- 10 6月, 2009 3 次提交
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由 Chris Mason 提交于
The btrfs IO submission threads try to service a bunch of devices with a small number of threads. They do a congestion check to try and avoid waiting on requests for a busy device. The checks make sure we've sent a few requests down to a given device just so that we aren't bouncing between busy devices without actually sending down any IO. The counter used to decide if we can switch to the next device is somewhat overloaded. It is also being used to decide if we've done a good batch of requests between the WRITE_SYNC or regular priority lists. It may get reset to zero often, leaving us hammering on a busy device instead of moving on to another disk. This commit adds a new counter for the number of bios sent while servicing a device. It doesn't get reset or fiddled with. On multi-device filesystems, this fixes IO stalls in streaming write workloads. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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由 Chris Mason 提交于
Btrfs uses dedicated threads to submit bios when checksumming is on, which allows us to make sure the threads dedicated to checksumming don't get stuck waiting for requests. For each btrfs device, there are two lists of bios. One list is for WRITE_SYNC bios and the other is for regular priority bios. The IO submission threads used to process all of the WRITE_SYNC bios first and then switch to the regular bios. This commit makes sure we don't completely starve the regular bios by rotating between the two lists. WRITE_SYNC bios are still favored 2:1 over the regular bios, and this tries to run in batches to avoid seeking. Benchmarking shows this eliminates stalls during streaming buffered writes on both multi-device and single device filesystems. If the regular bios starve, the system can end up with a large amount of ram pinned down in writeback pages. If we are a little more fair between the two classes, we're able to keep throughput up and make progress on the bulk of our dirty ram. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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由 Yan Zheng 提交于
This commit introduces a new kind of back reference for btrfs metadata. Once a filesystem has been mounted with this commit, IT WILL NO LONGER BE MOUNTABLE BY OLDER KERNELS. When a tree block in subvolume tree is cow'd, the reference counts of all extents it points to are increased by one. At transaction commit time, the old root of the subvolume is recorded in a "dead root" data structure, and the btree it points to is later walked, dropping reference counts and freeing any blocks where the reference count goes to 0. The increments done during cow and decrements done after commit cancel out, and the walk is a very expensive way to go about freeing the blocks that are no longer referenced by the new btree root. This commit reduces the transaction overhead by avoiding the need for dead root records. When a non-shared tree block is cow'd, we free the old block at once, and the new block inherits old block's references. When a tree block with reference count > 1 is cow'd, we increase the reference counts of all extents the new block points to by one, and decrease the old block's reference count by one. This dead tree avoidance code removes the need to modify the reference counts of lower level extents when a non-shared tree block is cow'd. But we still need to update back ref for all pointers in the block. This is because the location of the block is recorded in the back ref item. We can solve this by introducing a new type of back ref. The new back ref provides information about pointer's key, level and in which tree the pointer lives. This information allow us to find the pointer by searching the tree. The shortcoming of the new back ref is that it only works for pointers in tree blocks referenced by their owner trees. This is mostly a problem for snapshots, where resolving one of these fuzzy back references would be O(number_of_snapshots) and quite slow. The solution used here is to use the fuzzy back references in the common case where a given tree block is only referenced by one root, and use the full back references when multiple roots have a reference on a given block. This commit adds per subvolume red-black tree to keep trace of cached inodes. The red-black tree helps the balancing code to find cached inodes whose inode numbers within a given range. This commit improves the balancing code by introducing several data structures to keep the state of balancing. The most important one is the back ref cache. It caches how the upper level tree blocks are referenced. This greatly reduce the overhead of checking back ref. The improved balancing code scales significantly better with a large number of snapshots. This is a very large commit and was written in a number of pieces. But, they depend heavily on the disk format change and were squashed together to make sure git bisect didn't end up in a bad state wrt space balancing or the format change. Signed-off-by: NYan Zheng <zheng.yan@oracle.com> Signed-off-by: NChris Mason <chris.mason@oracle.com>
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- 04 6月, 2009 1 次提交
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由 Yan Zheng 提交于
