Dne 12. 11. 24 v 20:40 Shao Miller napsal(a):
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).
Hi
dm-cache should be likely your 'favorite' since it's designed to handle
different policies - for some reason not too many policies were ever written
(maybe existing one is good enough for most to bother), but the idea behind
was to be modular and handle different need of users - if they simply 'know
more' upfront then the current 'heuristic' smq policy might ever learn from
observing IO traffic within cached device.
So I'd highly recommend to read about those policies and eventually write a
better one for you specific case (like instantly promote any blocks until
cache is 100% full)
Although you should probably try to use the existing one first -and you might
be surprised how good it will get over the time in picking your hot spots of
your disks - although there might be some 'extra' reads from your origin
device before the cache considers a block to be 'promoted' to cache.
As for your 'write' concerns - they are actually irrelevant itself - since if
there is no write - there is no need to maintain the place for it....
Regards
Zdenek
