On Mon, 22 Mar 2010 16:03:10 -0600, Andreas Dilger <[email protected]> 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
On 2010-03-26, at 04:28, Aneesh Kumar K. V wrote:
> On Mon, 22 Mar 2010 16:03:10 -0600, Andreas Dilger <[email protected]>
> 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.
>
> @@ -2029,9 +2028,12 @@ repeat:
> + /* 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;
>
> 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 ?
ac_b_ex is the best currently ALLOCATED extent, so mballoc wouldn't
ever select an extent that is smaller than ac_b_ex.fe_len. That means
it is pointless to even look at a group which has fewer free blocks
than ac_b_ex.fe_len.
Later, after the group information is loaded, ldiskfs_mb_good_group()
will skip the group if the average fragment size is smaller than the
GOAL extent, but only for certain criterion levels. At the highest
criterion, any group with free blocks will be scanned.
Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.