Re: [Devel] Re: [PATCH 2.6.24-rc8-mm1 09/15] (RFC) IPC: new kernel API to change an ID

[Pavel Emelyanov <xemul@openvz.org>]

Daniel Lezcano wrote:
> Pavel Emelyanov wrote:
>> Kirill Korotaev wrote:
>>> Cedric Le Goater wrote:
>>>> Hello Kirill !
>>>>
>>>> Kirill Korotaev wrote:
>>>>> Pierre,
>>>>>
>>>>> my point is that after you've added interface "set IPCID", you'll need
>>>>> more and more for checkpointing:
>>>>> - "create/setup conntrack" (otherwise connections get dropped),
>>>>> - "set task start time" (needed for Oracle checkpointing BTW),
>>>>> - "set some statistics counters (e.g. networking or taskstats)"
>>>>> - "restore inotify"
>>>>> and so on and so forth.
>>>> right. we know that we will have to handle a lot of these
>>>> and more and we will need an API for it :) so how should we handle it ?
>>>> through a dedicated syscall that would be able to checkpoint and/or
>>>> restart a process, an ipc object, an ipc namespace, a full container ?
>>>> will it take a fd or a big binary blob ?  
>>>> I personally really liked Pavel idea's of filesystem. but we dropped the
>>>> thread.
>>> Imho having a file system interface means having all its problems.
>>> Imagine you have some information about tasks exported with a file system interface.
>>> Obviously to collect the information you have to hold some spinlock like tasklist_lock or similar.
>>> Obviously, you have to drop the lock between sys_read() syscalls.
>>> So interface gets much more complicated - you have to rescan the objects and somehow find the place where
>>> you stopped previous read. Or you have to to force reader to read everything at once.
>> To remember the place when we stopped previous read we have a "pos" counter
>> on the struct file.
>>
>> Actually, tar utility, that I propose to perform the most simple migration
>> reads the directory contents with 4Kb buffer - that's enough for ~500 tasks.
>>
>> Besides, is this a real problem for a frozen container?
> 
> I like the idea of a C/R filesystem. Does it implies a specific user 
> space program to orchestrate the checkpoint/restart of the different 
> subsystems ? I mean the checkpoint is easy but what about the restart ? 

I though about smth like "writing to this fs causes restore process".

> We must ensure, for example to restore a process before restoring the fd 
> associated to it, or restore a deleted file before restoring the fd 

This is achieved by tar automatically - it extracts files in the order
of archiving. Thus is we provide them in correct order we'll get them
in correct one as well.

> opened to it, no ?
> 
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