Re: [RFC] fadvise: add more flags to provide a hint for block allocation

On Mon, Mar 05, 2012 at 11:48:43AM -0800, Sunil Mushran wrote:

On 03/05/2012 04:50 AM, Zheng Liu wrote:

quoted

Hi list,

Block allocation is a key component of file system.  Every file systems try to
improve the performance with optimizing the block allocation of a file.  But no
matter what file system does, it just guesses what the user expects.  Thus, it
is not very accurate.  fadvise(2) provides a method to let the user to give a
hint to file system.  However, until now, only few flags are provided.  So we
can provide more flags to tell file system how to allocate the blocks for a
file.

For example:
we can add these flags into fadvise(2):
FADV_ALLOC_READ_SEQ
FADV_ALLOC_READ_RANDOM
FADV_ALLOC_WRITE_ONCE
FADV_ALLOC_WRITE_APPEND

FADV_ALLOC_READ_* are not similar with FADV_SEQUENTIAL and FADV_RANDOM.
FADV_ALLOC_READ_SEQ tells file system that this file need to allocate some
sequential blocks, and FADV_ALLOC_READ_RADOM tells file system that this file
can endure the fragmentation.

Hi Sunil,

Thank you for your feedback.

File systems typically allocate the best layout they can for a file
at the time of write. Does _RANDOM mean do not do that. Find single
bits scattered around the disk. If so, why will people use it. I
mean, random IOs are slow. What you are proposing it is a further
slowdown.
Hardly a feature that will be attractive to users.

No, _RANDOM means that file system doesn't need to try its best to find
a proper position to allocate some blocks for this file.  Furthermore,
currently random IOs seem that they are not obviously slower than
sequential IOs in Flash/SSD device.  For example, when users know a file
that is accessed infrequently, they can put this file in a corner, such
as in some discontinuously blocks.  Then sequential blocks are reserved
for the file that needs to be accessed frequently and users can obtain
the better performance.

quoted

FADV_ALLOC_WRITE_ONCE indicates that this file just is written once.  So file
system can allocate some sequential blocks for it to improve the read
performance.  FADV_ALLOC_WRITE_APPEND flag is set to point out that data will be
appended to the end of this file, and file system can reserve some blocks for it
to guarantee the sequence as much as possible.

Define ONCE. Is it one write(2)? I guess not. You probably mean
that once the file descriptor is closed, it will not be written
to. But we have no way of knowing how many writes there will be.
So it will be treated the same as APPEND. And file systems already
provide allocation reservation and/or delayed allocation to handle
APPEND write loads. So this flag does not offer much to the user
or the fs.

Sorry, I don't express clearly. _ONCE means that the size of a file
doesn't be chagned after it has been created. Certainly, you are right.
We can use fallocate(2) to obtain the same result. ;-)

Regards,
Zheng

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