It was not being properly initialized, and so the size saved to disk was not correct. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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- 27 4月, 2009 1 次提交
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由 Chris Ball 提交于
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: NChris Ball <cjb@laptop.org> Signed-off-by: NChris Mason <chris.mason@oracle.com>
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- 21 4月, 2009 1 次提交
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由 Chris Mason 提交于
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: NChris Mason <chris.mason@oracle.com>
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- 03 4月, 2009 2 次提交
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由 Chris Mason 提交于
Btrfs pages being written get set to writeback, and then may go through a number of steps before they hit the block layer. This includes compression, checksumming and async bio submission. The end result is that someone who writes a page and then does wait_on_page_writeback is likely to unplug the queue before the bio they cared about got there. We could fix this by marking bios sync, or by doing more frequent unplugs, but this commit just changes the async bio submission code to unplug after it has processed all the bios for a device. The async bio submission does a fair job of collection bios, so this shouldn't be a huge problem for reducing merging at the elevator. For streaming O_DIRECT writes on a 5 drive array, it boosts performance from 386MB/s to 460MB/s. Thanks to Hisashi Hifumi for helping with this work. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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由 Chris Mason 提交于
Btrfs uses async helper threads to submit write bios so the checksumming helper threads don't block on the disk. The submit bio threads may process bios for more than one block device, so when they find one device congested they try to move on to other devices instead of blocking in get_request_wait for one device. This does a pretty good job of keeping multiple devices busy, but the congested flag has a number of problems. A congested device may still give you a request, and other procs that aren't backing off the congested device may starve you out. This commit uses the io_context stored in current to decide if our process has been made a batching process by the block layer. If so, it keeps sending IO down for at least one batch. This helps make sure we do a good amount of work each time we visit a bdev, and avoids large IO stalls in multi-device workloads. It's also very ugly. A better solution is in the works with Jens Axboe. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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- 11 3月, 2009 2 次提交
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由 Chris Mason 提交于
The full flag on the space info structs tells the allocator not to try and allocate more chunks because the devices in the FS are fully allocated. When more devices are added, we need to clear the full flag so the allocator knows it has more space available. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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由 Chris Mason 提交于
Storage allocated to different raid levels in btrfs is tracked by a btrfs_space_info structure, and all of the current space_infos are collected into a list_head. Most filesystems have 3 or 4 of these structs total, and the list is only changed when new raid levels are added or at unmount time. This commit adds rcu locking on the list head, and properly frees things at unmount time. It also clears the space_info->full flag whenever new space is added to the FS. The locking for the space info list goes like this: reads: protected by rcu_read_lock() writes: protected by the chunk_mutex At unmount time we don't need special locking because all the readers are gone. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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- 13 2月, 2009 1 次提交
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由 Chris Mason 提交于
Btrfs is currently using spin_lock_nested with a nested value based on the tree depth of the block. But, this doesn't quite work because the max tree depth is bigger than what spin_lock_nested can deal with, and because locks are sometimes taken before the level field is filled in. The solution here is to use lockdep_set_class_and_name instead, and to set the class before unlocking the pages when the block is read from the disk and just after init of a freshly allocated tree block. btrfs_clear_path_blocking is also changed to take the locks in the proper order, and it also makes sure all the locks currently held are properly set to blocking before it tries to retake the spinlocks. Otherwise, lockdep gets upset about bad lock orderin. The lockdep magic cam from Peter Zijlstra <peterz@infradead.org> Signed-off-by: NChris Mason <chris.mason@oracle.com>
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- 12 2月, 2009 1 次提交
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由 Julia Lawall 提交于
