Fix a bug that triggered asserts in btrfs_balance() in both normal and
resume modes -- restriper state was not properly restored on read-only
mounts. This factors out resuming code from btrfs_restore_balance(),
which is now also called earlier in the mount sequence to avoid the
problem of some early writes getting the old profile.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Al pointed out that we can just toss out the old name on a device and add a
new one arbitrarily, so anybody who uses device->name in printk could
possibly use free'd memory. Instead of adding locking around all of this he
suggested doing it with RCU, so I've introduced a struct rcu_string that
does just that and have gone through and protected all accesses to
device->name that aren't under the uuid_mutex with rcu_read_lock(). This
protects us and I will use it for dealing with removing the device that we
used to mount the file system in a later patch. Thanks,
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <josef@redhat.com>
The device statistics are written into the device tree with each
transaction commit. Only modified statistics are written.
When a filesystem is mounted, the device statistics for each involved
device are read from the device tree and used to initialize the
counters.
Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
An ioctl interface is added to get the device statistic counters.
A second ioctl is added to atomically get and reset these counters.
Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
The goal is to detect when drives start to get an increased error rate,
when drives should be replaced soon. Therefore statistic counters are
added that count IO errors (read, write and flush). Additionally, the
software detected errors like checksum errors and corrupted blocks are
counted.
Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Implement an ioctl for canceling restriper. Currently we wait until
relocation of the current block group is finished, in future this can be
done by triggering a commit. Balance item is deleted and no memory
about the interrupted balance is kept.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Implement an ioctl for pausing restriper. This pauses the relocation,
but balance is still considered to be "in progress": balance item is
not deleted, other volume operations cannot be started, etc. If paused
in the middle of profile changing operation we will continue making
allocations with the target profile.
Add a hook to close_ctree() to pause restriper and free its data
structures on unmount. (It's safe to unmount when restriper is in
"paused" state, we will resume with the same parameters on the next
mount)
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
On mount, if balance item is found, resume balance in a separate
kernel thread.
Try to be smart to continue roughly where previous balance (or convert)
was interrupted. For chunk types that were being converted to some
profile we turn on soft convert, in case of a simple balance we turn on
usage filter and relocate only less-than-90%-full chunks of that type.
These are just heuristics but they help quite a bit, and can be improved
in future.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
When doing convert from one profile to another if soft mode is on
restriper won't touch chunks that already have the profile we are
converting to. This is useful if e.g. half of the FS was converted
earlier.
The soft mode switch is (like every other filter) per-type. This means
that we can convert for example meta chunks the "hard" way while
converting data chunks selectively with soft switch.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Profile changing is done by launching a balance with
BTRFS_BALANCE_CONVERT bits set and target fields of respective
btrfs_balance_args structs initialized. Profile reducing code in this
case will pick restriper's target profile if it's available instead of
doing a blind reduce. If target profile is not yet available it goes
back to a plain reduce.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Select chunks which have at least one byte located inside a given
[vstart, vend) virtual address space range.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Select chunks which have at least one byte of at least one stripe
located on a device with devid X in a given [pstart,pend) physical
address range.
This filter only works when devid filter is turned on.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
This allows to have a separate set of filters for each chunk type
(data,meta,sys). The code however is generic and switch on chunk type
is only done once.
This commit also adds a type filter: it allows to balance for example
meta and system chunks w/o touching data ones.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Add basic restriper infrastructure: extended balancing ioctl and all
related ioctl data structures, add data structure for tracking
restriper's state to fs_info, etc. The semantics of the old balancing
ioctl are fully preserved.
Explicitly disallow any volume operations when balance is in progress.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
When btrfs is writing the super blocks, it send barrier flushes to make
sure writeback caching drives get all the metadata on disk in the
right order.
But, we have two bugs in the way these are sent down. When doing
full commits (not via the tree log), we are sending the barrier down
before the last super when it should be going down before the first.
In multi-device setups, we should be waiting for the barriers to
complete on all devices before writing any of the supers.
Both of these bugs can cause corruptions on power failures. We fix it
with some new code to send down empty barriers to all devices before
writing the first super.
Alexandre Oliva found the multi-device bug. Arne Jansen did the async
barrier loop.
Signed-off-by: Chris Mason <chris.mason@oracle.com>
Reported-by: Alexandre Oliva <oliva@lsd.ic.unicamp.br>
Add state information for readahead to btrfs_fs_info and btrfs_device
Changes v2:
- don't wait in radix_trees
- add own set of workers for readahead
Reviewed-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Arne Jansen <sensille@gmx.net>
btrfs_bio is a bio abstraction able to split and not complete after the last
bio has returned (like the old btrfs_multi_bio). Additionally, btrfs_bio
tracks the mirror_num used to read data which can be used for error
correction purposes.
Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>
We have a problem where if a user specifies discard but doesn't actually support
it we will return EOPNOTSUPP from btrfs_discard_extent. This is a problem
because this gets called (in a fashion) from the tree log recovery code, which
has a nice little BUG_ON(ret) after it, which causes us to fail the tree log
replay. So instead detect wether our devices support discard when we're adding
them and then don't issue discards if we know that the device doesn't support
it. And just for good measure set ret = 0 in btrfs_issue_discard just in case
we still get EOPNOTSUPP so we don't screw anybody up like this again. Thanks,
Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
fs_devices->devices is only updated on remove and add device paths, so we can
use rcu to protect it in the reader side
Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
In a multi device setup, the chunk allocator currently always allocates
chunks on the devices in the same order. This leads to a very uneven
distribution, especially with RAID1 or RAID10 and an uneven number of
devices.
This patch always sorts the devices before allocating, and allocates the
stripes on the devices with the most available space, as long as there
is enough space available. In a low space situation, it first tries to
maximize striping.
The patch also simplifies the allocator and reduces the checks for
corner cases.
The simplification is done by several means. First, it defines the
properties of each RAID type upfront. These properties are used afterwards
instead of differentiating cases in several places.
Second, the old allocator defined a minimum stripe size for each block
group type, tried to find a large enough chunk, and if this fails just
allocates a smaller one. This is now done in one step. The largest possible
chunk (up to max_chunk_size) is searched and allocated.
Because we now have only one pass, the allocation of the map (struct
map_lookup) is moved down to the point where the number of stripes is
already known. This way we avoid reallocation of the map.
We still avoid allocating stripes that are not a multiple of STRIPE_SIZE.
This adds an initial implementation for scrub. It works quite
straightforward. The usermode issues an ioctl for each device in the
fs. For each device, it enumerates the allocated device chunks. For
each chunk, the contained extents are enumerated and the data checksums
fetched. The extents are read sequentially and the checksums verified.
If an error occurs (checksum or EIO), a good copy is searched for. If
one is found, the bad copy will be rewritten.
All enumerations happen from the commit roots. During a transaction
commit, the scrubs get paused and afterwards continue from the new
roots.
This commit is based on the series originally posted to linux-btrfs
with some improvements that resulted from comments from David Sterba,
Ilya Dryomov and Jan Schmidt.
Signed-off-by: Arne Jansen <sensille@gmx.net>
Remove static and global declarations and/or definitions. Reduces size
of btrfs.ko by ~3.4kB.
text data bss dec hex filename
402081 7464 200 409745 64091 btrfs.ko.base
398620 7144 200 405964 631cc btrfs.ko.remove-all
Signed-off-by: David Sterba <dsterba@suse.cz>
btrfs_map_block() will only return a single stripe length, but we want the
full extent be mapped to each disk when we are trimming the extent,
so we add length to btrfs_bio_stripe and fill it if we are mapping for REQ_DISCARD.
Signed-off-by: Li Dongyang <lidongyang@novell.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
Tracepoints can provide insight into why btrfs hits bugs and be greatly
helpful for debugging, e.g
dd-7822 [000] 2121.641088: btrfs_inode_request: root = 5(FS_TREE), gen = 4, ino = 256, blocks = 8, disk_i_size = 0, last_trans = 8, logged_trans = 0
dd-7822 [000] 2121.641100: btrfs_inode_new: root = 5(FS_TREE), gen = 8, ino = 257, blocks = 0, disk_i_size = 0, last_trans = 0, logged_trans = 0
btrfs-transacti-7804 [001] 2146.935420: btrfs_cow_block: root = 2(EXTENT_TREE), refs = 2, orig_buf = 29368320 (orig_level = 0), cow_buf = 29388800 (cow_level = 0)
btrfs-transacti-7804 [001] 2146.935473: btrfs_cow_block: root = 1(ROOT_TREE), refs = 2, orig_buf = 29364224 (orig_level = 0), cow_buf = 29392896 (cow_level = 0)
btrfs-transacti-7804 [001] 2146.972221: btrfs_transaction_commit: root = 1(ROOT_TREE), gen = 8
flush-btrfs-2-7821 [001] 2155.824210: btrfs_chunk_alloc: root = 3(CHUNK_TREE), offset = 1103101952, size = 1073741824, num_stripes = 1, sub_stripes = 0, type = DATA
flush-btrfs-2-7821 [001] 2155.824241: btrfs_cow_block: root = 2(EXTENT_TREE), refs = 2, orig_buf = 29388800 (orig_level = 0), cow_buf = 29396992 (cow_level = 0)
flush-btrfs-2-7821 [001] 2155.824255: btrfs_cow_block: root = 4(DEV_TREE), refs = 2, orig_buf = 29372416 (orig_level = 0), cow_buf = 29401088 (cow_level = 0)
flush-btrfs-2-7821 [000] 2155.824329: btrfs_cow_block: root = 3(CHUNK_TREE), refs = 2, orig_buf = 20971520 (orig_level = 0), cow_buf = 20975616 (cow_level = 0)
btrfs-endio-wri-7800 [001] 2155.898019: btrfs_cow_block: root = 5(FS_TREE), refs = 2, orig_buf = 29384704 (orig_level = 0), cow_buf = 29405184 (cow_level = 0)
btrfs-endio-wri-7800 [001] 2155.898043: btrfs_cow_block: root = 7(CSUM_TREE), refs = 2, orig_buf = 29376512 (orig_level = 0), cow_buf = 29409280 (cow_level = 0)
Here is what I have added:
1) ordere_extent:
btrfs_ordered_extent_add
btrfs_ordered_extent_remove
btrfs_ordered_extent_start
btrfs_ordered_extent_put
These provide critical information to understand how ordered_extents are
updated.
