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| author | Mike Snitzer <snitzer@kernel.org> | 2025-11-11 09:59:32 -0500 |
|---|---|---|
| committer | Chuck Lever <chuck.lever@oracle.com> | 2025-12-01 09:57:10 -0500 |
| commit | fa8d4e6784d1b6a6eaa3911bac993181631d2856 (patch) | |
| tree | f67598a43c700f3f4eb5f60789cf244c6bea9f2a /Documentation/filesystems | |
| parent | 06c5c97293e3fca99ce15da157068edf45a7c6e4 (diff) | |
| download | tip-fa8d4e6784d1b6a6eaa3911bac993181631d2856.tar.gz | |
NFSD: add Documentation/filesystems/nfs/nfsd-io-modes.rst
This document details the NFSD IO modes that are configurable using
NFSD's experimental debugfs interfaces:
/sys/kernel/debug/nfsd/io_cache_read
/sys/kernel/debug/nfsd/io_cache_write
This document will evolve as NFSD's interfaces do (e.g. if/when NFSD's
debugfs interfaces are replaced with per-export controls).
Future updates will provide more specific guidance and howto
information to help others use and evaluate NFSD's IO modes:
BUFFERED, DONTCACHE and DIRECT.
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Diffstat (limited to 'Documentation/filesystems')
| -rw-r--r-- | Documentation/filesystems/nfs/nfsd-io-modes.rst | 144 |
1 files changed, 144 insertions, 0 deletions
diff --git a/Documentation/filesystems/nfs/nfsd-io-modes.rst b/Documentation/filesystems/nfs/nfsd-io-modes.rst new file mode 100644 index 00000000000000..e3a522d097666b --- /dev/null +++ b/Documentation/filesystems/nfs/nfsd-io-modes.rst @@ -0,0 +1,144 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============= +NFSD IO MODES +============= + +Overview +======== + +NFSD has historically always used buffered IO when servicing READ and +WRITE operations. BUFFERED is NFSD's default IO mode, but it is possible +to override that default to use either DONTCACHE or DIRECT IO modes. + +Experimental NFSD debugfs interfaces are available to allow the NFSD IO +mode used for READ and WRITE to be configured independently. See both: +- /sys/kernel/debug/nfsd/io_cache_read +- /sys/kernel/debug/nfsd/io_cache_write + +The default value for both io_cache_read and io_cache_write reflects +NFSD's default IO mode (which is NFSD_IO_BUFFERED=0). + +Based on the configured settings, NFSD's IO will either be: +- cached using page cache (NFSD_IO_BUFFERED=0) +- cached but removed from page cache on completion (NFSD_IO_DONTCACHE=1) +- not cached stable_how=NFS_UNSTABLE (NFSD_IO_DIRECT=2) + +To set an NFSD IO mode, write a supported value (0 - 2) to the +corresponding IO operation's debugfs interface, e.g.: + echo 2 > /sys/kernel/debug/nfsd/io_cache_read + echo 2 > /sys/kernel/debug/nfsd/io_cache_write + +To check which IO mode NFSD is using for READ or WRITE, simply read the +corresponding IO operation's debugfs interface, e.g.: + cat /sys/kernel/debug/nfsd/io_cache_read + cat /sys/kernel/debug/nfsd/io_cache_write + +If you experiment with NFSD's IO modes on a recent kernel and have +interesting results, please report them to linux-nfs@vger.kernel.org + +NFSD DONTCACHE +============== + +DONTCACHE offers a hybrid approach to servicing IO that aims to offer +the benefits of using DIRECT IO without any of the strict alignment +requirements that DIRECT IO imposes. To achieve this buffered IO is used +but the IO is flagged to "drop behind" (meaning associated pages are +dropped from the page cache) when IO completes. + +DONTCACHE aims to avoid what has proven to be a fairly significant +limition of Linux's memory management subsystem if/when large amounts of +data is infrequently accessed (e.g. read once _or_ written once but not +read until much later). Such use-cases are particularly problematic +because the page cache will eventually become a bottleneck to servicing +new IO requests. + +For more context on DONTCACHE, please see these Linux commit headers: +- Overview: 9ad6344568cc3 ("mm/filemap: change filemap_create_folio() + to take a struct kiocb") +- for READ: 8026e49bff9b1 ("mm/filemap: add read support for + RWF_DONTCACHE") +- for WRITE: 974c5e6139db3 ("xfs: flag as supporting FOP_DONTCACHE") + +NFSD_IO_DONTCACHE will fall back to NFSD_IO_BUFFERED