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

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Message-ID: <bug-103111-13602-8MQRLFu2PJ@https.bugzilla.kernel.org/>
Date:	Tue, 25 Aug 2015 17:49:33 +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 #10 from Eric Sandeen <sandeen@...hat.com> ---
Ok, so there are 2 basic heuristics here.

One is that if we call ext4_truncate to size 0, we set the AUTO_DA_ALLOC flag
so that it'll call ext4_alloc_da_blocks in ext4_release_file (essentially on
close).

The other is that if we call rename, and we're overwriting an existing file, we
call ext4_alloc_da_blocks.

ext4_alloc_da_blocks will start writeback on the file (i.e. the file which was
truncated, or the new file overwriting the old file) if there are any delayed
allocations still pending; if not, it does nothing.

Note, we don't get to ext4_truncate if the file is already zero length when you
open it O_TRUNC.

Also, notice that if we strace your c++ program (with the rename call
included), we see:

open("example.txt", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
rename("example.txt", "example.txt1")   = 0
write(3, "Writing this to a file.\n", 24) = 24
close(3)                                = 0

so the rename happens before the write; even if that is overwriting an existing
file, there are no delalloc blocks on the new file yet, so the rename heuristic
does nothing in this case.

So there are a few prerequisites to make your c++ test work

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