2) extent_map:
btrfs_get_extent
extent_map is used in both read and write cases, and it is useful for tracking
how btrfs specific IO is running.
3) writepage:
__extent_writepage
btrfs_writepage_end_io_hook
Pages are cirtical resourses and produce a lot of corner cases during writeback,
so it is valuable to know how page is written to disk.
4) inode:
btrfs_inode_new
btrfs_inode_request
btrfs_inode_evict
These can show where and when a inode is created, when a inode is evicted.
5) sync:
btrfs_sync_file
btrfs_sync_fs
These show sync arguments.
6) transaction:
btrfs_transaction_commit
In transaction based filesystem, it will be useful to know the generation and
who does commit.
7) back reference and cow:
btrfs_delayed_tree_ref
btrfs_delayed_data_ref
btrfs_delayed_ref_head
btrfs_cow_block
Btrfs natively supports back references, these tracepoints are helpful on
understanding btrfs's COW mechanism.
8) chunk:
btrfs_chunk_alloc
btrfs_chunk_free
Chunk is a link between physical offset and logical offset, and stands for space
infomation in btrfs, and these are helpful on tracing space things.
9) reserved_extent:
btrfs_reserved_extent_alloc
btrfs_reserved_extent_free
These can show how btrfs uses its space.
Signed-off-by: Liu Bo <liubo2009@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
* 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mason/btrfs-unstable: (25 commits)
Btrfs: forced readonly mounts on errors
btrfs: Require CAP_SYS_ADMIN for filesystem rebalance
Btrfs: don't warn if we get ENOSPC in btrfs_block_rsv_check
btrfs: Fix memory leak in btrfs_read_fs_root_no_radix()
btrfs: check NULL or not
btrfs: Don't pass NULL ptr to func that may deref it.
btrfs: mount failure return value fix
btrfs: Mem leak in btrfs_get_acl()
btrfs: fix wrong free space information of btrfs
btrfs: make the chunk allocator utilize the devices better
btrfs: restructure find_free_dev_extent()
btrfs: fix wrong calculation of stripe size
btrfs: try to reclaim some space when chunk allocation fails
btrfs: fix wrong data space statistics
fs/btrfs: Fix build of ctree
Btrfs: fix off by one while setting block groups readonly
Btrfs: Add BTRFS_IOC_SUBVOL_GETFLAGS/SETFLAGS ioctls
Btrfs: Add readonly snapshots support
Btrfs: Refactor btrfs_ioctl_snap_create()
btrfs: Extract duplicate decompress code
...
When we store data by raid profile in btrfs with two or more different size
disks, df command shows there is some free space in the filesystem, but the
user can not write any data in fact, df command shows the wrong free space
information of btrfs.
# mkfs.btrfs -d raid1 /dev/sda9 /dev/sda10
# btrfs-show
Label: none uuid: a95cd49e-6e33-45b8-8741-a36153ce4b64
Total devices 2 FS bytes used 28.00KB
devid 1 size 5.01GB used 2.03GB path /dev/sda9
devid 2 size 10.00GB used 2.01GB path /dev/sda10
# btrfs device scan /dev/sda9 /dev/sda10
# mount /dev/sda9 /mnt
# dd if=/dev/zero of=tmpfile0 bs=4K count=9999999999
(fill the filesystem)
# sync
# df -TH
Filesystem Type Size Used Avail Use% Mounted on
/dev/sda9 btrfs 17G 8.6G 5.4G 62% /mnt
# btrfs-show
Label: none uuid: a95cd49e-6e33-45b8-8741-a36153ce4b64
Total devices 2 FS bytes used 3.99GB
devid 1 size 5.01GB used 5.01GB path /dev/sda9
devid 2 size 10.00GB used 4.99GB path /dev/sda10
It is because btrfs cannot allocate chunks when one of the pairing disks has
no space, the free space on the other disks can not be used for ever, and should
be subtracted from the total space, but btrfs doesn't subtract this space from
the total. It is strange to the user.
