Good day, Device Mapper Folks.
In the format of an X Y Problem:
[Almost] Real goal (X): I would like to read storage-blocks locally from
a far-away (remote, high latency) source and keep costs (for local
storage, for network-traffic) to a minimum. 95% of the remote source
will never be read and 5% will be read frequently. If I read a
storage-block 1,000,000 times over 10 years, ideally it should only
traverse the network once during that time.
There is no mention of writes, so far.
Sub-goal (Y): Identify whether or not there is a device-mapper target
with copy-on-read, read-only behaviour. This seems like it could be
useful in certain forensic situations in which "accesses" ought to be
recorded. "We" (device-mapper users) have the "write half" of
"accesses" covered, already, with snapshots.
dm-cache might work out, except that it appears to've been designed with
considerations for writes, which are irrelevant to the goal(s).
dm-snapshot might work out, except that I'd like to copy-on-read,
instead of copy-on-write.
dm-thin appears to pass reads to the source, so doesn't seem to be
appropriate.
dm-clone doesn't keep costs low because it copies the 95% unused
storage-blocks. dm-clone without any writes happening and with
"hydration" disabled might work out, except that it appears to pass
reads to the source, with "hydration" disabled.
Mirroring doesn't keep costs low because it copies the 95% unused
storage-blocks.
Please forgive my ignorance/forgetfulness: is there already a
device-mapper target or a stack of targets which seem(s) appropriate for
the goal(s)? The idea is pretty much read-caching, which might be
argued is also available at filesystem layers, but files aren't
mentioned in the goal(s).
Thank you for your generous device-mapper efforts.
- Shao Miller
Synthetel Corporation
