RE: Replicating directories - Intercepting write/modify system calls

["Siddhartha Jain" <sid@netmagicsolutions.com>]

Hey Joseph,

>Too complicated.  Too invasive.

In what way? Will it slow down the FS too much? What other approach can be adopted to keep the implementation FS-independant and still track the changes in real-time.

>Replicating the entire partition rather than just a specific directory is more feasible.

How would I do that? Will the implementation still be FS-independant?

>Does it need to be replicated in real-time, or would an incremental synchronization be acceptable (i.e. synced every 15 >>minutes or so)?

Synced every fifteen minutes is ok but anything that scans the whole partition/directory file by file would be nighmarish given that the mail toasters I am mirroring have tens of thousands of files. Therefore, IMHO, the solution needs to track changes as and when they happen to the file and mirror them immiediately.

For eg, a rsync/mirrordir takes upto half-an-hour to sync about 100,000 files. Not to mention the 40% CPU load on the file server while the rsync runs (a host mounts the NFS folder from a NetApp filer and rsyncs it to a local dir). This is just one server. If I were to put all my 6 mail toasters on the filer and mirror the filer's main volume to a host, I guess either would croak.

>Does it need to be a two-way synchronization, or is a one-way synchronization acceptable?  In other words, do changes on the >replica need to be synced back to the original?

For my purpose, one-way is fine. But two-way would be cool and in the greater good of humankind ;)

>Are you willing to commission someone (i.e. pay them money) to do this work? 8-D

Yep, I will give you a free mail account on my mail servers and if the code is really good, maybe a rksh account too :P

Btw, I am willing to do all the dirty work of wrestling with C code and testing but I need guidance.

Sorry, if my questions sound too un-informed or stupid. I feel like a mere human roaming amongst gods of FS. :)

Siddhartha Jain (CISSP)


> -----Original Message-----
> From: Siddhartha Jain [mailto:sid@netmagicsolutions.com]
> Sent: Thursday, February 12, 2004 6:39 AM
> To: linux-fsdevel@vger.kernel.org
> Subject: Replicating directories - Intercepting write/modify system calls
> 
> Hello,
> 
> I need a module/tool to replicate directories locally or across a network.
> Locally, the master and the mirror directories may not belong to the same
> FS. My immiediate need is to replicate a NFS-mounted directory to another
> local/NFS directory. So here is what I figured:
> 
> 1. A config file contains the master and mirror paths
> 2. Expand the vnode structure to include two things: a bit-flag to
> indicate
> that the vnode has a replica and a pointer to the replica vnode.
> 3. When an open() is called,
> 	a. check if the read/modify option is set in the open() (I am not
> interested in replicating access-time at the moment).
> 	b. If yes, check the path of the file against the known masters in
> the
> config.
> 	c. If it matches, copy the file to a destination specified by the
> mirror's
> path.
> 	d. Set the flag in the master vnode for replica and place a pointer
> to the
> replica vnode in the master's structure.
> 	e. After this replicate all write() calls for the master vnode to
> the
> replica vnode.
> 	f. close() the replica vnode when the master is close()-ed.
> 
> 
> Being a sysadmin and not really a C/Kernel programmer, I do not know how
> much is this feasible and practical.
> 
> Given that it is ok, how do I avoid the copy at open() and do only
> incremental changes everytime a file is closed and re-opened.
> 
> Regards,
> 
> Siddhartha
> 
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