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

On 03/06/2012 06:29 AM, Lukas Czerner wrote:

However the file system do not have the information which part of the
device it resides on is faster. It might be the beginning of the file
system, but it might not be the case at all.

Think HSM and flash storage as the hot region. Remember these are
hints and not guaranteed to work in all cases.

Moreover the flag which is stating that the file does not have to be
allocated sequentially is not particularly helpful, I can not imagine
people using it. Why would someone want to lower their performance ?
Well, they might think that it will increase performance of the other
files, but that is highly disputable and there are better solutions like
using faster storage for the files that actually needs it.

Additionally *_HOT* flag does not say anything about the allocation
policy. It might be accessed often ,but no in sequential manner, or it
can be written to a lot, it can be appended a lot, or it the content
might be changed without changing its size etc... *Hot* might mean so
many thing that this is just not useful for the file system. It would
certainly be better to come up with something less esoteric which would
actually address concrete user issues and help file system to deal with
them better, like, I do not know, do not fsync/force allocation on
rename maybe...(or whatever we are doing right now).

_HOT/_COLD is descriptive for allocation policy though fadvise() is
the wrong call as it pertains to access patterns.

Sunil