cc92e9d0c3
3 Commits
| Author | SHA1 | Message | Date | |
|---|---|---|---|---|
Etienne Dechamps
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920dd524fb |
Add FASTWRITE algorithm for synchronous writes.
Currently, ZIL blocks are spread over vdevs using hint block pointers managed by the ZIL commit code and passed to metaslab_alloc(). Spreading log blocks accross vdevs is important for performance: indeed, using mutliple disks in parallel decreases the ZIL commit latency, which is the main performance metric for synchronous writes. However, the current implementation suffers from the following issues: 1) It would be best if the ZIL module was not aware of such low-level details. They should be handled by the ZIO and metaslab modules; 2) Because the hint block pointer is managed per log, simultaneous commits from multiple logs might use the same vdevs at the same time, which is inefficient; 3) Because dmu_write() does not honor the block pointer hint, indirect writes are not spread. The naive solution of rotating the metaslab rotor each time a block is allocated for the ZIL or dmu_sync() doesn't work in practice because the first ZIL block to be written is actually allocated during the previous commit. Consequently, when metaslab_alloc() decides the vdev for this block, it will do so while a bunch of other allocations are happening at the same time (from dmu_sync() and other ZILs). This means the vdev for this block is chosen more or less at random. When the next commit happens, there is a high chance (especially when the number of blocks per commit is slightly less than the number of the disks) that one disk will have to write two blocks (with a potential seek) while other disks are sitting idle, which defeats spreading and increases the commit latency. This commit introduces a new concept in the metaslab allocator: fastwrites. Basically, each top-level vdev maintains a counter indicating the number of synchronous writes (from dmu_sync() and the ZIL) which have been allocated but not yet completed. When the metaslab is called with the FASTWRITE flag, it will choose the vdev with the least amount of pending synchronous writes. If there are multiple vdevs with the same value, the first matching vdev (starting from the rotor) is used. Once metaslab_alloc() has decided which vdev the block is allocated to, it updates the fastwrite counter for this vdev. The rationale goes like this: when an allocation is done with FASTWRITE, it "reserves" the vdev until the data is written. Until then, all future allocations will naturally avoid this vdev, even after a full rotation of the rotor. As a result, pending synchronous writes at a given point in time will be nicely spread over all vdevs. This contrasts with the previous algorithm, which is based on the implicit assumption that blocks are written instantaneously after they're allocated. metaslab_fastwrite_mark() and metaslab_fastwrite_unmark() are used to manually increase or decrease fastwrite counters, respectively. They should be used with caution, as there is no per-BP tracking of fastwrite information, so leaks and "double-unmarks" are possible. There is, however, an assert in the vdev teardown code which will fire if the fastwrite counters are not zero when the pool is exported or the vdev removed. Note that as stated above, marking is also done implictly by metaslab_alloc(). ZIO also got a new FASTWRITE flag; when it is used, ZIO will pass it to the metaslab when allocating (assuming ZIO does the allocation, which is only true in the case of dmu_sync). This flag will also trigger an unmark when zio_done() fires. A side-effect of the new algorithm is that when a ZIL stops being used, its last block can stay in the pending state (allocated but not yet written) for a long time, polluting the fastwrite counters. To avoid that, I've implemented a somewhat crude but working solution which unmarks these pending blocks in zil_sync(), thus guaranteeing that linguering fastwrites will get pruned at each sync event. The best performance improvements are observed with pools using a large number of top-level vdevs and heavy synchronous write workflows (especially indirect writes and concurrent writes from multiple ZILs). Real-life testing shows a 200% to 300% performance increase with indirect writes and various commit sizes. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Issue #1013 |
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George Wilson
|
6d974228ef |
Illumos #1051: zfs should handle imbalanced luns
Today zfs tries to allocate blocks evenly across all devices. This means when devices are imbalanced zfs will use lots of CPU searching for space on devices which tend to be pretty full. It should instead fail quickly on the full LUNs and move onto devices which have more availability. Reviewed by: Eric Schrock <Eric.Schrock@delphix.com> Reviewed by: Matt Ahrens <Matt.Ahrens@delphix.com> Reviewed by: Adam Leventhal <Adam.Leventhal@delphix.com> Reviewed by: Albert Lee <trisk@nexenta.com> Reviewed by: Gordon Ross <gwr@nexenta.com> Approved by: Garrett D'Amore <garrett@nexenta.com> References to Illumos issue and patch: - https://www.illumos.org/issues/510 - https://github.com/illumos/illumos-gate/commit/5ead3ed965 Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Issue #340 |
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Brian Behlendorf
|
6283f55ea1 |
Support custom build directories and move includes
One of the neat tricks an autoconf style project is capable of is allow configurion/building in a directory other than the source directory. The major advantage to this is that you can build the project various different ways while making changes in a single source tree. For example, this project is designed to work on various different Linux distributions each of which work slightly differently. This means that changes need to verified on each of those supported distributions perferably before the change is committed to the public git repo. Using nfs and custom build directories makes this much easier. I now have a single source tree in nfs mounted on several different systems each running a supported distribution. When I make a change to the source base I suspect may break things I can concurrently build from the same source on all the systems each in their own subdirectory. wget -c http://github.com/downloads/behlendorf/zfs/zfs-x.y.z.tar.gz tar -xzf zfs-x.y.z.tar.gz cd zfs-x-y-z ------------------------- run concurrently ---------------------- <ubuntu system> <fedora system> <debian system> <rhel6 system> mkdir ubuntu mkdir fedora mkdir debian mkdir rhel6 cd ubuntu cd fedora cd debian cd rhel6 ../configure ../configure ../configure ../configure make make make make make check make check make check make check This change also moves many of the include headers from individual incude/sys directories under the modules directory in to a single top level include directory. This has the advantage of making the build rules cleaner and logically it makes a bit more sense. |