linux-ext4 - Aw: Re: Ext4: Slow performance on first write after mount

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Date:	Tue, 21 May 2013 20:02:36 +0200 (CEST)
From:	frankcmoeller@...or.de
To:	adilger@...ger.ca
Cc:	linux-ext4@...r.kernel.org
Subject: Aw: Re: Ext4: Slow performance on first write after mount

Hi Andreas,

only a short question: 

> I like the idea of keeping the high bits of the buddy bitmap in the group
> descriptor, instead of just the largest free order. It takes the same
> amount of space, but provides more information. 
More informations for what? The allocator or better the good_group function
needs bb_largest_free_order and in some cases fragment count. Do you 
want to use the bitmap for a not 100% correct fragment count calculation? 
Or is there another use for it?

Best regards,
Frank

> 
> > On Sun, 19 May 2013 21:36:02 +0200 (CEST) Frank C Moeller wrote:
> >> From my point (end user) I would prefer a builtin solution. I'm also a
> >> programmer and I can therefore understand why you don't want to change
> >> anything.
> > 
> > It's not that I don't want to change anything, it's that I'm very
> > hesitant to add new mount options or new code paths that now need more
> > testing unless there's no other way of addressing a particular use
> > case.  Another consideration is how to do it in such a way that it
> > doesn't degrade other users' performance.
> > 
> > Issuing readahead requests for the bitmap blocks might be good
> > compromise; since they are readahead requests, as low priority
> > requests they won't interfere with anything else going on, and in
> > practice, unless you are starting your video recording **immediately**
> > after the reboot, it should address your concern.
> 
> Right. Some of our users do something similar in userspace to avoid
> slowdown on first write, which doesn't _usually_ happen immediately
> after mount, but this isn't always helpful. 
> 
> >  (Also note that for
> > most people willing to hack a DVR, adding a line to /etc/rc.local is
> > usually considered easier than building a new kernel from sources and
> > then after making file system format changes, requiring a reformat of
> > their data disk!)
> 
> I think storing the buddy bitmap top bits in the GDT could be a COMPAT
> feature.  It is just a hint that could be ignored or incorrect, since
> the actual bitmap would be authoritative. 
> 
> Cheers, Andreas
> 
> > So it's not that I'm against solutions that involve kernel changes or
> > file system format changes.  It's just that I want to make sure we
> > explore the entire solution space, since there are costs in terms of
> > testing costs, the need to do a backup-reformat-restore pass, etc,
> > etc., to some of the solutions that have been suggested so far.
> > 
> > Regards,
> > 
> >                        - Ted
> 
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