The call to kzalloc is followed by a kmalloc whose result is stored in the same variable. The semantic match that finds the problem is as follows: (http://www.emn.fr/x-info/coccinelle/) // <smpl> @r exists@ local idexpression x; statement S; expression E; identifier f,l; position p1,p2; expression *ptr != NULL; @@ ( if ((x@p1 = \(kmalloc\|kzalloc\|kcalloc\)(...)) == NULL) S | x@p1 = \(kmalloc\|kzalloc\|kcalloc\)(...); ... if (x == NULL) S ) <... when != x when != if (...) { <+...x...+> } x->f = E ...> ( return \(0\|<+...x...+>\|ptr\); | return@p2 ...; ) @script:python@ p1 << r.p1; p2 << r.p2; @@ print "* file: %s kmalloc %s return %s" % (p1[0].file,p1[0].line,p2[0].line) // </smpl> Signed-off-by: NJulia Lawall <julia@diku.dk> Signed-off-by: NChris Mason <chris.mason@oracle.com>
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- 04 2月, 2009 1 次提交
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由 Chris Mason 提交于
The async bio submission thread was missing some bios that were added after it had decided there was no work left to do. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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- 21 1月, 2009 3 次提交
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由 Qinghuang Feng 提交于
Merge list_for_each* and list_entry to list_for_each_entry* Signed-off-by: NQinghuang Feng <qhfeng.kernel@gmail.com> Signed-off-by: NChris Mason <chris.mason@oracle.com>
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由 Roland Dreier 提交于
The "devid <xxx> transid <xxx>" printk in btrfs_scan_one_device() actually follows another printk that doesn't end in a newline (since the intention is for the two printks to make one line of output), so the KERN_INFO just ends up messing up the output: device label exp <6>devid 1 transid 9 /dev/sda5 Fix this by changing the extra KERN_INFO to KERN_CONT. Signed-off-by: NRoland Dreier <rolandd@cisco.com> Signed-off-by: NChris Mason <chris.mason@oracle.com>
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由 Huang Weiyi 提交于
Removed unused #include <version.h>'s in btrfs Signed-off-by: NHuang Weiyi <weiyi.huang@gmail.com> Signed-off-by: NChris Mason <chris.mason@oracle.com>
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- 17 1月, 2009 1 次提交
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由 Chris Mason 提交于
Btrfs maintains a queue of async bio submissions so the checksumming threads don't have to wait on get_request_wait. In order to avoid extra wakeups, this code has a running_pending flag that is used to tell new submissions they don't need to wake the thread. When the threads notice congestion on a single device, they may decide to requeue the job and move on to other devices. This makes sure the running_pending flag is cleared before the job is requeued. It should help avoid IO stalls by making sure the task is woken up when new submissions come in. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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- 06 1月, 2009 1 次提交
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由 Chris Mason 提交于
There were many, most are fixed now. struct-funcs.c generates some warnings but these are bogus. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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- 12 12月, 2008 1 次提交
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由 Yan Zheng 提交于
This patch makes seed device possible to be shared by multiple mounted file systems. The sharing is achieved by cloning seed device's btrfs_fs_devices structure. Thanks you, Signed-off-by: NYan Zheng <zheng.yan@oracle.com>
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- 09 12月, 2008 4 次提交
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由 Chris Mason 提交于
This adds a sequence number to the btrfs inode that is increased on every update. NFS will be able to use that to detect when an inode has changed, without relying on inaccurate time fields. While we're here, this also: Puts reserved space into the super block and inode Adds a log root transid to the super so we can pick the newest super based on the fsync log as well as the main transaction ID. For now the log root transid is always zero, but that'll get fixed. Adds a starting offset to the dev_item. This will let us do better alignment calculations if we know the start of a partition on the disk. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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由 Chris Mason 提交于
It is possible that generic_bin_search will be called on a tree block that has not been locked. This happens because cache_block_block skips locking on the tree blocks. Since the tree block isn't locked, we aren't allowed to change the extent_buffer->map_token field. Using map_private_extent_buffer avoids any changes to the internal extent buffer fields. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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由 Yan Zheng 提交于
This patch implements superblock duplication. Superblocks are stored at offset 16K, 64M and 256G on every devices. Spaces used by superblocks are preserved by the allocator, which uses a reverse mapping function to find the logical addresses that correspond to superblocks. Thank you, Signed-off-by: NYan Zheng <zheng.yan@oracle.com>
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由 Chris Mason 提交于
