Austin S Hemmelgarn posted on Thu, 09 Oct 2014 09:18:22 -0400 as excerpted:
> On 2014-10-09 08:34, Duncan wrote: >> The only way a read-only >> mount should be writable is if it's mounted (bind-mounted or >> btrfs-subvolume-mounted) read-write elsewhere, and the write occurs to >> that mount, not the read-only mounted location. > In theory yes, but there are caveats to this, namely: > * atime updates still happen unless you have mounted the fs with noatime I've been mounting noatime for well over a decade now, exactly due to such problems. But I believe at least /some/ filesystems are truly read- only when they're mounted as such, and atime updates don't happen on them. These days I actually apply a patch that changes the default relatime to noatime, so I don't even have to have it in my mount-options. =:^) > * The superblock gets updated if there are 'any' writes Yeah. At least in theory, there shouldn't be, however. As I said, in theory, even journal replay and orphan delete shouldn't hit media, altho handling it in memory and dirtying the cache, so if the filesystem is ever remounted read-write they get written, is reasonable. > * The free space cache 'might' be updated if there are any writes Makes sense. But of course that's what I'm arguing, there shouldn't /be/ any writes. Read-only should mean exactly that, don't touch media, period. I remember at one point activating an mdraid1 degraded, read-only, just a single device of the 4-way raid1 I was running at the time, to recover data from it after the system it was running in died. The idea was don't write to the device at all, because I was still testing the new system, and in case I decided to try to reassemble the raid at some point. Read- only really NEEDS to be read-only, under such conditions. Similarly for forensic examination, of course. If there's a write, any write, it's evidence tampering. Read-only needs to MEAN read-only! -- Duncan - List replies preferred. No HTML msgs. "Every nonfree program has a lord, a master -- and if you use the program, he is your master." Richard Stallman -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
