如果我将外部硬盘插入 RasberryPi4，然后运行：
time du -sh /mnt/disk/thisfolderis300gb.dir
然后大约需要6分钟，后续所有运行也需要6分钟，
但是，如果我将相同的硬盘插入我老化的 Linux 笔记本电脑，并运行相同的命令，第一次运行也需要 6 分钟，但所有后续运行只需要2秒。
显然某种缓存在这里起作用。这里到底发生了什么，我怎样才能在 RPi 上获得相同的“性能”？ （我尝试复制挂载选项，没有效果）
我注意到这两款设备的内存都接近耗尽。
硬件详细信息：
硬盘：外置 2.5 英寸 2TB USB3、ext4。RaspberryPi4
：4GB RAM、USB3。
老化笔记本电脑：6 年以上、16GB RAM、linux mint、仅 USB2。
RaPi 挂载选项：(rw,relatime)
Linux mint 挂载选项：(rw,nosuid,nodev,uhelper=udisks2)
*我尝试使用上述两种安装选项安装在RaPi上，结果是相同的。
笔记本电脑/proc/meminfo，在运行之前
MemTotal: 16386892 kB
MemFree: 11127292 kB
MemAvailable: 13237468 kB
Buffers: 192568 kB
Cached: 2331040 kB
SwapCached: 0 kB
Active: 3166152 kB
Inactive: 1523380 kB
Active(anon): 2166896 kB
Inactive(anon): 312204 kB
Active(file): 999256 kB
Inactive(file): 1211176 kB
Unevictable: 96 kB
Mlocked: 96 kB
SwapTotal: 2097148 kB
SwapFree: 2097148 kB
Dirty: 124 kB
Writeback: 0 kB
AnonPages: 2166040 kB
Mapped: 918028 kB
Shmem: 314208 kB
KReclaimable: 238016 kB
Slab: 377336 kB
SReclaimable: 238016 kB
SUnreclaim: 139320 kB
KernelStack: 15360 kB
PageTables: 38840 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 10290592 kB
Committed_AS: 10813140 kB
VmallocTotal: 34359738367 kB
VmallocUsed: 42604 kB
VmallocChunk: 0 kB
Percpu: 6208 kB
HardwareCorrupted: 0 kB
AnonHugePages: 0 kB
ShmemHugePages: 0 kB
ShmemPmdMapped: 0 kB
CmaTotal: 0 kB
CmaFree: 0 kB
HugePages_Total: 0
HugePages_Free: 0
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB
Hugetlb: 0 kB
DirectMap4k: 362240 kB
DirectMap2M: 16404480 kB
笔记本电脑/proc/meminfo，运行du后
MemTotal: 16386892 kB
MemFree: 8264124 kB
MemAvailable: 12752864 kB
Buffers: 1291636 kB
Cached: 2430124 kB
SwapCached: 0 kB
Active: 4706588 kB
Inactive: 1605828 kB
Active(anon): 2591628 kB
Inactive(anon): 346244 kB
Active(file): 2114960 kB
Inactive(file): 1259584 kB
Unevictable: 96 kB
Mlocked: 96 kB
SwapTotal: 2097148 kB
SwapFree: 2097148 kB
Dirty: 5784 kB
Writeback: 0 kB
AnonPages: 2590316 kB
Mapped: 949516 kB
Shmem: 348248 kB
KReclaimable: 1452468 kB
Slab: 1595388 kB
SReclaimable: 1452468 kB
SUnreclaim: 142920 kB
KernelStack: 16544 kB
PageTables: 45964 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 10290592 kB
Committed_AS: 12123736 kB
VmallocTotal: 34359738367 kB
VmallocUsed: 43836 kB
VmallocChunk: 0 kB
Percpu: 6208 kB
HardwareCorrupted: 0 kB
AnonHugePages: 0 kB
ShmemHugePages: 0 kB
ShmemPmdMapped: 0 kB
CmaTotal: 0 kB
CmaFree: 0 kB
HugePages_Total: 0
HugePages_Free: 0
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB
Hugetlb: 0 kB
DirectMap4k: 376576 kB
DirectMap2M: 16390144 kB
RasPi4 /proc/meminfo，在运行 du 之前
MemTotal: 3999784 kB
MemFree: 3828048 kB
MemAvailable: 3789780 kB
Buffers: 14584 kB
Cached: 51556 kB
SwapCached: 0 kB
Active: 48208 kB
Inactive: 32684 kB
Active(anon): 14984 kB
Inactive(anon): 8336 kB
Active(file): 33224 kB
Inactive(file): 24348 kB
Unevictable: 16 kB
Mlocked: 16 kB
HighTotal: 3264512 kB
HighFree: 3189372 kB
LowTotal: 735272 kB
LowFree: 638676 kB
SwapTotal: 524284 kB
SwapFree: 524284 kB
Dirty: 4 kB
Writeback: 0 kB
AnonPages: 14792 kB
Mapped: 19408 kB
Shmem: 8576 kB
Slab: 34932 kB
SReclaimable: 14160 kB
SUnreclaim: 20772 kB
KernelStack: 1072 kB
PageTables: 1108 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 2524176 kB
Committed_AS: 85956 kB
VmallocTotal: 245760 kB
VmallocUsed: 0 kB
VmallocChunk: 0 kB
Percpu: 608 kB
CmaTotal: 262144 kB
CmaFree: 222908 kB
RasPi4 /proc/meminfo，运行 du 后
MemTotal: 3999784 kB
MemFree: 3274908 kB
MemAvailable: 3742736 kB
Buffers: 428308 kB
Cached: 37832 kB
SwapCached: 0 kB
Active: 254960 kB
Inactive: 227440 kB
Active(anon): 16504 kB
Inactive(anon): 8336 kB
Active(file): 238456 kB
Inactive(file): 219104 kB
Unevictable: 16 kB
Mlocked: 16 kB
HighTotal: 3264512 kB
HighFree: 3200564 kB
LowTotal: 735272 kB
LowFree: 74344 kB
SwapTotal: 524284 kB
SwapFree: 524284 kB
Dirty: 0 kB
Writeback: 0 kB
AnonPages: 16284 kB
Mapped: 19976 kB
Shmem: 8580 kB
Slab: 185740 kB
SReclaimable: 158128 kB
SUnreclaim: 27612 kB
KernelStack: 1056 kB
PageTables: 1172 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 2524176 kB
Committed_AS: 89956 kB
VmallocTotal: 245760 kB
VmallocUsed: 0 kB
VmallocChunk: 0 kB
Percpu: 608 kB
CmaTotal: 262144 kB
CmaFree: 51616 kB
答案1
答案是，你猜对了，它是缓存。
如何改进缓存：更多 RAM。
- 停止运行使用大量 RAM 的进程。
- 获取更多 RAM（Pi 已达到最大容量，因此不是一个选项）。