This patch fixes it by calcing the free space that can be used to allocate
chunks.
Implementation:
1. get all the devices free space, and align them by stripe length.
2. sort the devices by the free space.
3. check the free space of the devices,
3.1. if it is not zero, and then check the number of the devices that has
more free space than this device,
if the number of the devices is beyond the min stripe number, the free
space can be used, and add into total free space.
if the number of the devices is below the min stripe number, we can not
use the free space, the check ends.
3.2. if the free space is zero, check the next devices, goto 3.1
This implementation is just likely fake chunk allocation.
After appling this patch, df can show correct space information:
# df -TH
Filesystem Type Size Used Avail Use% Mounted on
/dev/sda9 btrfs 17G 8.6G 0 100% /mnt
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
With this patch, we change the handling method when we can not get enough free
extents with default size.
Implementation:
1. Look up the suitable free extent on each device and keep the search result.
If not find a suitable free extent, keep the max free extent
2. If we get enough suitable free extents with default size, chunk allocation
succeeds.
3. If we can not get enough free extents, but the number of the extent with
default size is >= min_stripes, we just change the mapping information
(reduce the number of stripes in the extent map), and chunk allocation
succeeds.
4. If the number of the extent with default size is < min_stripes, sort the
devices by its max free extent's size descending
5. Use the size of the max free extent on the (num_stripes - 1)th device as the
stripe size to allocate the device space
By this way, the chunk allocator can allocate chunks as large as possible when
the devices' space is not enough and make full use of the devices.
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
* 'for-2.6.38/core' of git://git.kernel.dk/linux-2.6-block: (43 commits)
block: ensure that completion error gets properly traced
blktrace: add missing probe argument to block_bio_complete
block cfq: don't use atomic_t for cfq_group
block cfq: don't use atomic_t for cfq_queue
block: trace event block fix unassigned field
block: add internal hd part table references
block: fix accounting bug on cross partition merges
kref: add kref_test_and_get
bio-integrity: mark kintegrityd_wq highpri and CPU intensive
block: make kblockd_workqueue smarter
Revert "sd: implement sd_check_events()"
block: Clean up exit_io_context() source code.
Fix compile warnings due to missing removal of a 'ret' variable
fs/block: type signature of major_to_index(int) to major_to_index(unsigned)
block: convert !IS_ERR(p) && p to !IS_ERR_NOR_NULL(p)
cfq-iosched: don't check cfqg in choose_service_tree()
fs/splice: Pull buf->ops->confirm() from splice_from_pipe actors
cdrom: export cdrom_check_events()
sd: implement sd_check_events()
sr: implement sr_check_events()
...
* git://git.kernel.org/pub/scm/linux/kernel/git/mason/btrfs-unstable:
Btrfs: prevent RAID level downgrades when space is low
Btrfs: account for missing devices in RAID allocation profiles
Btrfs: EIO when we fail to read tree roots
Btrfs: fix compiler warnings
Btrfs: Make async snapshot ioctl more generic
Btrfs: pwrite blocked when writing from the mmaped buffer of the same page
Btrfs: Fix a crash when mounting a subvolume
Btrfs: fix sync subvol/snapshot creation
Btrfs: Fix page leak in compressed writeback path
Btrfs: do not BUG if we fail to remove the orphan item for dead snapshots
Btrfs: fixup return code for btrfs_del_orphan_item
Btrfs: do not do fast caching if we are allocating blocks for tree_root
Btrfs: deal with space cache errors better
Btrfs: fix use after free in O_DIRECT
When we mount in RAID degraded mode without adding a new device to
replace the failed one, we can end up using the wrong RAID flags for
allocations.
This results in strange combinations of block groups (raid1 in a raid10
filesystem) and corruptions when we try to allocate blocks from single
spindle chunks on drives that are actually missing.
The first device has two small 4MB chunks in it that mkfs creates and
these are usually unused in a raid1 or raid10 setup. But, in -o degraded,
the allocator will fall back to these because the mask of desired raid groups
isn't correct.
