linux-ext4 - [Bug 103111] auto_da_alloc mount option not working

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Date:	Tue, 25 Aug 2015 18:05:08 +0000
From:	bugzilla-daemon@...zilla.kernel.org
To:	linux-ext4@...r.kernel.org
Subject: [Bug 103111] auto_da_alloc mount option not working

https://bugzilla.kernel.org/show_bug.cgi?id=103111

--- Comment #11 from Eric Sandeen <sandeen@...hat.com> ---
IOWS, it handles these two cases:

fd = open("foo.new")
write(fd,..)
close(fd)
rename("foo.new", "foo") // syncs out foo.new if it has delalloc blocks

and

fd = open("foo", O_TRUNC)
write(fd,..)
close(fd) // syncs out foo if "foo" had blocks prior to the O_TRUNC truncate

Your testcase does this if foo.new doesn't already exist:

fd = open("foo.new", O_TRUNC) // if foo.new has no blocks, O_TRUNC does nothing
rename("foo.new", "foo") // foo.new has no delalloc blocks, does nothing
write(fd)
close(fd) 

If "foo.new" does exist, 

fd = open("foo.new", O_TRUNC) // if foo.new has blocks, sets da_alloc flag
rename("foo.new", "foo") // foo.new has no delalloc blocks, does nothing
write(fd)
close(fd) // syncs out the data

IOWS, if example.txt starts with allocated blocks, this:

# rm example.txt1
# echo foobar > example.txt
# sync
# ./testcase

works as you hope, because testcase does:

fd = open("example.txt", O_TRUNC) // example.txt has blocks, sets da_alloc flag
rename("example.txt", "example.txt1") // "example.txt" no has delalloc blocks
nothing happens
write(fd) // now we have delalloc blocks
close(fd) // syncs out the data


So it's not that the heuristic is broken; your testcase just doesn't
necessarily meet the conditions of the heuristic.

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