User`s guide

IST-033576 D2.1.4

• Files must exist at their original paths and have permissions that per-

mit them to be opened for the original access modes. It is permissible

to use a copy of the original ﬁle (for instance when performing migra-

tion).

• Files open for reading should not be modiﬁed relative to their contents

when the checkpoint was taken in any way observable by the applica-

tion.

• Files that are open for writing which are written in an append-only

manner (regardless of the O_APPEND ﬂag) may have data appended

after the checkpoint was taken. Such data will be truncated away when

the application is restarted.

• Files that are open for both reading and writing should be restored to

exactly their state at the time of the checkpoint.

• An exception to the open ﬁles rules above exists for ﬁles already unlinked

(deleted) at the time the checkpoint is taken. When BLCR encounters such

a ﬁle, its entire contents is saved in the checkpoint context ﬁle and is later

restored as a new (also unlinked) ﬁle.

Of these requirements, BLCR is only able to verify the availability of the PIDs

and the existence and permissions of the executable, libraries and open ﬁles. Fail-

ure to satisfy those constraints will lead to an explicit failure from BLCR. Viola-

tion of the rules against modiﬁcation to any ﬁles will not be detected by BLCR

and the resulting effects on the restarted application are unpredictable.

You may restart a program on a different machine than the one it was check-

pointed on if all of these conditions are met (they often are on cluster systems,

especially if you are using a shared network ﬁlesystem), and the kernels are the

same. The restriction on executables and their shared libraries being the same can

be a problem for systems using prelinking; see the BLCR FAQ for information on

dealing with systems that prelink.

You can restart a process by using cr_restart on its context ﬁle:

% c r _ r e s t a r t c o n t e x t .1 5 0 0 5

The original process will be restored, and resume running in the exact state

it was in at checkpoint time. Note that this includes restoring its process ID, so

you cannot restart a program unless the original copy of it has exited (otherwise

cr_restart will fail with a message that the PID is already in use). You may restart

a process from a particular context ﬁle as many times as you wish. The context

ﬁle is not automatically removed at any point, so you should delete it if/when it is

no longer useful to you.

XtreemOS–Integrated Project 46/49