On 2013-05-21, at 12:02 PM, frankcmoeller@xxxxxxxx wrote: >> I like the idea of keeping the high bits of the buddy bitmap >> in the group descriptor, instead of just the largest free order. >> It takes the same amount of space, but provides more information. > More informations for what? Sorry, what I meant to write was that it provides more information than just recording e.g. the number of blocks in the largest free extent. > The allocator or better the good_group function > needs bb_largest_free_order and in some cases fragment count. Do you > want to use the bitmap for a not 100% correct fragment count calculation? Or is there another use for it? The bitmap would provide the largest_free_order value directly (assuming it is at least 4MB in size). Cheers, Andreas >>> On Sun, 19 May 2013 21:36:02 +0200 (CEST) Frank C Moeller wrote: >>>> From my point (end user) I would prefer a builtin solution. I'm also a >>>> programmer and I can therefore understand why you don't want to change >>>> anything. >>> >>> It's not that I don't want to change anything, it's that I'm very >>> hesitant to add new mount options or new code paths that now need more >>> testing unless there's no other way of addressing a particular use >>> case. Another consideration is how to do it in such a way that it >>> doesn't degrade other users' performance. >>> >>> Issuing readahead requests for the bitmap blocks might be good >>> compromise; since they are readahead requests, as low priority >>> requests they won't interfere with anything else going on, and in >>> practice, unless you are starting your video recording **immediately** >>> after the reboot, it should address your concern. >> >> Right. Some of our users do something similar in userspace to avoid >> slowdown on first write, which doesn't _usually_ happen immediately >> after mount, but this isn't always helpful. >> >>> (Also note that for >>> most people willing to hack a DVR, adding a line to /etc/rc.local is >>> usually considered easier than building a new kernel from sources and >>> then after making file system format changes, requiring a reformat of >>> their data disk!) >> >> I think storing the buddy bitmap top bits in the GDT could be a COMPAT >> feature. It is just a hint that could be ignored or incorrect, since >> the actual bitmap would be authoritative. >> >> Cheers, Andreas >> >>> So it's not that I'm against solutions that involve kernel changes or >>> file system format changes. It's just that I want to make sure we >>> explore the entire solution space, since there are costs in terms of >>> testing costs, the need to do a backup-reformat-restore pass, etc, >>> etc., to some of the solutions that have been suggested so far. >>> >>> Regards, >>> >>> - Ted >> Cheers, Andreas -- 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
