Albretch Mueller writes: [...]
does dd actually hit the bare metal drive or is it just reaching the
disks cache
This is what I am consistently getting from my code doing intesive IO
on the RAM drive:
// __ write speed test
# time dd if=/dev/zero of="${_RAM_MNT}"/zero bs=4k count=100000
[...]
409600000 Bytes (410 MB, 391 MiB) kopiert, 0,756943 s, 541 MB/s
[...]Just to add a datapoint: I have /tmp on a ramdisk and initially thought you might be using too little data to test properly? However, after testing, one can see that my system outputs higher transfer rates even when using small amounts of data...
### Ramdisk (tmpfs) Write Speed ~$ time dd if=/dev/zero of=/tmp/ramtest.bin bs=4k count=1536000 1536000+0 records in 1536000+0 records out 6291456000 bytes (6.3 GB, 5.9 GiB) copied, 2.64634 s, 2.4 GB/s real 0m2.648s user 0m0.816s sys 0m1.832s ~$ time dd if=/dev/zero of=/tmp/ramtest.bin bs=4k count=100000 100000+0 records in 100000+0 records out 409600000 bytes (410 MB, 391 MiB) copied, 0.179949 s, 2.3 GB/s real 0m0.182s user 0m0.033s sys 0m0.149s ### HDD (MDADM RAID1) Write Speed $ time dd if=/dev/zero of=/fs/e01/nobak/test.bin bs=4k count=5242880 conv=fdatasync 5242880+0 records in 5242880+0 records out 21474836480 bytes (21 GB, 20 GiB) copied, 168.831 s, 127 MB/s real 2m48.833s user 0m3.137s sys 0m12.492s ### SSD (MDADM RAID1) Write Speed $ time dd if=/dev/zero of=/home/linux-fan/wd/test.bin bs=4k count=5242880 conv=fdatasync 5242880+0 records in 5242880+0 records out 21474836480 bytes (21 GB, 20 GiB) copied, 47.0841 s, 456 MB/s real 0m47.086s user 0m3.049s sys 0m12.246sCould it be possible that on your system, writing to the ramdisk is somehow bound by CPU speed? What hardware are you using? My tests were on this system:
$ syssheet -g ┌─────── System Sheet Script 1.2.3, Copyright (c) 2012-2020 Ma_Sys.ma ─────────┐ │ linux-fan (id 1000) on rxvt-unicode-256color Debian GNU/Linux 10 (buster) │ │ Linux 4.19.0-9-amd64 x86_64 │ │ 17.06.2020 13:15:16 pte5 │ │ up 31 min, 8 users, load avg: 1.29, 2.83, 1.73 2004/32121 MiB │ │ 8 Intel(R) Xeon(R) CPU E3-1231 v3 @ 3.40GHz │ ├────────────────────────────────── Network ───────────────────────────────────┤ │ Interface Sent/MiB Received/MiB Address │ │ eno1 0 0 │ │ eno2 47 2641 192.168.1.16/24 │ │ ens2 0 0 192.168.2.1/30 │ ├─────────────────────────────── File systems ─────────────────────────────────┤ │ Mountpoint Used/GiB Of/GiB Percentage │ │ / 204 251 84% │ │ /fs/ll 1324 1563 86% │ │ /fs/e01 1413 1615 93% │ │ /data 100 216 50% │ ├─────────────────────────────────── Users ────────────────────────────────────┤ │ Username MEM/MiB Top/MEM CPU Top/CPU Time/min │ │ colord 13 colord 0% colord 0 │ │ Debian-exim 3 exim4 0% exim4 0 │ │ rtkit 2 rtkit-daemon 0% rtkit-daemon 0 │ └──────────────────────────────────────────────────────────────────────────────┘ $ mount | grep '/tmp' tmpfs on /tmp type tmpfs (rw,relatime,size=6291456k,nr_inodes=1048576) HTH Linux-Fan
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