On Thu, Feb 16, 2023 at 10:07:39AM -0700, Andreas Dilger wrote: > On Feb 10, 2023, at 7:37 AM, Jan Kara <jack@xxxxxxx> wrote: > > So I belive mballoc tries to align everything (offsets & lengths) > > to powers of two to reduce fragmentation and simplify the work for > > the buddy allocator. If ac->ac_b_ex.fe_len is a power-of-two, the > > alignment makes sense. But once we had to resort to higher allocator > > passes and just got some random-length extent, the alignment stops > > making sense. > > In addition to optimizing for the buddy allocator, the other reason that > the allocations are aligned to power-of-two offsets is to better align > with underlying RAID stripes. Otherwise, unaligned writes will cause > parity read-modify-write updates to multiple RAID stripes. This alignment > can also help (though to a lesser degree) with NAND flash erase blocks. > > Cheers, Andreas > Got it, thanks. So from my limited understanding of RAID, if the write is stripe aligned and the (length % stripe == 0) then we won't need a RMW cycle for parity bits and thats one of the reasons to pay attention to alignment and length in mballoc code. Then I think Jan's reasoning still holds that if ac_b_ex.fe_len is already not of a proper size then we'll anyways be ending with a RMW write in RAID so no point of paying attention to its alignment, right? Regards, ojaswin
