03cc316001
The last three phases of the filesystem, which were written together and
touch the same files throughout -- kmount.c, objw.c and vfs.c are each
rewritten by all three -- so they land as one commit rather than three
that could not be built separately.
Durability. A transaction that reaches the platter out of order is worse
than one that never reached it: the uberblock can name a tree whose
blocks are still in the drive's write cache. bdirty/bflush give the
buffer layer an explicit barrier, ideflush issues ATA FLUSH CACHE and
usb_storage_flush the SCSI SYNCHRONIZE CACHE that means the same thing,
and a commit now waits for the data before it writes the root that
points at it. The uberblock groups are read newest-first with fallback,
so a volume whose newest roots are torn mounts at an older one and says
which generations were lost.
Reflink. Extents carry a refcount, so two objects can name the same
blocks and neither frees them while the other holds them. Exposed as
FICLONE -- Linux's ioctl value, so `cp --reflink` works unmodified --
and as cmd/reflink for the case where there is no cp to hand.
Attributes, and the one genuinely new idea here. attr.c stores named
values on an object, reachable through setxattr/getxattr/removexattr at
Linux's syscall numbers. On top of that, schfs_index declares an index
over an attribute name and schfs_query answers "which objects carry this
value" from the B-tree rather than by walking. A query against an
unindexed attribute is refused with an error, never silently answered by
a full scan: a query that quietly becomes a scan is worse than one that
admits it cannot help. cmd/schattr drives both.
Three bugs fixed on the way, each found by the numbers being wrong
rather than by inspection:
- statfs's f_fsid was declared int64[2]. Linux's fsid_t is two *ints*,
so every field after it landed one slot late: userland read f_frsize
out of f_namelen, got 255, and df reported a 32 MB root filesystem
as 1992 KB. It hid for as long as it did because the fields used to
be invented constants, so wrong numbers looked like the usual ones.
- struct inode's nlink was a short against SchFS's uint32, so a file
past 32767 names came back negative -- and iput frees an object
whose count reaches zero. It is an int now, signed on purpose, so an
underflow lands somewhere a check can see rather than wrapping.
- filewrite split every write to fit v1's log. That limit belongs to
the journal, not to writing, and SchFS makes one transaction of
whatever it is handed: a 1 MB write was becoming 170 commits and
170 flushes for nothing. fs_ops carries max_write now, and SchFS
sets no limit.
Also here: a real mount table, so `mount -t schfs /dev/hdb /mnt` shows
up in /proc/mounts -- df looks a path up there before it calls statfs at
all, so a mount missing from that list was invisible to it -- several
volumes mounted at once, snapshots mounted by name, and /proc/fsstat
with an rdtsc-based counter, which is the only instrument fine enough to
see inside a single syscall.
Verified: `make schfs-check` passes in full, including the sweep that
crashes the volume at each of 119 points mid-transaction and requires
every one to mount with exactly its committed data, and the case where
both newest uberblocks are rubbish and the volume still comes back. The
headless boot check passes with 27 [ok] and no failures, on a SchFS root.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
46 lines
1.6 KiB
C
46 lines
1.6 KiB
C
// SPDX-License-Identifier: Apache-2.0
|
|
//
|
|
// Counters for the filesystem and block paths, read through /proc/fsstat.
|
|
//
|
|
// These exist because "the system feels slow" is not a bug report anyone can
|
|
// act on, and every guess about where the time goes in a filesystem is wrong
|
|
// until it is measured. `ls /bin` taking three seconds could be the directory
|
|
// walk, the tree descents, the copy path, or the disk -- and those four call
|
|
// for completely different fixes.
|
|
//
|
|
// Deliberately plain `uint64` with no locking. A counter that is off by one
|
|
// because two CPUs raced tells you the same thing as one that is exact, and a
|
|
// lock here would change the very timings it is meant to report.
|
|
#pragma once
|
|
|
|
struct fsstat {
|
|
// Buffer cache. `bread_miss` is the one that matters: a miss is a real
|
|
// transfer, a hit is a memcpy.
|
|
uint64 bread_calls;
|
|
uint64 bread_miss;
|
|
uint64 bget_evict_dirty; // had to write a buffer out to make room
|
|
|
|
// Actual device transfers, and the cycles spent inside them.
|
|
uint64 disk_reads;
|
|
uint64 disk_writes;
|
|
uint64 disk_cycles;
|
|
|
|
// The VFS read path: one call is one read(2) reaching a SchFS file.
|
|
uint64 read_calls;
|
|
uint64 read_bytes;
|
|
uint64 read_cycles;
|
|
|
|
// Directory listing.
|
|
uint64 getdents_calls;
|
|
uint64 getdents_cycles;
|
|
uint64 getdents_walked; // entries stepped over, including ones re-walked
|
|
|
|
// B-tree descents from the root, and the block reads they cost.
|
|
uint64 descents;
|
|
|
|
// exec(2), which on this system means reading an 800 KB multicall binary.
|
|
uint64 exec_calls;
|
|
uint64 exec_cycles;
|
|
};
|
|
|
|
extern struct fsstat fsstat;
|