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The zone append operation processing for zloop devices is similar to any other command, that is, the operation is processed as a command work item, without any special serialization between the work items (beside the zone mutex for mutually exclusive code sections). This processing is fine and gives excellent performance. However, it has a side effect: zone append operation are very often reordered and processed in a sequence that is very different from their issuing order by the user. This effect is very visible using an XFS file system on top of a zloop device. A simple file write leads to many file extents as the data writes using zone append are reordered and so result in the physical order being different than the file logical order. E.g. executing: $ dd if=/dev/zero of=/mnt/test bs=1M count=10 && sync $ xfs_bmap /mnt/test /mnt/test: 0: [0..4095]: 2162688..2166783 1: [4096..6143]: 2168832..2170879 2: [6144..8191]: 2166784..2168831 3: [8192..10239]: 2170880..2172927 4: [10240..12287]: 2174976..2177023 5: [12288..14335]: 2172928..2174975 6: [14336..20479]: 2177024..2183167 For 10 IOs, 6 extents are created. This is fine and actually allows to exercise XFS zone garbage collection very well. However, this also makes debugging/working on XFS data placement harder as the underlying device will most of the time reorder IOs, resulting in many file extents. Allow a user to mitigate this with the new ordered_zone_append configuration parameter. For a zloop device created with this parameter specified, the sector of a zone append command is set early, when the command is submitted by the block layer with the zloop_queue_rq() function, instead of in the zloop_rw() function which is exectued later in the command work item context. This change ensures that more often than not, zone append operations data end up being written in the same order as the command submission by the user. In the case of XFS, this leads to far less file data extents. E.g., for the previous example, we get a single file data extent for the written file. $ dd if=/dev/zero of=/mnt/test bs=1M count=10 && sync $ xfs_bmap /mnt/test /mnt/test: 0: [0..20479]: 2162688..2183167 Since we cannot use a mutex in the context of the zloop_queue_rq() function to atomically set a zone append operation sector to the target zone write pointer location and increment that the write pointer, a new per-zone spinlock is introduced to protect a zone write pointer access and modifications. To check a zone write pointer location and set a zone append operation target sector to that value, the function zloop_set_zone_append_sector() is introduced and called from zloop_queue_rq(). Signed-off-by: Damien Le Moal <dlemoal@kernel.org> Signed-off-by: Jens Axboe <axboe@kernel.dk> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.