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!
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