Date:	Mon, 27 Feb 2012 16:24:24 +0100 (CET)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Lukas Czerner <lczerner@...hat.com>
cc:	Zheng Liu <gnehzuil.liu@...il.com>, linux-ext4@...r.kernel.org,
	Yongqiang Yang <xiaoqiangnk@...il.com>
Subject: Re: [RFC] ext4: block reservation allocation

On Mon, 27 Feb 2012, Lukas Czerner wrote:

> On Mon, 27 Feb 2012, Zheng Liu wrote:
> 
> > On Mon, Feb 27, 2012 at 02:33:28PM +0100, Lukas Czerner wrote:
> > > On Mon, 27 Feb 2012, Zheng Liu wrote:
> > > 
> > > > On Mon, Feb 27, 2012 at 01:00:07PM +0100, Lukas Czerner wrote:
> > > > > On Mon, 27 Feb 2012, Zheng Liu wrote:
> > > > > 
> > > > > > Hi list,
> > > > > > 
> > > > > > Now, in ext4, we have multi-block allocation and delay allocation. They work
> > > > > > well for most scenarios. However, in some specific scenarios, they cannot help
> > > > > > us to optimize block allocation. For example, the user may want to indicate some
> > > > > > file set to be allocated at the beginning of the disk because its speed in this
> > > > > > position is faster than its speed at the end of disk.
> > > > > > 
> > > > > > I have done the following experiment. The experiment is on my own server, which
> > > > > > has 16 Intel(R) Xeon(R) CPU E5620 @ 2.40GHz, 48G memory and a 1T sas disk. I
> > > > > > split this disk into two partitions, one has 900G, and another has 100G. Then I
> > > > > > use dd to get the speed of read/write. The result is as following.
> > > > > > 
> > > > > > [READ]
> > > > > > # dd if=/dev/sdk1 of=/dev/null bs=128k count=10000 iflag=direct
> > > > > > 1310720000 bytes (1.3 GB) copied, 9.41151 s, 139 MB/s
> > > > > > 
> > > > > > # dd if=/dev/sdk2 of=/dev/null bs=128k count=10000 iflag=direct
> > > > > > 1310720000 bytes (1.3 GB) copied, 17.952 s, 73.0 MB/s
> > > > > > 
> > > > > > [WRITE]
> > > > > > # dd if=/dev/zero of=/dev/sdk1 bs=128k count=10000 oflag=direct
> > > > > > 1310720000 bytes (1.3 GB) copied, 8.46005 s, 155 MB/s
> > > > > > 
> > > > > > # dd if=/dev/zero of=/dev/sdk2 bs=128k count=10000 oflag=direct
> > > > > > 1310720000 bytes (1.3 GB) copied, 15.8493 s, 82.7 MB/s
> > > > > > 
> > > > > > So filesystem can provide a new feature to let the user to indicate a value
> > > > > > for reserving some blocks from the beginning of the disk. When the user needs
> > > > > > to allocate some blocks for an important file that needs to be read/write as
> > > > > > quick as possible, the user can use ioctl(2) and/or other ways to notify
> > > > > > filesystem to allocate these blocks in the reservation area. Thereby, the user
> > > > > > can obtain the higher performance for manipulating this file set.
> > > > > > 
> > > > > > This idea is very trivial. So any comments or suggestions are appreciated.
> > > > > > 
> > > > > > Regards,
> > > > > > Zheng
> > > > > > --
> > > > > > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
> > > > > > the body of a message to majordomo@...r.kernel.org
> > > > > > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > > > > > 
> > > > > 
> > > > > Hi Zheng,
> > > > > 
> > > > > I have to admit I do not like it :). I think that this kind of
> > > > > optimization is useless in the long run. There are several reasons for
> > > > > this:
> > > > 
> > > > Hi Lukas,
> > > > 
> > > > Thank you for your opinion. ;-)
> > > > 
> > > > > 
> > > > >  - the test you've done is purely fabricated and does not respond to
> > > > >    real workload at all. Especially because it is done on a huge files.
> > > > >    I can imagine this approach improving boot speed, but you will
> > > > >    usually have to load just small files, so for single file it does not
> > > > >    make much sense. Moreover with small files more seeks would have to
> > > > >    be done hugely reducing the advantage you can see with dd.
> > > > 
> > > > I will describe the problem that we encounter. the problem shows that
> > > > even if files are small, the performance can be improved in some
> > > > specific scenarios using this block allocation.
> > > > 
> > > > >  - HDD might have more platters than just one
> > > > >  - Your file system might span across several drives
> > > > >  - On thinly provisioned storage this does not make sense at all
> > > > >  - SSD's are more and more common and this optimization is useless for
> > > > >    them.
> > > > > 
> > > > > Is there any 'real' problem you would want to solve with this ? Or is it
> > > > > just something that came to you mind ? I agree that we want to improve
> > > > > our allocators, but IMHO especially for better scalability, not to cover
> > > > > this disputable niche.
> > > > 
> > > > We encounter a problem in our product system. In a 2TB sata disk, the
> > > > file can be divided into two categories. One is index file, and another
> > > > is block file. The average size of index files is about 128k and will
> > > > increase as time goes on. The size of block files is 70M and they are
> > > > created by fallocate(2). Thus, index file is allocated at the end of the
> > > > disk. When application starts up, it needs to load all of index files
> > > > into memory. So it costs too much time. If we can allocate index files
> > > > at the beginning of the disk, we will cut down the startup time and
> > > > increase the service time of this application.
> > > > 
> > > > Therefore, I think that it might be as a generic mechanism to provide
> > > > other users that have the similar requirement.
> > > 
> > > Ok, so this seems like a valid use case. However I think that this is
> > > exactly something that can be quite easily solved without having to
> > > modify file system code, right ?
> > > 
> > > You can simply use separate drive for the index files, or even raid. Or
> > > you can actually use an SSD for this, which I believe will give you *a
> > > lot* better performance improvements and you wont be bothered by the
> > > size/price ratio for SSD as you would only store indexes there, right ?
> > > 
> > > Or, if you really do not want to, or can not, but a new hardware for
> > > some reason, you can always partition a 2TB disk and put all your index
> > > files on the smaller, close to the disk center partition. I really do
> > > not see a reason to modify the code.
> > > 
> > > What might be even more interesting is, that you might generally benefit
> > > from splitting the index/data file systems. The reason is that your data
> > > file and your index file filesystem might benefit from bigalloc if you
> > > split them, because you can set different cluster sizes on both file
> > > system depending on the file sizes you would actually store there, since
> > > as I understand the index and data files differs in size significantly.
> > 
> > You are right. I am trying this solution in our test environment. I have
> > splitted a 2TB disk into 2 partitions. One is for index file and is
> > formated with big alloc, and another is for block file.
> 
> That's good to hear. So you have your solution maybe ?

