Currently the ARC state (MFU/MRU) of cached L2ARC buffer and their
content type is unknown. Knowing this information may prove beneficial
in adjusting the L2ARC caching policy.
This commit adds L2ARC arcstats that display the aligned size
(in bytes) of L2ARC buffers according to their content type
(data/metadata) and according to their ARC state (MRU/MFU or
prefetch). It also expands the existing evict_l2_eligible arcstat to
differentiate between MFU and MRU buffers.
L2ARC caches buffers from the MRU and MFU lists of ARC. Upon caching a
buffer, its ARC state (MRU/MFU) is stored in the L2 header
(b_arcs_state). The l2_m{f,r}u_asize arcstats reflect the aligned size
(in bytes) of L2ARC buffers according to their ARC state (based on
b_arcs_state). We also account for the case where an L2ARC and ARC
cached MRU or MRU_ghost buffer transitions to MFU. The l2_prefetch_asize
reflects the alinged size (in bytes) of L2ARC buffers that were cached
while they had the prefetch flag set in ARC. This is dynamically updated
as the prefetch flag of L2ARC buffers changes.
When buffers are evicted from ARC, if they are determined to be L2ARC
eligible then their logical size is recorded in
evict_l2_eligible_m{r,f}u arcstats according to their ARC state upon
eviction.
Persistent L2ARC:
When committing an L2ARC buffer to a log block (L2ARC metadata) its
b_arcs_state and prefetch flag is also stored. If the buffer changes
its arcstate or prefetch flag this is reflected in the above arcstats.
However, the L2ARC metadata cannot currently be updated to reflect this
change.
Example: L2ARC caches an MRU buffer. L2ARC metadata and arcstats count
this as an MRU buffer. The buffer transitions to MFU. The arcstats are
updated to reflect this. Upon pool re-import or on/offlining the L2ARC
device the arcstats are cleared and the buffer will now be counted as an
MRU buffer, as the L2ARC metadata were not updated.
Bug fix:
- If l2arc_noprefetch is set, arc_read_done clears the L2CACHE flag of
an ARC buffer. However, prefetches may be issued in a way that
arc_read_done() is bypassed. Instead, move the related code in
l2arc_write_eligible() to account for those cases too.
Also add a test and update manpages for l2arc_mfuonly module parameter,
and update the manpages and code comments for l2arc_noprefetch.
Move persist_l2arc tests to l2arc.
Reviewed-by: Ryan Moeller <freqlabs@FreeBSD.org>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes#10743
The ARC caches data in scatter ABD's, which are collections of pages,
which are typically 4K. Therefore, the space used to cache each block
is rounded up to a multiple of 4K. The ABD subsystem tracks this wasted
memory in the `scatter_chunk_waste` kstat. However, the ARC's `size` is
not aware of the memory used by this round-up, it only accounts for the
size that it requested from the ABD subsystem.
Therefore, the ARC is effectively using more memory than it is aware of,
due to the `scatter_chunk_waste`. This impacts observability, e.g.
`arcstat` will show that the ARC is using less memory than it
effectively is. It also impacts how the ARC responds to memory
pressure. As the amount of `scatter_chunk_waste` changes, it appears to
the ARC as memory pressure, so it needs to resize `arc_c`.
If the sector size (`1<<ashift`) is the same as the page size (or
larger), there won't be any waste. If the (compressed) block size is
relatively large compared to the page size, the amount of
`scatter_chunk_waste` will be small, so the problematic effects are
minimal.
However, if using 512B sectors (`ashift=9`), and the (compressed) block
size is small (e.g. `compression=on` with the default `volblocksize=8k`
or a decreased `recordsize`), the amount of `scatter_chunk_waste` can be
very large. On a production system, with `arc_size` at a constant 50%
of memory, `scatter_chunk_waste` has been been observed to be 10-30% of
memory.
This commit adds `scatter_chunk_waste` to `arc_size`, and adds a new
`waste` field to `arcstat`. As a result, the ARC's memory usage is more
observable, and `arc_c` does not need to be adjusted as frequently.
Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Wilson <gwilson@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes#10701
A bunch of places need to edit files to incorporate the configured paths
i.e. bindir, sbindir etc. Move this logic into a common file.
Create arc_summary by copying arc_summary[23] as appropriate at build
time instead of install time.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Closes#10559
