Re: file system defragmentation

Re: file system defragmentation

[Cosmo Nova]

Hi, you mentioned delayed allocation. What is the size of the buffer holding
the data, before they're actually written to disk? How can it tackle growing
files?

If I have a DVR system of 16 channels. They keep writing data to the disk in
pieces of video files. I read some spec of xfs. Apart from extend-based
allocation, there're allocaiton groups in xfs. I would like to ask, does
XFS's allocation groups work similar as JFS's, which would lock an
allocation group for individual file write? How does the allocation group in
XFS work? And how would it help the fragmentation problem?

Thankyou!
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Re: file system defragmentation

[David Chatterton]

Cosmo Nova wrote:
> Hi, you mentioned delayed allocation. What is the size of the buffer holding
> the data, before they're actually written to disk? How can it tackle growing
> files?

I'll leave the details for Nathan/Dave...

> 
> If I have a DVR system of 16 channels. They keep writing data to the disk in
> pieces of video files. I read some spec of xfs. Apart from extend-based
> allocation, there're allocaiton groups in xfs. I would like to ask, does
> XFS's allocation groups work similar as JFS's, which would lock an
> allocation group for individual file write? How does the allocation group in
> XFS work? And how would it help the fragmentation problem?
> 

Locking down an allocation group is part of what a new "filestream"
mount option will provide. This feature is already in use at a
number of IRIX sites with media apps (mainly in conjunction with CXFS
since you have more writers in a clustered filesystem and more need to
lock down contiguous space), and is coming to Linux XFS soon.

The best way to tackle fragementation with streaming data is to
preallocate space for the files. This allows the allocator to
try to find the space you require using as large contiguous chunks
as it can find. See xfsctl(3).

David

-- 
David Chatterton                   Phone: +61 3 9963 1934
XFS Engineering Manager            Mobile: +61 409 154 121
SGI Australia          http://www.sgi.com/products/storage

Re: file system defragmentation

[David Chinner]

On Mon, Jul 17, 2006 at 12:36:09AM -0700, Cosmo Nova wrote:
> 
> Hi, you mentioned delayed allocation. What is the size of the buffer holding
> the data, before they're actually written to disk?  How can it tackle growing
> files?

No special buffering is needed in XFS as the page cache does it all for us.
delalloc means that XFS does not allocate until the data gets written to disk
(i.e. flushed from the cache). The size of the allocation is dependent on the
amount of contiguous dirty data in the file. That is, The more sequential
dirty pages in the page cache for a file, the larger the allocation...

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group

Re: file system defragmentation

[Chris Wedgwood]

On Mon, Jul 17, 2006 at 12:36:09AM -0700, Cosmo Nova wrote:

> If I have a DVR system of 16 channels. They keep writing data to the
> disk in pieces of video files.

I did some work for someone who does a similar thing (they write 96
channels in parallel and have to be able to do read back up to 32 of
them at the same time of something).

By default concurrent writes into the same directory will cause the
files to get badly interleaved, and trying to get one file per-ag
doesn't work so well if the agcount < n-files.

I ended up getting them to change their code to preallocate and AFAIK
it works very well now (you can throw away any space you over allocate
when you're closing the file too).

Re: file system defragmentation

[Cosmo Nova]

Hi, can I summarize by saying that, XFS still partition the volume into
different allocation groups, but there is NO locking. Two files may write in
the same allocation group and introduce fragments?
What is the maximum and typical number of allocation groups please? I can't
find the numbers in the source code...

And for the buffer, Is preallocation the same as delayed allocation? page
cache, as I remember belongs to Linux VFS. What are the memory / disk
requirement and sizing for the cache please? If a file space allocation is
delayed, what is the upper bound of the delayed size? Can files of MBs, GBs
being delayed allocate, and multiple channels/threads files all store in
memory?? How XFS's allocation works (with number please if possible) to help
tackle fragmentation?

Thankyou!
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Re: file system defragmentation

[Chris Wedgwood]

On Tue, Jul 18, 2006 at 12:59:39AM -0700, Cosmo Nova wrote:

> Hi, can I summarize by saying that, XFS still partition the volume
> into different allocation groups, but there is NO locking.

the allocator has per-AG locks

> Two files may write in the same allocation group and introduce
> fragments?

i'm not sure what you mean, but any number of files can be writing to
a single AG at once, the degree of fragmentation you will see depends
on the IO patterns and where there are writing

in the case of writing multiple large files in parallel in the same
directory the blocks do tend to interleave

> What is the maximum and typical number of allocation groups please?
> I can't find the numbers in the source code...

It's a 32-bit value, I'm not sure how practical very large values are
though.

> And for the buffer, Is preallocation the same as delayed allocation?

No, preallocation allocates space a head of time on disk so that later
when it's written the allocation process is simpler (or not needed)
and ideally also to reduce fragmentation by doing as much allocation
as possible at once.

Delayed allocation is simply the idea of doing the allocation as late
as possible so when you do have to flush the data to disk you can try
to make better decisions about where to place the blocks.

> If a file space allocation is delayed, what is the upper bound of
> the delayed size?

It depends on the platform and how often data is flushed from the OS.

Re: file system defragmentation

[Cosmo Nova]

If there are AG locks, but multiple files can write in the same AG, how "AG
lock" is interpreted by XFS then? What does it really do?

For pre-allocation and delayed allocation, do they belong to the feature set
of XFS? Or are they application dependant?

I am trying to compare filesystems, especially xfs vs jfs. I found that
jfs's per AG locking would only allow one file to be written per AG, which
helps a lot to prevent fragmentation (according to experiment results).
There're no pre-allocation and delayed allocation in JFS though. Comparing
the experiment results, XFS is doing a good job, giving 6-10 fragments
(compare to majority of single fragment in JFS...). So answers of the above
questions with numbers would help a lot. Thanks!
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Re: file system defragmentation

[Cosmo Nova]

does the preallocation belong to xfs layer? or it's a "suggested feature" for
applications?
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Re: file system defragmentation

[Cosmo Nova]

what is the maximum tolerance for the delayed allocations pls? I guess it's
not possible to buf a GB size file and delay its allocation?
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Re: file system defragmentation

[David Chinner]

On Wed, Jul 19, 2006 at 09:23:42PM -0700, Cosmo Nova wrote:
> 
> what is the maximum tolerance for the delayed allocations pls? I guess it's
> not possible to buf a GB size file and delay its allocation?

Sure it is. The limit on how much is buffered before writeback (and therefore
allocation) is determined by how much memory you have and your
/proc/sys/vm/dirty* tunable settings. This can range from a few MB to a few TB
of data depending on how big your machine is ;)

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group