[DRBD-user] loopback devices and DRBD w/Hearbeat

Florian Haas florian.haas at linbit.com
Fri May 6 09:41:44 CEST 2011

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.


On 2011-05-05 21:47, Jeff Humby wrote:
> Has anyone achieved success with loopback devices? Any suggestions

Suppose you have a filesystem on top of DRBD and that DRBD is a
loop device in another filesystem, and you're getting memory pressure.

Any process: Memory, this pid whatever. Request a page.
Memory allocator (malloc): pid whatever, copy, stand by.
malloc: kernel threads, this is malloc, perform writeout.
Filesystem: roger that malloc, stand by.
Filesystem: DRBD, write this page.
DRBD: 10-4, writing page (no need to talk to malloc, we have a mempool).
Stand by.
DRBD: loop driver, here's a page, write it.
Loop driver: Roger that DRBD, processing.
Loop driver: filesystem, here's a page you need to write
Filesystem: Roger that loop.
Filesystem: malloc, I need some memory.
malloc: copy that Filesystem, stand by until we free some pages.
Filesystem: malloc, are you telling me to bugger off? You just caused a
writeout! Do you expect pages to be freed magically?
malloc: Affirmative.
Filesystem: Damn.
Loop driver: Damn.
DRBD: Damn.
Process which requested page: Damn.

See my point?

Don't put DRBD on loop devices.

Cheers,
Florian

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