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

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Date:	Mon, 7 Jul 2014 21:35:11 -0400
From:	Benjamin LaHaise <bcrl@...ck.org>
To:	Theodore Ts'o <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: ext4: first write to large ext3 filesystem takes 96 seconds

Hi Ted,

On Mon, Jul 07, 2014 at 08:16:55PM -0400, Theodore Ts'o wrote:
> 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.

Yeah, we're kinda stuck with ext3 on disk for now due to a bunch of reasons.  
The main reason for using the ext4 codebase instead of ext3 has mostly to do 
with slightly better performance for some metadata intensive operations 
(like unlink and sync writes).

> 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

Sure -- I put a copy at http://www.kvack.org/~bcrl/mb_groups as it's a bit 
too big for the mailing list.  The filesystem in question has a couple of 
11GB files on it, with the remainder of the space being taken up by files 
7200016 bytes in size.  Cheers,

		-ben

> Thanks!!
> 
> 					- Ted

-- 
"Thought is the essence of where you are now."
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