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(New list member, just subscribed. Apologies if this
is not the right list for this question.) Is there a way to see the updated contents of a DM target’s
underlying device while the DM target is in use? I Googled and searched
in documentation and existing code for an answer but was not able to find it. Background: I’m debugging a DM target that does
change-tracking. It’s a combination of code from dm-linear, Alberto
Bertogli’s dm-csum, dm-delay, and some original code. (The project
architect does intend to properly release it under GPL per the license of the incorporated
GPL pieces.) The target reserves an early portion of the underlying
device and exposes/creates/supports a device that is a linear mapping of the
rest of the underlying device. The idea is each write to the exposed
device sets a corresponding bit in the reserved portion, using the write barrier
trick from dm-csum’s ‘same’ mode, to indicate which parts of
the exposed device have been modified and which have not. (I had earlier
tried to use the userspace dirty-logging facility but found it to not meet our
needs.) The target is intended to be used with LVM snapshots to do
block-device-level backups. (I’m aware of filesystem-level issues
with that, but the requirement is the requirement.) The underlying device
for the DM target will be an LVM logical volume. The backup manager will
send a dmsetup message to the DM target to hold all writes (and maybe reads) on
indefinite delay, create an LVM snapshot of the DM target’s underlying
device, then flush the held bios and resume normal operation. Problem: The current problem I have is many updates are not visible
in the DM target’s underlying device until I do a ‘dmsetup remove’.
When I look at the underlying device (an LVM LV) prior to ‘dmsetup remove’,
it’s not reliable whether I see what has been written by the DM
target. I have verified that the bios go out and the callbacks are
called. Any suggestions toward being able to reliably see (as in
copy out) data from the underlying device or making an LVM snapshot of it while
my change-tracking DM target module is still in use? Thanks, Robert Riches Azad Consultant at Intel |
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