891568c990
To make better predictions on parallel workloads dmu_zfetch() should be called as early as possible to reduce possible request reordering. In particular, it should be called before dmu_buf_hold_array_by_dnode() calls dbuf_hold(), which may sleep waiting for indirect blocks, waking up multiple threads same time on completion, that can significantly reorder the requests, making the stream look like random. But we should not issue prefetch requests before the on-demand ones, since they may get to the disks first despite the I/O scheduler, increasing on-demand request latency. This patch splits dmu_zfetch() into two functions: dmu_zfetch_prepare() and dmu_zfetch_run(). The first can be executed as early as needed. It only updates statistics and makes predictions without issuing any I/Os. The I/O issuance is handled by dmu_zfetch_run(), which can be called later when all on-demand I/Os are already issued. It even tracks the activity of other concurrent threads, issuing the prefetch only when _all_ on-demand requests are issued. For many years it was a big problem for storage servers, handling deeper request queues from their clients, having to either serialize consequential reads to make ZFS prefetcher usable, or execute the incoming requests as-is and get almost no prefetch from ZFS, relying only on deep enough prefetch by the clients. Benefits of those ways varied, but neither was perfect. With this patch deeper queue sequential read benchmarks with CrystalDiskMark from Windows via iSCSI to FreeBSD target show me much better throughput with almost 100% prefetcher hit rate, comparing to almost zero before. While there, I also removed per-stream zs_lock as useless, completely covered by parent zf_lock. Also I reused zs_blocks refcount to track zf_stream linkage of the stream, since I believe previous zs_fetch == NULL check in dmu_zfetch_stream_done() was racy. Delete prefetch streams when they reach ends of files. It saves up to 1KB of RAM per file, plus reduces searches through the stream list. Block data prefetch (speculation and indirect block prefetch is still done since they are cheaper) if all dbufs of the stream are already in DMU cache. First cache miss immediately fires all the prefetch that would be done for the stream by that time. It saves some CPU time if same files within DMU cache capacity are read over and over. Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: Adam Moss <c@yotes.com> Reviewed-by: Matthew Ahrens <mahrens@delphix.com> Signed-off-by: Alexander Motin <mav@FreeBSD.org> Sponsored-By: iXsystems, Inc. Closes #11652 |
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README.md
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.