The fix here is to count the missing devices as we build up the list
of devices in the system. This count is used when picking the
raid level to make sure we continue using the same levels that were
in place before we lost a drive.
Signed-off-by: Chris Mason <chris.mason@oracle.com>
After recent blkdev_get() modifications, open_by_devnum() and
open_bdev_exclusive() are simple wrappers around blkdev_get().
Replace them with blkdev_get_by_dev() and blkdev_get_by_path().
blkdev_get_by_dev() is identical to open_by_devnum().
blkdev_get_by_path() is slightly different in that it doesn't
automatically add %FMODE_EXCL to @mode.
All users are converted. Most conversions are mechanical and don't
introduce any behavior difference. There are several exceptions.
* btrfs now sets FMODE_EXCL in btrfs_device->mode, so there's no
reason to OR it explicitly on blkdev_put().
* gfs2, nilfs2 and the generic mount_bdev() now set FMODE_EXCL in
sb->s_mode.
* With the above changes, sb->s_mode now always should contain
FMODE_EXCL. WARN_ON_ONCE() added to kill_block_super() to detect
errors.
The new blkdev_get_*() functions are with proper docbook comments.
While at it, add function description to blkdev_get() too.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Philipp Reisner <philipp.reisner@linbit.com>
Cc: Neil Brown <neilb@suse.de>
Cc: Mike Snitzer <snitzer@redhat.com>
Cc: Joern Engel <joern@lazybastard.org>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: Jan Kara <jack@suse.cz>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: KONISHI Ryusuke <konishi.ryusuke@lab.ntt.co.jp>
Cc: reiserfs-devel@vger.kernel.org
Cc: xfs-masters@oss.sgi.com
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Switch to the WRITE_FLUSH_FUA flag for log writes, remove the EOPNOTSUPP
detection for barriers and stop setting the barrier flag for discards.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Chris Mason <chris.mason@oracle.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
Currently, we can panic the box if the first block group we go to move is of a
type where there is no space left to move those extents. For example, if we
fill the disk up with data, and then we try to balance and we have no room to
move the data nor room to allocate new chunks, we will panic. Change this by
checking to see if we have room to move this chunk around, and if not, return
-ENOSPC and move on to the next chunk. This will make sure we remove block
groups that are moveable, like if we have alot of empty metadata block groups,
and then that way we make room to be able to balance our data chunks as well.
Tested this with an fs that would panic on btrfs-vol -b normally, but no longer
panics with this patch.
V1->V2:
-actually search for a free extent on the device to make sure we can allocate a
chunk if need be.
-fix btrfs_shrink_device to make sure we actually try to relocate all the
chunks, and then if we can't return -ENOSPC so if we are doing a btrfs-vol -r
we don't remove the device with data still on it.
-check to make sure the block group we are going to relocate isn't the last one
in that particular space
-fix a bug in btrfs_shrink_device where we would change the device's size and
not fix it if we fail to do our relocate
Signed-off-by: Josef Bacik <jbacik@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
On multi-device filesystems, btrfs writes supers to all of the devices
before considering a sync complete. There wasn't any additional
locking between super writeout and the device list management code
because device management was done inside a transaction and
super writeout only happened with no transation writers running.
With the btrfs fsync log and other async transaction updates, this
has been racey for some time. This adds a mutex to protect
the device list. The existing volume mutex could not be reused due to
transaction lock ordering requirements.
Signed-off-by: Chris Mason <chris.mason@oracle.com>
During mount, btrfs will check the queue nonrot flag
for all the devices found in the FS. If they are all
non-rotating, SSD mode is enabled by default.
If the FS was mounted with -o nossd, the non-rotating
flag is ignored.
Signed-off-by: Chris Mason <chris.mason@oracle.com>
Previously, we updated a device's size prior to attempting a shrink
operation. This patch moves the device resizing logic to only happen if
the shrink completes successfully. In the process, it introduces a new
field to btrfs_device -- disk_total_bytes -- to track the on-disk size.
Signed-off-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
Part of reducing fsync/O_SYNC/O_DIRECT latencies is using WRITE_SYNC for
writes we plan on waiting on in the near future. This patch
mirrors recent changes in other filesystems and the generic code to
use WRITE_SYNC when WB_SYNC_ALL is passed and to use WRITE_SYNC for
other latency critical writes.
Btrfs uses async worker threads for checksumming before the write is done,
and then again to actually submit the bios. The bio submission code just
runs a per-device list of bios that need to be sent down the pipe.
This list is split into low priority and high priority lists so the
WRITE_SYNC IO happens first.
Signed-off-by: Chris Mason <chris.mason@oracle.com>