if the underlying +filesystem doesn't indicate support by setting FOP_DONTCACHE. + +NFSD DIRECT +=========== + +DIRECT IO doesn't make use of the page cache, as such it is able to +avoid the Linux memory management's page reclaim scalability problems +without resorting to the hybrid use of page cache that DONTCACHE does. + +Some workloads benefit from NFSD avoiding the page cache, particularly +those with a working set that is significantly larger than available +system memory. The pathological worst-case workload that NFSD DIRECT has +proven to help most is: NFS client issuing large sequential IO to a file +that is 2-3 times larger than the NFS server's available system memory. +The reason for such improvement is NFSD DIRECT eliminates a lot of work +that the memory management subsystem would otherwise be required to +perform (e.g. page allocation, dirty writeback, page reclaim). When +using NFSD DIRECT, kswapd and kcompactd are no longer commanding CPU +time trying to find adequate free pages so that forward IO progress can +be made. + +The performance win associated with using NFSD DIRECT was previously +discussed on linux-nfs, see: +https://lore.kernel.org/linux-nfs/aEslwqa9iMeZjjlV@kernel.org/ +But in summary: +- NFSD DIRECT can significantly reduce memory requirements +- NFSD DIRECT can reduce CPU load by avoiding costly page reclaim work +- NFSD DIRECT can offer more deterministic IO performance + +As always, your mileage may vary and so it is important to carefully +consider if/when it is beneficial to make use of NFSD DIRECT. When +assessing comparative performance of your workload please be sure to log +relevant performance metrics during testing (e.g. memory usage, cpu +usage, IO performance). Using perf to collect perf data that may be used +to generate a "flamegraph" for work Linux must perform on behalf of your +test is a really meaningful way to compare the relative health of the +system and how switching NFSD's IO mode changes what is observed. + +If NFSD_IO_DIRECT is specified by writing 2 (or 3 and 4 for WRITE) to +NFSD's debugfs interfaces, ideally the IO will be aligned relative to +the underlying block device's logical_block_size. Also the memory buffer +used to store the READ or WRITE payload must be aligned relative to the +underlying block device's dma_alignment. + +But NFSD DIRECT does handle misaligned IO in terms of O_DIRECT as best +it can: + +Misaligned READ: + If NFSD_IO_DIRECT is used, expand any misaligned READ to the next + DIO-aligned block (on either end of the READ). The expanded READ is + verified to have proper offset/len (logical_block_size) and + dma_alignment checking. + +Misaligned WRITE: + If NFSD_IO_DIRECT is used, split any misaligned WRITE into a start, + middle and end as needed. The large middle segment is DIO-aligned + and the start and/or end are misaligned. Buffered IO is used for the + misaligned segments and O_DIRECT is used for the middle DIO-aligned + segment. DONTCACHE buffered IO is _not_ used for the misaligned + segments because using normal buffered IO offers significant RMW + performance benefit when handling streaming misaligned WRITEs. + +Tracing: + The nfsd_read_direct trace event shows how NFSD expands any + misaligned READ to the next DIO-aligned block (on either end of the + original READ, as needed). + + This combination of trace events is useful for READs: + echo 1 > /sys/kernel/tracing/events/nfsd/nfsd_read_vector/enable + echo 1 > /sys/kernel/tracing/events/nfsd/nfsd_read_direct/enable + echo 1 > /sys/kernel/tracing/events/nfsd/nfsd_read_io_done/enable + echo 1 > /sys/kernel/tracing/events/xfs/xfs_file_direct_read/enable + + The nfsd_write_direct trace event shows how NFSD splits a given + misaligned WRITE into a DIO-aligned middle segment. + + This combination of trace events is useful for WRITEs: + echo 1 > /sys/kernel/tracing/events/nfsd/nfsd_write_opened/enable + echo 1 > /sys/kernel/tracing/events/nfsd/nfsd_write_direct/enable + echo 1 > /sys/kernel/tracing/events/nfsd/nfsd_write_io_done/enable + echo 1 > /sys/kernel/tracing/events/xfs/xfs_file_direct_write/enable |