Btrfs stores checksums for each data block. Until now, they have been stored in the subvolume trees, indexed by the inode that is referencing the data block. This means that when we read the inode, we've probably read in at least some checksums as well. But, this has a few problems: * The checksums are indexed by logical offset in the file. When compression is on, this means we have to do the expensive checksumming on the uncompressed data. It would be faster if we could checksum the compressed data instead. * If we implement encryption, we'll be checksumming the plain text and storing that on disk. This is significantly less secure. * For either compression or encryption, we have to get the plain text back before we can verify the checksum as correct. This makes the raid layer balancing and extent moving much more expensive. * It makes the front end caching code more complex, as we have touch the subvolume and inodes as we cache extents. * There is potentitally one copy of the checksum in each subvolume referencing an extent. The solution used here is to store the extent checksums in a dedicated tree. This allows us to index the checksums by phyiscal extent start and length. It means: * The checksum is against the data stored on disk, after any compression or encryption is done. * The checksum is stored in a central location, and can be verified without following back references, or reading inodes. This makes compression significantly faster by reducing the amount of data that needs to be checksummed. It will also allow much faster raid management code in general. The checksums are indexed by a key with a fixed objectid (a magic value in ctree.h) and offset set to the starting byte of the extent. This allows us to copy the checksum items into the fsync log tree directly (or any other tree), without having to invent a second format for them. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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- 02 12月, 2008 2 次提交
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由 Christoph Hellwig 提交于
Make sure to propagate fmode_t properly and use the right constants for it. Signed-off-by: NChristoph Hellwig <hch@lst.de>
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由 Christoph Hellwig 提交于
Shut up various sparse warnings about symbols that should be either static or have their declarations in scope. Signed-off-by: NChristoph Hellwig <hch@lst.de>
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- 20 11月, 2008 2 次提交
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由 Chris Mason 提交于
The btrfs git kernel trees is used to build a standalone tree for compiling against older kernels. This commit makes the standalone tree work with 2.6.27 Signed-off-by: NChris Mason <chris.mason@oracle.com>
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由 Chris Mason 提交于
* open/close_bdev_excl -> open/close_bdev_exclusive * blkdev_issue_discard takes a GFP mask now * Fix blkdev_issue_discard usage now that it is enabled Signed-off-by: NChris Mason <chris.mason@oracle.com>
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- 13 11月, 2008 1 次提交
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由 yanhai zhu 提交于
Add a missing kzalloc() return pointer check in add_missing_dev(). Signed-off-by: NChris Mason <chris.mason@oracle.com>
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- 18 11月, 2008 1 次提交
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由 Yan Zheng 提交于
Seed device is a special btrfs with SEEDING super flag set and can only be mounted in read-only mode. Seed devices allow people to create new btrfs on top of it. The new FS contains the same contents as the seed device, but it can be mounted in read-write mode. This patch does the following: 1) split code in btrfs_alloc_chunk into two parts. The first part does makes the newly allocated chunk usable, but does not do any operation that modifies the chunk tree. The second part does the the chunk tree modifications. This division is for the bootstrap step of adding storage to the seed device. 2) Update device management code to handle seed device. The basic idea is: For an FS grown from seed devices, its seed devices are put into a list. Seed devices are opened on demand at mounting time. If any seed device is missing or has been changed, btrfs kernel module will refuse to mount the FS. 3) make btrfs_find_block_group not return NULL when all block groups are read-only. Signed-off-by: NYan Zheng <zheng.yan@oracle.com>
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- 08 11月, 2008 1 次提交
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由 Chris Mason 提交于
While doing a commit, btrfs makes sure all the metadata blocks were properly written to disk, calling wait_on_page_writeback for each page. This writeback happens after allowing another transaction to start, so it competes for the disk with other processes in the FS. If the page writeback bit is still set, each wait_on_page_writeback might trigger an unplug, even though the page might be waiting for checksumming to finish or might be waiting for the async work queue to submit the bio. This trades wait_on_page_writeback for waiting on the extent writeback bits. It won't trigger any unplugs and substantially improves performance in a number of workloads. This also changes the async bio submission to avoid requeueing if there is only one device. The requeue just wastes CPU time because there are no other devices to service. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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- 30 10月, 2008 2 次提交
