linux-ext4 - Re: ext4: first write to large ext3 filesystem takes 96 seconds

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Message-ID: <20140708001655.GI8254@thunk.org>
Date:	Mon, 7 Jul 2014 20:16:55 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Benjamin LaHaise <bcrl@...ck.org>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: ext4: first write to large ext3 filesystem takes 96 seconds

On Mon, Jul 07, 2014 at 05:13:49PM -0400, Benjamin LaHaise wrote:s
> Hi folks,
> 
> I've just ran into a bug with the ext4 codebase in 3.4.91 that doesn't seem 
> to exist in ext3, and was wondering if anyone has encountered this before.  
> I have a 7.4TB ext3 filesystem that has been filled with 1.8TB of data.  
> When this filesystem is freshly mounted, the first write to the filesystem 
> takes a whopping 96 seconds to complete, during which time the system is 
> reading about 1000 blocks per second.  Subsequent writes are much quicker.  
> The problem seems to be that ext4 is loading all of the bitmaps on the 
> filesystem before the first write proceeds.  The backtrace looks roughly as 
> follows:

So the issue is that ext3 will just allocate the first free block it
can find, even if it is a single free block in block group #1001,
followed by a single free block in block group #2002.  Ext4 tries a
harder to find contiguous blocks.

If you are using an ext3 file system format, the block allocation
bitmaps are scattered across the entire file system, so we end up
doing a lot random 4k seeks.

We can try to be a bit smarter about how we try to search the file
system for free blocks.

Out of curiosity, can you send me a copy of the contents of:

/proc/fs/ext4/dm-XX/mb_groups

Thanks!!

					- Ted
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