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551bf13ba8
Currently we track space used in the log by grant heads. These store the reserved space as a physical log location and combine both space reserved for future use with space already used in the log in a single variable. The amount of space consumed in the log is then calculated as the distance between the log tail and the grant head. The problem with tracking the grant head as a physical location comes from the fact that it tracks both log cycle count and offset into the log in bytes in a single 64 bit variable. because the cycle count on disk is a 32 bit number, this also limits the offset into the log to 32 bits. ANd because that is in bytes, we are limited to being able to track only 2GB of log space in the grant head. Hence to support larger physical logs, we need to track used space differently in the grant head. We no longer use the grant head for guiding AIL pushing, so the only thing it is now used for is determining if we've run out of reservation space via the calculation in xlog_space_left(). What we really need to do is move the grant heads away from tracking physical space in the log. The issue here is that space consumed in the log is not directly tracked by the current mechanism - the space consumed in the log by grant head reservations gets returned to the free pool by the tail of the log moving forward. i.e. the space isn't directly tracked or calculated, but the used grant space gets "freed" as the physical limits of the log are updated without actually needing to update the grant heads. Hence to move away from implicit, zero-update log space tracking we need to explicitly track the amount of physical space the log actually consumes separately to the in-memory reservations for operations that will be committed to the journal. Luckily, we already track the information we need to calculate this in the AIL itself. That is, the space currently consumed by the journal is the maximum LSN that the AIL has seen minus the current log tail. As we update both of these items dynamically as the head and tail of the log moves, we always know exactly how much space the journal consumes. This means that we also know exactly how much space the currently active reservations require, and exactly how much free space we have remaining for new reservations to be made. Most importantly, we know what these spaces are indepedently of the physical locations of the head and tail of the log. Hence by separating out the physical space consumed by the journal, we can now track reservations in the grant heads purely as a byte count, and the log can be considered full when the tail space + reservation space exceeds the size of the log. This means we can use the full 64 bits of grant head space for reservation space, completely removing the 32 bit byte count limitation on log size that they impose. Hence the first step in this conversion is to track and update the "log tail space" every time the AIL tail or maximum seen LSN changes. Signed-off-by: Dave Chinner <dchinner@redhat.com> Reviewed-by: Darrick J. Wong <djwong@kernel.org> Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Chandan Babu R <chandanbabu@kernel.org> |
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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.