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由 Josef Bacik 提交于
This patch removes the giant fs_info->alloc_mutex and replaces it with a bunch of little locks. There is now a pinned_mutex, which is used when messing with the pinned_extents extent io tree, and the extent_ins_mutex which is used with the pending_del and extent_ins extent io trees. The locking for the extent tree stuff was inspired by a patch that Yan Zheng wrote to fix a race condition, I cleaned it up some and changed the locking around a little bit, but the idea remains the same. Basically instead of holding the extent_ins_mutex throughout the processing of an extent on the extent_ins or pending_del trees, we just hold it while we're searching and when we clear the bits on those trees, and lock the extent for the duration of the operations on the extent. Also to keep from getting hung up waiting to lock an extent, I've added a try_lock_extent so if we cannot lock the extent, move on to the next one in the tree and we'll come back to that one. I have tested this heavily and it does not appear to break anything. This has to be applied on top of my find_free_extent redo patch. I tested this patch on top of Yan's space reblancing code and it worked fine. The only thing that has changed since the last version is I pulled out all my debugging stuff, apparently I forgot to run guilt refresh before I sent the last patch out. Thank you, Signed-off-by: NJosef Bacik <jbacik@redhat.com>
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由 Chris Mason 提交于
This is a large change for adding compression on reading and writing, both for inline and regular extents. It does some fairly large surgery to the writeback paths. Compression is off by default and enabled by mount -o compress. Even when the -o compress mount option is not used, it is possible to read compressed extents off the disk. If compression for a given set of pages fails to make them smaller, the file is flagged to avoid future compression attempts later. * While finding delalloc extents, the pages are locked before being sent down to the delalloc handler. This allows the delalloc handler to do complex things such as cleaning the pages, marking them writeback and starting IO on their behalf. * Inline extents are inserted at delalloc time now. This allows us to compress the data before inserting the inline extent, and it allows us to insert an inline extent that spans multiple pages. * All of the in-memory extent representations (extent_map.c, ordered-data.c etc) are changed to record both an in-memory size and an on disk size, as well as a flag for compression. From a disk format point of view, the extent pointers in the file are changed to record the on disk size of a given extent and some encoding flags. Space in the disk format is allocated for compression encoding, as well as encryption and a generic 'other' field. Neither the encryption or the 'other' field are currently used. In order to limit the amount of data read for a single random read in the file, the size of a compressed extent is limited to 128k. This is a software only limit, the disk format supports u64 sized compressed extents. In order to limit the ram consumed while processing extents, the uncompressed size of a compressed extent is limited to 256k. This is a software only limit and will be subject to tuning later. Checksumming is still done on compressed extents, and it is done on the uncompressed version of the data. This way additional encodings can be layered on without having to figure out which encoding to checksum. Compression happens at delalloc time, which is basically singled threaded because it is usually done by a single pdflush thread. This makes it tricky to spread the compression load across all the cpus on the box. We'll have to look at parallel pdflush walks of dirty inodes at a later time. Decompression is hooked into readpages and it does spread across CPUs nicely. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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- 04 10月, 2008 1 次提交
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由 Chris Mason 提交于
On 32 bit machines without CONFIG_LBD, the bi_sector field is only 32 bits. Btrfs needs to cast it before shifting up, or we end up doing IO into the wrong place. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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- 29 9月, 2008 1 次提交
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由 Chris Mason 提交于
btrfs-vol -a /dev/xxx will zero the first and last two MB of the device. The kernel code needs to wait for this IO to finish before it adds the device. btrfs metadata IO does not happen through the block device inode. A separate address space is used, allowing the zero filled buffer heads in the block device inode to be written to disk after FS metadata starts going down to the disk via the btrfs metadata inode. The end result is zero filled metadata blocks after adding new devices into the filesystem. The fix is a simple filemap_write_and_wait on the block device inode before actually inserting it into the pool of available devices. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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- 26 9月, 2008 2 次提交
