Date:	Fri, 11 Nov 2011 23:32:46 +0800
From:	Zheng Liu <gnehzuil.liu@...il.com>
To:	Steven Whitehouse <swhiteho@...hat.com>
Cc:	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2 0/8] Filesystem io types statistic

On Fri, Nov 11, 2011 at 10:55:26AM +0000, Steven Whitehouse wrote:
> Hi,
> 
> On Thu, 2011-11-10 at 18:34 +0800, Zheng Liu wrote:
> > Hi all,
> > 
> > v1->v2: totally redesign this mechanism
> > 
> > This patchset implements an io types statistic mechanism for filesystem
> > and it has been added into ext4 to let us know how the ext4 is used by
> > applications. It is useful for us to analyze how to improve the filesystem
> > and applications. Nowadays, I have added it into ext4, but other filesytems
> > also can use it to count the io types by themselves.
> > 
> > A 'Issue' flag is added into buffer_head and will be set in submit_bh().
> > Thus, we can check this flag in filesystem to know that a request is issued
> > to the disk when this flag is set. Filesystems just need to check it in
> > read operation because filesystem should know whehter a write request hits
> > cache or not, at least in ext4. In filesystem, buffer needs to be locked in
> > checking and clearing this flag, but it doesn't cost much overhead.
> > 

Hi Steve,

Thank you for your attention.

> There is already a REQ_META flag available which allows distinction
> between data and metadata I/O (at least when they are not contained
> within the same block). If that was to be extended to allow some
> filesystem specific bits that would solve the problem that you appear to
> be addressing with these patches in a fs independent way.

You are right. REQ_META flag quite can distinguish between metadata and
data. But it is difficulty to check this flag in filesystem because
buffer_head doesn't use it and most of filesystems still use buffer_head
to submit a IO request. This is the reason why I added a new flag into
buffer_head.

> 
> That would probably have already been done, except that the REQ_ flags
> field is already almost full - so it might need the addition of an extra
> field or some other solution.

In v1[1], a structure called ios is defined. This structure saves some
information (e.g. IO type) and a callback function. Some interfaces in
buffer layer are modifed to add a new argument that points to this
structure. When this request doesn't hit cache and is issued to the
disk, the callback function in this structure will be called. Filesystem
can define a function to do some operations. A defect in this solution
is that it needs to change some interfaces, such bread, breadahead and
so on. So in v2, I re-implement a new mechanism.

> 
> Either way, an fs independent solution to this problem would be worth
> considering,

Yes, I am willing to implement an fs independent solution. This is my
original intention too. So any suggestions are welcome. Thank you.

[1] http://www.spinics.net/lists/linux-ext4/msg28608.html

Regards,
Zheng

> 
> Steve.
> 
> 
> > In ext4, a per-cpu counter is defined and some functions are added to count
> > the io types of buffered/direct io. An exception is __breadahead() due to
> > this function doesn't need a buffer_head as argument or return value. So now
> > we cannot handle these requests calling __breadahead().
> > 
> > The IO types in ext4 have shown as following:
> > Metadata:
> >  - super block
> >  - group descriptor
> >  - inode bitmap
> >  - block bitmap
> >  - inode table
> >  - extent block
> >  - indirect block
> >  - dir index and entry
> >  - extended attribute
> > Data:
> >  - regular data block
> > 
> > The result is shown in sysfs. We can read from /sys/fs/ext4/$DEVICE/io_stats
> > to see the result. We can understand how much metadata or data requests are
> > issued to the disk according to the result.
> > 
> > I have finished some benchmarks to test its overhead that calling lock_buffer()
> > brings. The following fio script is used to run on a SSD. The result shows that
> > the ovheread can be ignored.
> > 
> > FIO config file:
> > [global]
> > ioengineshortync
> > bs=4k
> > filename=/mnt/sda1/testfile
> > size=64G
> > runtime=300
> > group_reporting
> > loops=500
> > 
> > [read]
> > rw=randread
> > numjobs=4
> > 
> > [write]
> > rw=randwrite
> > numjobs=1
> > 
> > The result (iops):
> >         w/o         w/
> > READ:  16304      15906 (-2.44%)
> > WRITE:  1332       1353 (+1.58%)
> > 
> > Any comments or suggestions are welcome.
> > 
> > Regards,
> > Zheng
> > --
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> 
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