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

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Message-Id: <5FF1E777-4701-4F9E-A2C2-48092510428C@dilger.ca>
Date:	Tue, 21 May 2013 18:27:13 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	frankcmoeller@...or.de
Cc:	linux-ext4@...r.kernel.org
Subject: Re: Aw: Re: Ext4: Slow performance on first write after mount

On 2013-05-21, at 12:02 PM, frankcmoeller@...or.de wrote:
>> 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?

Sorry, what I meant to write was that it provides more information
than just recording e.g. the number of blocks in the largest free
extent.

> 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?

The bitmap would provide the largest_free_order value directly
(assuming it is at least 4MB in size).

Cheers, Andreas

>>> 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
>> 


Cheers, Andreas





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