Ted Rodriguez-Bell wrote:
The first thing we tried was just to have B mount the disks read-only and check for a flag file; if that file (we'll call it IN_USE) exists, don't do anything. If it doesn't, then A has shut down nicely and it's safe to grab the filesystem read-only.
Hrmm... Wait a minute ! Are you saying side A is using the filesystem R/W and you are attempting to mount the filesystem on side B R/O ? And *THEN* you are trying to see any modifications made on side A from side B ? .... That's very *VERY* unreliable ! First, as you noted, ext3 is a journaled filesystem.. So this means that any changes made by side A only have meaning if you can replay the log. Second (as Mark noted) - you may think you can mount the FS R/O and prevent the log from being replayed by specifying you want a non-journaled FS (aka EXT2) - which may (and will) lead to the FS being marked dirty (as Shane noted) - requiring a 'fsck' to be performed - requiring WRITE access (bad.. very bad !) Third.. If you mount the fs R/O and read data off of it - there is NO GUARANTEE - that the FS manager will reread that very same block if it is already cached.. So your view of the FS is likely going to be skewed.. Or you could be reading something completely different and unrelated... Which in the event you are trying to read an hyperblock, alloc map or other forms of meta data, could lead to a system panic if the system believes the FS has gone completely insane (which it could since it cannot relate the changes made by side A). ... ext2 and/or ext3 is (as I see it) never been designed to be used with concurrent access in mind. Personally, I would *NOT* tread in those waters (whatever the architecture !) I would strongly suggest you use a filesystem that is designed to be accessed concurrently by multiple systems (OCFS2 maybe ?) --Ivan ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
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