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Subject[PATCH] ext4: Change the handling of RAID stripe width
Date
Previously, the stripe width was blindly used for determining the size
of allocations. Now, the stripe width is used as a hint for the initial
mb_group_prealloc; if it is greater than 1, then we make sure that
mb_group_prealloc is some multiple of it, and otherwise it is ignored.
mb_group_prealloc is always usable to adjust the preallocation strategy,
not just when the stripe-width is 0 as before.

Signed-off-by: Dan Ehrenberg <dehrenberg@google.com>
---
fs/ext4/mballoc.c | 40 +++++++++++++++++++++++++++++-----------
1 files changed, 29 insertions(+), 11 deletions(-)
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 6ed859d..710c27f 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -127,13 +127,14 @@
* 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
- * /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
+ * 512 blocks. If we have mounted the file system with -O
* stripe=<value> option the group prealloc request is normalized to the
- * stripe value (sbi->s_stripe)
+ * the smallest multiple of the stripe value (sbi->s_stripe) which is
+ * greater than the default 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.
*
- * The regular allocator(using the buddy cache) supports few tunables.
+ * The regular allocator (using the buddy cache) supports a few tunables.
*
* /sys/fs/ext4/<partition>/mb_min_to_scan
* /sys/fs/ext4/<partition>/mb_max_to_scan
@@ -2471,7 +2472,26 @@ int ext4_mb_init(struct super_block *sb, int needs_recovery)
sbi->s_mb_stats = MB_DEFAULT_STATS;
sbi->s_mb_stream_request = MB_DEFAULT_STREAM_THRESHOLD;
sbi->s_mb_order2_reqs = MB_DEFAULT_ORDER2_REQS;
+ /*
+ * If the stripe width is 1, this makes no sense and
+ * we set it to 0 to turn off stripe handling code.
+ */
+ if (sbi->s_stripe == 1)
+ sbi->s_stripe = 0;
+
sbi->s_mb_group_prealloc = MB_DEFAULT_GROUP_PREALLOC;
+ /*
+ * If there is a s_stripe > 1, then we set the s_mb_group_prealloc
+ * to the lowest multiple of s_stripe which is bigger than
+ * the s_mb_group_prealloc as determined above. We want
+ * the preallocation size to be an exact multiple of the
+ * RAID stripe size so that preallocations don't fragment
+ * the stripes.
+ */
+ if (sbi->s_stripe > 1) {
+ sbi->s_mb_group_prealloc = roundup(
+ sbi->s_mb_group_prealloc, sbi->s_stripe);
+ }

sbi->s_locality_groups = alloc_percpu(struct ext4_locality_group);
if (sbi->s_locality_groups == NULL) {
@@ -2830,8 +2850,9 @@ out_err:

/*
* here we normalize request for locality group
- * Group request are normalized to s_strip size if we set the same via mount
- * option. If not we set it to s_mb_group_prealloc which can be configured via
+ * Group request are normalized to s_mb_group_prealloc, which goes to
+ * s_strip if we set the same via mount option.
+ * s_mb_group_prealloc can be configured via
* /sys/fs/ext4/<partition>/mb_group_prealloc
*
* XXX: should we try to preallocate more than the group has now?
@@ -2842,10 +2863,7 @@ static void ext4_mb_normalize_group_request(struct ext4_allocation_context *ac)
struct ext4_locality_group *lg = ac->ac_lg;

BUG_ON(lg == NULL);
- if (EXT4_SB(sb)->s_stripe)
- ac->ac_g_ex.fe_len = EXT4_SB(sb)->s_stripe;
- else
- ac->ac_g_ex.fe_len = EXT4_SB(sb)->s_mb_group_prealloc;
+ ac->ac_g_ex.fe_len = EXT4_SB(sb)->s_mb_group_prealloc;
mb_debug(1, "#%u: goal %u blocks for locality group\n",
current->pid, ac->ac_g_ex.fe_len);
}
--
1.7.3.1


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