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Prefetched buffers are currently read from L2ARC if, and only if, l2arc_noprefetch is set to non-default value of 0. This means that a streaming read which can be served from L2ARC will instead engage the main pool. For example, consider what happens when a file is sequentially read: - application requests contiguous data, engaging the prefetcher; - ARC buffers are initially marked as prefetched but, as the calling application consumes data, the prefetch tag is cleared; - these "normal" buffers become eligible for L2ARC and are copied to it; - re-reading the same file will *not* engage L2ARC even if it contains the required buffers; - main pool has to suffer another sequential read load, which (due to most NCQ-enabled HDDs preferring sequential loads) can dramatically increase latency for uncached random reads. In other words, current behavior is to write data to L2ARC (wearing it) without using the very same cache when reading back the same data. This was probably useful many years ago to preserve L2ARC read bandwidth but, with current SSD speed/size/price, it is vastly sub-optimal. Setting l2arc_noprefetch=1, while enabling L2ARC to serve these reads, means that even prefetched but unused buffers will be copied into L2ARC, further increasing wear and load for potentially not-useful data. This patch enable prefetched buffer to be read from L2ARC even when l2arc_noprefetch=1 (default), increasing sequential read speed and reducing load on the main pool without polluting L2ARC with not-useful (ie: unused) prefetched data. Moreover, it clear users confusion about L2ARC size increasing but not serving any IO when doing sequential reads. Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: Alexander Motin <mav@FreeBSD.org> Signed-off-by: Gionatan Danti <g.danti@assyoma.it> Closes #15451 |
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OpenZFS is an advanced file system and volume manager which was originally developed for Solaris and is now maintained by the OpenZFS community. This repository contains the code for running OpenZFS on Linux and FreeBSD.
Official Resources
- Documentation - for using and developing this repo
- ZoL Site - Linux release info & links
- Mailing lists
- OpenZFS site - for conference videos and info on other platforms (illumos, OSX, Windows, etc)
Installation
Full documentation for installing OpenZFS on your favorite operating system can be found at the Getting Started Page.
Contribute & Develop
We have a separate document with contribution guidelines.
We have a Code of Conduct.
Release
OpenZFS is released under a CDDL license.
For more details see the NOTICE, LICENSE and COPYRIGHT files; UCRL-CODE-235197
Supported Kernels
- The
METAfile contains the officially recognized supported Linux kernel versions. - Supported FreeBSD versions are any supported branches and releases starting from 12.2-RELEASE.