You probably know all of that that already, but just in case... for the
sake of good performance make sure that you partitions are properly aligned,
because drive might very well have 4k sector size.

-Lukas

> 
> > 
> > > 
> > > How much of the performance boost do you expect by doing this your way -
> > > modifying the file system? Note that dd will not tell you that, as I
> > > explained earlier. I surely would not match using SSD for index files by
> > > far.
> > > 
> > > What do you think?
> > 
> > As Yongqiang said, maybe we can allocate faster block for the file which
> > needs to be fast read/write when the user sets a flag to notify the file
> > system. Maybe we don't need to implement a new block allocation
> > algorithm. We only need to modify the current block allocation to
> > provide this mechansim.
> > 
> > Regards,
> > Zheng
> 
> I am not sure what Yongqiang meant by that. I know that there is a
> REQ_META flag which is supposed to set higher priority for metadata
> reads. However how do you expect this to work ? It would have to be set
> *only* by root, because from user perspective *every* file is a priority
> above other users files :). But doing this as root greatly limits it
> use.
> 
> If the REQ_META thing is what Yongqiang meant, I am not sure if it is
> such a good idea to exploit this flag like that.
> 
> Thanks!
> -Lukas
> 
> > 
> > > 
> > > Thanks!
> > > -Lukas
> > > 
> > > 
> > > 
> > > > 
> > > > Regards,
> > > > Zheng
> > > > 
> > > > > 
> > > > > Anyway, you may try to come up with better experiment. Something which
> > > > > would actually show how much can we get from the more realistic workload
> > > > > rather than showing that contiguous serial writes are faster closely to
> > > > > the center of the disk platter, we know that.
> > > > > 
> > > > > Thanks!
> > > > > -Lukas
> > > > 
> > > 
> > > -- 
> > 
> 
> 

-- 
--
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

Powered by blists - more mailing lists