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Old 03-26-2010, 11:35 PM
Mikulas Patocka
Default shared snapshots release 18


I released version 18 of shared snapshots. See

It contains a variable to tune cache memory consumption.

I decided to place it as a module parameter, to
/sys/module/dm_store_mikulas/parameters/dm_bufio_cache_size It is one
setting, the cache is divided fairly for all the instances. I may come up
with some sort of adaptive sharing (i.e. shrink cache for unused caches
and allow the space to be used by used)

Other options, where to place the configuration are:

*** On lvm command line, passed through dm table line (or messages):
+ could be saved in lvm2 metadata
- the cache management policy may change and require additional parameters
or drop the existing parameter, if memory management people provide us
with a callback to free cache. This leads to a situation where kernel and
userspace needs synchronization. For the same reason, vm or tcpip tunables
are exported in /proc and are not set with an userspace utility --- the
tunables change from kernel to kernel.

*** Create a directory and file in /proc
+ /proc is disorganized, so we can create what we want there
- /proc is disorganized, so people don't want to add new stuff there

*** Set it individually for each block device
+ it is understandable for the admin
- it is logicaly wrong, because dm targets don't correspond to block
devices (for example, if we have two targets in a table for one device,
how does one setting affect the targets?). dm-bufio doesn't know
anything about the device it is used for and it would need a lot of glue


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