Date:	Mon, 27 Feb 2012 14:16:58 -0700
From:	Andreas Dilger <adilger@...ger.ca>
To:	Lukas Czerner <lczerner@...hat.com>
Cc:	Zheng Liu <gnehzuil.liu@...il.com>, linux-ext4@...r.kernel.org
Subject: Re: [RFC] ext4: block reservation allocation

On 2012-02-27, at 6:33 AM, 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.
>>> 
>>> 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.

This introduces more administration complexity, and is one of the reasons
why desktop systems install with a single huge partition instead of many
separate partitions.  Having a slightly slower index is much less harmful
to an application than the index partition becoming full while the data
partition has free space.

This also introduces gratuitous overhead due to separate journals, multiple
cache flush commands for the same disk, etc.

Cheers, Andreas

> 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.
> 
> 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?
> 
> 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
>> 
> 
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Cheers, Andreas





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