I found that mkfs.btrfs aborts when assigned multi volumes contain
a small volume:
# parted /dev/sdf p
Model: LSI MegaRAID SAS RMB (scsi)
Disk /dev/sdf: 72.8GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Number Start End Size Type File system Flags
1 32.3kB 72.4GB 72.4GB primary
2 72.4GB 72.8GB 461MB primary
# ./mkfs.btrfs -f /dev/sdf1 /dev/sdf2
:
SMALL VOLUME: forcing mixed metadata/data groups
adding device /dev/sdf2 id 2
mkfs.btrfs: volumes.c:852: btrfs_alloc_chunk: Assertion `!(ret)' failed.
Aborted (core dumped)
This failure of btrfs_alloc_chunk was caused by following steps:
1) since there is only small space in the small device, mkfs was
going to allocate a chunk from free space as much as available.
So mkfs called btrfs_alloc_chunk with
size = device->total_bytes - device->used_bytes.
2) To avoid overwriting superblock, btrfs_alloc_chunk starts taking
chunks at an offset of 1MB. It means that the layout of a disk
will be like:
[[1MB at begging for sb][allocated chunks]* ... free space ... ]
and you can see that the available free space for allocation is:
avail = device->total_bytes - device->used_bytes - 1MB.
3) Therefore there is only free space 1MB less than requested. damn.
So this fix let mkfs know how much spaces are really there.
Signed-off-by: Hidetoshi Seto <seto.hidetoshi@xxxxxxxxxxxxxx>
---
ctree.h | 3 +++
volumes.c | 7 ++++++-
2 files changed, 9 insertions(+), 1 deletions(-)
diff --git a/ctree.h b/ctree.h
index 0b0d701..791bd14 100644
--- a/ctree.h
+++ b/ctree.h
@@ -811,6 +811,9 @@ struct btrfs_csum_item {
u8 csum;
} __attribute__ ((__packed__));
+/* we have reserved 1M for superblock at the begging of device */
+#define BTRFS_BLOCK_RESERVED_1M_FOR_SUPER ((u64)1024 * 1024)
+
/* tag for the radix tree of block groups in ram */
#define BTRFS_BLOCK_GROUP_DATA (1ULL << 0)
#define BTRFS_BLOCK_GROUP_SYSTEM (1ULL << 1)
diff --git a/volumes.c b/volumes.c
index 0ff2283..bf6b2e1 100644
--- a/volumes.c
+++ b/volumes.c
@@ -283,7 +283,7 @@ static int find_free_dev_extent(struct btrfs_trans_handle *trans,
/* we don't want to overwrite the superblock on the drive,
* so we make sure to start at an offset of at least 1MB
*/
- search_start = max((u64)1024 * 1024, search_start);
+ search_start = max(BTRFS_BLOCK_RESERVED_1M_FOR_SUPER, search_start);
if (root->fs_info->alloc_start + num_bytes <= device->total_bytes)
search_start = max(root->fs_info->alloc_start, search_start);
@@ -783,6 +783,11 @@ again:
while(index < num_stripes) {
device = list_entry(cur, struct btrfs_device, dev_list);
avail = device->total_bytes - device->bytes_used;
+ /* we have reserved 1M for superblock at the head of device */
+ if (avail > BTRFS_BLOCK_RESERVED_1M_FOR_SUPER)
+ avail -= BTRFS_BLOCK_RESERVED_1M_FOR_SUPER;
+ else
+ avail = 0;
cur = cur->next;
if (avail >= min_free) {
list_move_tail(&device->dev_list, &private_devs);
--
1.7.1
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