On Tue, Mar 06, 2012 at 09:27:16AM +0100, Lukas Czerner wrote: > On Mon, 5 Mar 2012, 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. > > > > 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. > > Hi Zheng, > > those two flags does not make sense to me. The FADV_ALLOC_WRITE_ONCE is > actually the same as fallocate, and we certainly do not need more ways > to do fallocate, one is more than enough. > > FADV_ALLOC_WRITE_APPEND seems weird. File systems already do some > preallocations for the files, so we do not fragment them as much. So > what might be more interesting is to be able to set how much space we > want to keep preallocated for the particular file, however strictly > speaking it is not something we would not achieve with fallocate, but it > would certainly be more convenient. > > -Lukas > Hi Lukas, I have realized that these two flags seem redundant, and we don't need them. As we discussed previously and Sunil's suggestions. The key issue is that user provides a hint to file system, and file system can know whether or not this file can be stored in a corner or be allocated in non-sequential blocks. Then the sequential blocks are reserved for the particular file that has a *_HOT* flag. Although fallocate(2) can preallocate some blocks for a file, it cannot put a file at the beginning of the disk to obtain a better performance. So maybe file system can use these flags to optimize the layout of a file. Regards, Zheng > > > > File systems can support a subset of these flags according to its design. These > > flags provide a rich interface that lets the user to control block allocation of > > files. The user could precisely control the allocation of their files to > > improve the performance of appliatons. > > > > Any comments or suggestions are appreciated. Thank you. > > > > 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
