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From: bugzilla-daemon@bugzilla.kernel.org
Subject: [Bug 16223] New: ext4 in data=journal mode does not support delayed
allocation
Date: Wed, 16 Jun 2010 00:18:02 GMT
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https://bugzilla.kernel.org/show_bug.cgi?id=16223
Summary: ext4 in data=journal mode does not support delayed
allocation
Product: File System
Version: 2.5
Kernel Version: 2.6.34
Platform: All
OS/Version: Linux
Tree: Mainline
Status: NEW
Severity: normal
Priority: P1
Component: ext4
AssignedTo: fs_ext4@kernel-bugs.osdl.org
ReportedBy: theosib@gmail.com
Regression: No
Perhaps you could argue that someone using data=journal mode doesn't care about
write performance, but that would not necessarily be the case. It would be
really nice if I could use ext4 with data journaling AND get delayed
allocation. I'm just mentioning this because I'm probably not the only one out
there with this feeling. In fact, I know I'm not because I've seen the
question asked in forums.
I suspect it might be nontrivial to modify the journaling system so that you
can write data to the journal that has not yet been allocated a location on the
disk, but it seems to me that's basically what you'd have to do. (Otherwise it
wouldn't be DELAYED allocation, and we need data to hit the journal before it
hits the fs proper.) Data would be written to the journal, then when it's
going to be committed to the fs, it would be allocated (and that fact added to
the journal?). If the system goes down, and a journal replay is required, then
the data would be allocated space at that time. One thing to be concerned
about is having an allocation happen, the data not written to where it's
allocated, and the allocation not logged in the journal--on replay, you might
allocate yet more space for the data, and you'd need some means to recover the
first allocation that's now bogus.
Thanks.
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