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由 Zheng Yan 提交于
This patch updates the space balancing code to utilize the new backref format. Before, btrfs-vol -b would break any COW links on data blocks or metadata. This was slow and caused the amount of space used to explode if a large number of snapshots were present. The new code can keeps the sharing of all data extents and most of the tree blocks. To maintain the sharing of data extents, the space balance code uses a seperate inode hold data extent pointers, then updates the references to point to the new location. To maintain the sharing of tree blocks, the space balance code uses reloc trees to relocate tree blocks in reference counted roots. There is one reloc tree for each subvol, and all reloc trees share same root key objectid. Reloc trees are snapshots of the latest committed roots of subvols (root->commit_root). To relocate a tree block referenced by a subvol, there are two steps. COW the block through subvol's reloc tree, then update block pointer in the subvol to point to the new block. Since all reloc trees share same root key objectid, doing special handing for tree blocks owned by them is easy. Once a tree block has been COWed in one reloc tree, we can use the resulting new block directly when the same block is required to COW again through other reloc trees. In this way, relocated tree blocks are shared between reloc trees, so they are also shared between subvols. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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由 Chris Mason 提交于
Btrfs had compatibility code for kernels back to 2.6.18. These have been removed, and will be maintained in a separate backport git tree from now on. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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- 25 9月, 2008 4 次提交
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由 Josef Bacik 提交于
1) replace the per fs_info extent_io_tree that tracked free space with two rb-trees per block group to track free space areas via offset and size. The reason to do this is because most allocations come with a hint byte where to start, so we can usually find a chunk of free space at that hint byte to satisfy the allocation and get good space packing. If we cannot find free space at or after the given offset we fall back on looking for a chunk of the given size as close to that given offset as possible. When we fall back on the size search we also try to find a slot as close to the size we want as possible, to avoid breaking small chunks off of huge areas if possible. 2) remove the extent_io_tree that tracked the block group cache from fs_info and replaced it with an rb-tree thats tracks block group cache via offset. also added a per space_info list that tracks the block group cache for the particular space so we can lookup related block groups easily. 3) cleaned up the allocation code to make it a little easier to read and a little less complicated. Basically there are 3 steps, first look from our provided hint. If we couldn't find from that given hint, start back at our original search start and look for space from there. If that fails try to allocate space if we can and start looking again. If not we're screwed and need to start over again. 4) small fixes. there were some issues in volumes.c where we wouldn't allocate the rest of the disk. fixed cow_file_range to actually pass the alloc_hint, which has helped a good bit in making the fs_mark test I run have semi-normal results as we run out of space. Generally with data allocations we don't track where we last allocated from, so everytime we did a data allocation we'd search through every block group that we have looking for free space. Now searching a block group with no free space isn't terribly time consuming, it was causing a slight degradation as we got more data block groups. The alloc_hint has fixed this slight degredation and made things semi-normal. There is still one nagging problem I'm working on where we will get ENOSPC when there is definitely plenty of space. This only happens with metadata allocations, and only when we are almost full. So you generally hit the 85% mark first, but sometimes you'll hit the BUG before you hit the 85% wall. I'm still tracking it down, but until then this seems to be pretty stable and make a significant performance gain. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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由 Zheng Yan 提交于
--- Signed-off-by: NChris Mason <chris.mason@oracle.com>
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由 Chris Mason 提交于
Signed-off-by: NChris Mason <chris.mason@oracle.com>
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由 Chris Mason 提交于
The current code waits for the count of async bio submits to get below a given threshold if it is too high right after adding the latest bio to the work queue. This isn't optimal because the caller may have sequential adjacent bios pending they are waiting to send down the pipe. This changeset requires the caller to wait on the async bio count, and changes the async checksumming submits to wait for async bios any time they self throttle. The end result is much higher sequential throughput. Signed-off-by: NChris Mason <chris.mason@oracle.com>
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