Perhaps "gracefully" wasn't quite the right word. I've also noticed that ext3 complains when running off of a read-only disk. But other than spitting out a lot of error/warning messages, it does exactly what it should.
Mostly, I consider this useful for cloning/recovery kinds of problems where the disk is normally used read-write but you want to access it in a completely read-only manner from another guest. If the disk has static contents that are only written once and can be handled read-only thereafter, ext2 would yield a bit of a space and performance improvement by not trying to maintain a journal that you don't need. On Wed, Aug 25, 2010 at 11:08 AM, Richard Troth <[email protected]> wrote: > I have always expected the fall-back behavior of EXT3/2, but recently > have not seen it. To be specific, the last time I tried to mount an > EXT3 RO, it insisted on trying journal replay. When I marked the disk > RO in the block driver, still got attempted replay. (And I/O errors > to match the failing write-back of the replay.) When I tried to force > mounting as EXT2 (with a "-t ext2"), it still played journal. That > was last week. I think the kernel was 2.6.22 which should have a > mature EXT3. > > So ... I'm not arguing about what EXT3 is supposed to do, but ... my > experience does not match my expectation. I therefore recommend EXT2 > for any shared RO disk or volume or LUN or partition. (Unless you > want to go with ISO-9660, which also works.) > > -- R; <>< > > > > > > On Wed, Aug 25, 2010 at 08:26, Christian Paro <[email protected]> > wrote: > > Difference is that ext3 is prepared to fall back to ext2 functionality > when > > being read from a read-only device - which makes perfect sense, given > that > > the journal's purpose is to protect the integrity of writes and (as Mark > > pointed out) it is of no value on a disk which will never again be > written > > to. > > > > ReiserFS, not recognizing that the journal is only relevant when it is > > possible to write to the disk, doesn't gracefully fall back to a > > journal-less version of itself the way ext3 does. > > > > As for ext2 vs ext3 for the read-only file system, there is a question of > > whether it's a disk which is to be written to once and then linked to and > > mounted read-only by everyone (in which case ext2 makes the most sense, > with > > ext3 adding nothing but unnecessary overhead), or a disk which is > regularly > > written to by one system - which you for some reason want be able to link > to > > and mount read-only from another (in which case the ext3 journal does > still > > serve a purpose while the disk is being accessed in write mode by that > first > > system). > > > > On Wed, Aug 25, 2010 at 8:07 AM, Shane <[email protected]> wrote: > > > >> On Tue, 2010-08-24 at 11:00 -0400, Richard Troth wrote: > >> > >> > As others have said, ReiserFS wants to replay the journal. EXT3 does > >> > too. (Not sure about EXT4.) So for RO I recommend EXT2. > >> > >> Whilst I'm prepared to believe anything of reiserfs (not having > >> attempted to use it since before it was named thus), I query the > >> assertion re ext3. Were it to be mounted r/o everywhere, I fail to see > >> how the filesystem could have meta-data that would need replaying. > >> But I admit to not having specifically tested that - maybe someday. > >> > >> Shane ... > >> > >> ---------------------------------------------------------------------- > >> 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 > >> ---------------------------------------------------------------------- > >> For more information on Linux on System z, visit > >> http://wiki.linuxvm.org/ > >> > > > > ---------------------------------------------------------------------- > > 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 > > ---------------------------------------------------------------------- > > For more information on Linux on System z, visit > > http://wiki.linuxvm.org/ > > > > ---------------------------------------------------------------------- > 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 > ---------------------------------------------------------------------- > For more information on Linux on System z, visit > http://wiki.linuxvm.org/ > ---------------------------------------------------------------------- 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 ---------------------------------------------------------------------- For more information on Linux on System z, visit http://wiki.linuxvm.org/
