On Mon, 22 Mar 2010 16:03:10 -0600, Andreas Dilger <adilger@xxxxxxx> wrote: > Here is the patch I mentioned today on the call. It avoids (or at > least reduces) serious latency (10 minutes or more) on a large > filesystem (8TB+) on the first write, if the filesystem is nearly > full. The latency is entirely due to seeking to read the block > bitmaps, so is considerably less serious on flex_bg formatted > filesystems. > > A better long-term approach would be to store in the superblock the > last group that had space to allocate a stripe-sized chunk and/or flag > in the group descriptor if there is not a large amount of contiguous > free space therein (cleared on freeing blocks in the group). > > Having the mount-time buddy-bitmap (and checksum verifying) scanning > thread start at mount would only help if the first write to the > filesystem is not immediately after mount (which it is in Lustre at > least). Having a filesystem-wide (r)btree for the freespace (ala XFS) > would also only help if the btree could be (at least partially) built > from bitmaps before the first write, unless we cache the bitmap on > disk, which caused Lustre plenty in the past and I'm leery to do it. > > @@ -125,8 +125,7 @@ * list. In case of inode preallocation we follow a list of heuristics * based on file size. This can be found in ext4_mb_normalize_request. If * we are doing a group prealloc we try to normalize the request to - * sbi->s_mb_group_prealloc. Default value of s_mb_group_prealloc is - * 512 blocks. This can be tuned via + * sbi->s_mb_group_prealloc. This can be tuned via * /sys/fs/ext4/<partition/mb_group_prealloc. The value is represented in * terms of number of blocks. If we have mounted the file system with -O * stripe=<value> option the group prealloc request is normalized to the @@ -2029,9 +2028,12 @@ repeat: if (group == ngroups) group = 0; - /* quick check to skip empty groups */ + /* If there's no chance that this group has a better + * extent, just skip it instead of seeking to read + * block bitmap from disk. Initially ac_b_ex.fe_len = 0, + * so this always skips groups with no free space. */ grp = ext4_get_group_info(sb, group); - if (grp->bb_free == 0) + if (grp->bb_free <= ac->ac_b_ex.fe_len) continue; err = ext4_mb_load_buddy(sb, group, &e4b); I was wondering whether we need to make sure we also use criteria value when checking for bb_free. If we are really low on space we may want to return what is left right ?. Or does ac_b_ex take care of that ? -aneesh -- 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
