Re: What represent 646345728 bytes

[Eric Sandeen <sandeen@redhat.com>]

paul.chavent@fnac.net wrote:
> Hi
> 
> I'am writing an application that write a stream of pictures of fixed
> size on a disk.
> 
> My app run on a self integrated gnu/linux (based on a 2.6.31.6-rt19
> kernel).
> 
> My media is formated with
> 
> # mke2fs -t ext4 -L DATA -O large_file,^has_journal,extent -v
> /dev/sda3 [...]
> 
> And it is mounted with
> 
> # mount -t ext4 /dev/sda3 /var/data/ EXT4-fs (sda3): no journal 
> EXT4-fs (sda3): delayed allocation enabled EXT4-fs: file extents
> enabled EXT4-fs: mballoc enabled EXT4-fs (sda3): mounted filesystem
> without journal
> 
> My app opens the file with "O_WRONLY | O_CREAT | O_TRUNC | O_SYNC |
> O_DIRECT" flags.
> 
> Each write takes ~4.2ms for 304K (it is very good since it is the
> write bandwidth of my hard drive). There is a write every 100ms.

So you are doing streaming writes in 304k chunks?  Or each file
gets 304K?  It sounds like you are writing multiple 304k pictures
to a single file, right?

> But every exactly 646345728 bytes, the write takes ~46ms.

Do you mean every 304K written to the fs, or to the file?

304k doesn't divide evenly into 646345728 bytes so I'm not sure...

> I had the same problem with ext2 but every ~620M (the amount wasn't
> so constant).
> 
> Also i tryed to "posix_fallocate" with (eg 2G), and the first write
> overhead comes at this limit. I would like to avoid to preallocate.

Preallocation -should- be helpful in general, so if it's not ...

> I suppose it is a kind of block allocation issue. But i would like to
> have your opinion : - what is exatcly this amount of bytes ? - can i
> do something for having a "constant" write time from the user space
> point of view ? - is it a "probem" only for me ?

It might be interesting to know what the geometry of your filesystem is,
dumpe2fs -h would provide that.  Also if you could mock up a testcase
that demonstrates the behavior, it would help in debugging.

Actually... as a first step I would redo the test yourself with
-O ^uninit_bg at mkfs time to see if the bitmap init is causing the
delay.

-Eric

> Thank you for your reading.
> 
> Paul.
> 
> 
> 
> 
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