On Mon, Mar 05, 2012 at 11:48:43AM -0800, Sunil Mushran wrote: > On 03/05/2012 04:50 AM, Zheng Liu wrote: > >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. > > > >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 > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
