> limitation (the block layer doesn't allow pages to be shared across
> block devices, and dm-thinp snapshot volumes are each a block device).
> Modifying the VM, block and DM subsystems to provide this capability is
> not an easy task and as such is really not a near-term priority.

I sincerely appreciate your response, Mike.  I figured as much, but 
hopefully this thread will be useful to future searchers.

> Interestingly BTRFS does _not_ offer this page sharing either.  I'm told
> that the only emerging solution for this is overlayfs.
This is very surprising.  I've just done some testing and /proc/PID/maps 
for the same binary from two different snapshots deriving from the same 
base do show the same block-device major and minor, with the same 
offset.  Do you happen to have a reference for that detail about BTRFS, 
or perhaps an idea for the right venue to learn more about that?  
Unfortunately, I guess it's off-topic for device-mapper.

Also unfortunately, it seems that overlayfs isn't mainline Linux nor 
out-of-the-box for some distributions.  (Like CentOS 7.)

I enjoy device-mapper for CoW iSCSI and AoE.  Keep up the great work, folks!

-- 

Shao Miller
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